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120/208V or 277/480V.... huh?
what does it mean when something is rated 120/208V or 277/480V? What do the two numbers mean?

what does it mean when something is rated 120/208V or 277/480V? What do the two numbers mean?

Help, very confused!

It means it is rated to operate at those two voltages.

Either 120V or 208V.

0 out of 1 members thought this post was helpful...

It means that your source power is 120v from a 2 wire system, or single phase 2 wire. The combination of 120v from the 2 wire system will operate at 208volts. When 120 is added to 120 it essentially equals 208, not 240.If you would like to know, why? I can give you an explanation...

By W.L. Mostia on 11 February, 2009 - 7:25 pm

I don't believe that this post is correct. 120/208 VAC is nominal 3 phase voltage (i.e. 120 VAC * SQRT 3) as is 277/480 VAC with the phase to neutral/ground voltage first.

Single phase 2-wire w/neutral like is in most people's houses is nominally 120 VAC to neutral/ground and 240 VAC phase to phase.

William (Bill) L. Mostia, Jr., PE
Sr. Consultant
SIS-Tech Solutions

Can you get 208v to supply something (as per the nameplate requirement)from a 240/120 household system?

> Can you get 208v to supply something (as per the nameplate requirement)from a
> 240/120 household system?

I am also looking this answer but did not get it yet

By Curt Wuollet on 17 December, 2011 - 5:42 pm

Yes, with a transformer. You need a center tapped secondary to give 120/240 single phase with neutral.

The primary connects to any two 3 phase wires. That's why machine tools
for world wide use are so full of transformers.

Regards
cww

> Can you get 208v to supply something (as per the nameplate requirement) from a 240/120 household system?

if the load is resistive it is possible

0 out of 1 members thought this post was helpful...

>> Can you get 208v to supply something
>(as per the nameplate requirement) from
>a 240/120 household system?
>
>if the load is resistive it is possible

Remember the rule that your conductors gauge or thickness directly correlates with its maximum current carrying capacity (amps). A resistive load could be powered on 208v even though it's rated for 240v but this lower voltage of 32 less volts going to this 'heater' (I'll call it) now needs 'a little' more amps to keep its wattage rating where it is (for example, a plug-in heater at 1kW).

1 kW heater rated for 240 volts will draw 4.2 amps; if this same heater is then loaded up with 208 volts the amps now become 4.8 (0.6 amp increase, slight increase, but something worth noting when using the other voltage). 208 volt will now draw more amps in its supply conductors.

If these conductors (wires) are too thin for this additional ampacity, a fire could happen. I'm sure that most of the time it's ok because in North America this is such a common problem and I'm sure the manufacturers safeguard against this theoretical risk. But this risk is a real risk trust me.

Likewise say, you have the opposite problem, where you have a 208v rating and want to hook up with the higher 240v. Overvoltage to an appliance can be just as bad as overamperage talked about above. I've had a hot water tank rated for 208v or 240v (dual rating) but it was hooked up to single-phase 480v, this is an extreme example of overvoltage because the person wiring it in didn't know what he was doing, and what happens? The bottom of the tank blows out and leaks everywhere below (there was a restaurant below).

Overvoltage can damage equipment. Just like overamperage, the more you deviate from your rating (10s of volts different, then worse yet 100s of volts different, and so on), the more likely damage will occur. Keep your appliances voltage rating exactly as labeled or as close to as possible.

As for induction motors, they sometimes can be dual-tapped for both voltage choices. If they are only one of the other, then I wouldn't recommend using the unlabeled voltage. For the same reason as the resistive load, you could overamp or overvolt, causing overheating or damage to motor. If the voltage is too low, the motor might not start, or spin slower and likely overheat from trying to get to its rated r.p.m. (this needs to suck more amps now to spin at its speed).

By Steve Myres on 15 August, 2012 - 9:43 pm

Excellent answer, if by "heater" you mean "motor". Actual "heaters" draw LESS current at lower supply voltage, thus conveniently enabling them to obey the P = E^2/R rule.

> Excellent answer, if by "heater" you mean "motor". Actual "heaters" draw
> LESS current at lower supply voltage, thus conveniently enabling them to obey
> the P = E^2/R rule.

You are correct ONLY if it is a pure resistive load... P = E * I and for this case you use this version of Ohms Law: I = E/R but again, you are correct only if this is a pure resistive load...

"You are correct ONLY if it is a pure resistive load... P = E * I and for this case you use this version of Ohms Law: I = E/R but again, you are correct only if this is a pure resistive load..."

Which is more correct than the guy who's talking about heaters and then tries to apply rules about inductive loads to them. Heaters are just resistance elements and you feed them power. Very simple. Ohms law applies just fine.

What about the reverse? Can you power something rated for 208v with 220v single phase service?

> As for induction motors, they sometimes can be dual-tapped for both voltage choices. If they are only one of the
> other, then I wouldn't recommend using the unlabeled voltage. For the same reason as the resistive load, you could
> overamp or overvolt, causing overheating or damage to motor. If the voltage is too low, the motor might not start, or
> spin slower and likely overheat from trying to get to its rated r.p.m. (this needs to suck more amps now to spin at its speed).

undervoltage will cause more damage than overvoltage, especially when dealing with ac motors.

By Espartacuss on 22 August, 2013 - 3:23 pm

> I don't believe that this post is correct. 120/208 VAC is nominal 3 phase voltage (i.e. 120 VAC * SQRT 3) as is
> 277/480 VAC with the phase to neutral/ground voltage first.

Yeah you are correct BUT !!! And I have to emphasis this: As follows for purposes of clarification and education.

277/480volt is always 3 phase with 4 wire system
This system uses 4 physical wires
Neutral, (white striped) Hot (black, common) Blue (Blue also common) and Grounding wire conductor (color Green)

120/208volt is also a 3 phase 4 wire system
For this the grounded conductor must be white or gray only !!!

> 277/480volt is always 3 phase with 4 wire system
> This system uses 4 physical wires Neutral, (white striped) Hot (black,
> common) Blue (Blue also common) and Grounding wire conductor (color Green)

> 120/208volt is also a 3 phase 4 wire system
> For this the grounded conductor must be white or gray only !!!

Be wary - those colour codes do not apply all over the world.

> 277/480volt is always 3 phase with 4 wire system

> This system uses 4 physical wires Neutral, (white striped) Hot (black, common) Blue (Blue also common) and
> Grounding wire conductor (color Green)

> 120/208volt is also a 3 phase 4 wire system
> For this the grounded conductor must be white or gray only !!!

I believe this to be incorrect. At least where I work.
277/480
Hots are
Brown, Orange, Yellow
Neutral
Grey
Ground Green

120/208 3 ph 4 w
Hots
Black Red Blue
Neutral
White
Ground
Green

By Bob Peterson on 11 September, 2013 - 5:56 pm

277/480 indicates a 480V wye system - 3 hots and a neutral (plus ground).
480 V L-L and 277 V L-N.

208/120 indicates a 208V wye system - 3 hots and a neutral (plus ground).
208 V L-L and 120 V L-N.

In the US there is no national standard for the color code of the hots. Most plants have some kind of practice they follow, but it is not mandated.

neutral is always white or grey. Ground is always green.

> In the US there is no national standard for the color code of the hots. Most
> plants have some kind of practice they follow, but it is not mandated.

> neutral is always white or grey. Ground is always green.

Some three phase systems have a high leg that will read 208V to ground while the other two will read 120V. 2014 NEC requires this high leg to be marked with orange tape and it is recommended to place it in the middle.

480 volt 3 phase is B O Y colors = brown orange yellow easy to remember.

By Curt Wuollet on 11 February, 2009 - 9:25 pm

I'd like to see that.

Regards
cww

Hello that's not it at all. 208 is three phases defend type like motors work at 480. They are a lot of voltages out there and everyone in the world thinks all they are to life is house voltage. Many three phase has also taps in them like 120,208 277,480 always read on this this might come default lower voltage then what you are going to use. Please guys don't answer electricity questions you have no clue about you could get someone killed. I lost a good friend over this. He went to take copper and pulled the meter. on commercial buildings they use a CT and when you pull the meter it does not kill the voltage. He got into 480 volts the service was a 2000 amp service. Yes 2000 that why commercial electricians hate residential ones. They have no clue what an electrician is.

Ken I hope you come back and read this.

You took the words right out of my mouth I hear people talk about electricity all the time and all they think in the whole world is 120/240 and also most of them are like 110/220. I have worked as a commercial electrician for many years I get so mad when I try to tell someone about it. I believe in doing things right. I help older people with this when someone goes to their house and hooked stuff up wrong. I tell then just because it works do not make it right. Commercial electric is no game I have seen people grab a neutral on a 277 and knock them out. Also ground and neutral is not the same. I think before someone should be able to buy with or say romex they should have to take a class. Where I work at many that thought they were electricians they were in a house but not with this. They did not know how many amps on a wire types and this all goes back to voltage. Like computer we are on if you're at a house you can run 12/2 to a 240 and your computer will use less amps and that means less your electric bill will be. I have wired buildings that have had over 25 panels in them. Guys in this they are no romex here you make your own circuits and you have to know what you are doing. When you see something that like 120/208 or 277/480 this means something a lot of motors have taps to use these voltages. If you put 240 on the tap that's made for 120 well say buy to it. Electricity is not something to play with. Like ken said if you don't know what you are doing don't tell someone else how to do it. You could kill someone or mess something up. I know I don't spell well but you guys could learn this stuff it takes years to really understand this stuff and still some of it don't really make any since. Like 480/277 think about it/

By David Davis on 15 January, 2012 - 6:19 pm

Ken Ken, You say that you "can't spel" as if that doesn't matter. I am not very good at spelling either, so I use a dictionary as needed. Since you didn't bother to use a dictionary as you wrote your post, I can't help but wonder if you might not be a bit slow to look up details in the NEC as appropriate. The original poster may not have as much experience as you do in wiring, but he is at least trying to look up the information that he lacks. Who would you rather trust, the guy who thinks he knows it all and won't look it up, or the guy who knows he doesn't know and double checks?

By Bob Peterson on 15 January, 2012 - 8:53 pm

> Ken Ken, You say that you "can't spel" as if that doesn't matter. I am not
> very good at spelling either, so I use a dictionary as needed. Since you
> didn't bother to use a dictionary as you wrote your post, I can't help but
> wonder if you might not be a bit slow to look up details in the NEC as
> appropriate. The original poster may not have as much experience as you do
> in wiring, but he is at least trying to look up the information that he
> lacks. Who would you rather trust, the guy who thinks he knows it all and
> won't look it up, or the guy who knows he doesn't know and double checks?

Unfortunately, many chunks of the NEC are not for amateurs. Looking it up does not mean a guy who does not know any better will get clued in correctly. The guy who does not know and looks it up may end up thinking he knows, but still not know.

Let me ask what should be a simple question.

What color should the grounding electrode color be?

Hint - the grounding electrode conductor is the conductor that connects the grounded conductor (often the neutral - but not always) to whatever grounding electrodes(s)such as ground rods you may have.

I have a question.

I have an air compressor that calls for 208 delta or 360Y both three phase on the data plate of the motor. The compressor itself calls for 208/3 phase 155 amp.

I have 240/3 phase and my local compressor guy and two electricians say it will work fine. one electrician says I need two BUCK transformers to knock the power down.

I went with the majority and it does not work when connected to three phase 240. the contactor clangs back and forth. I replaced the contactor and coil and get the same clanging. The motor ohms good and not grounded.

ANy thoughts.

2 out of 2 members thought this post was helpful...

Kbob... the 240Vac source, represents only a 5% over-voltage above typical design values! The motor, when connected in delta should tolerate it!

More significant, however, is your observation about the contactor "clanging!" It suggests a starter problem.

How is the motor started, DOL, D-Y, Part-winding, or Soft-Start?

Regards, Phil Corso (cepsicon@aol.com)

> I have a question.

> I have an air compressor that calls for 208 delta or 360Y both three phase on the data plate of the motor. The
> compressor itself calls for 208/3 phase 155 amp.

> I have 240/3 phase and my local compressor guy and two electricians say it will work fine. one electrician says
> I need two BUCK transformers to knock the power down.

> I went with the majority and it does not work when connected to three phase 240. the contactor clangs back and
> forth. I replaced the contactor and coil and get the same clanging. The motor ohms good and not grounded.

> ANy thoughts.

you aren't able to carry the motor's starting current; that's why the contactor chatters. You need to look up the starting current and select wire gauge per NEC for your application and probably delayed trip over current protection.

Probably best to hire somebody or go ask a local electrician so you don't run your motor backwards and ruin your compressor. (no kidding)

BTW- three phase power isnt 120/240, it is 120/208 or 277/480 etc. THREE phase or hot wires, neutral and ground. in the UK it's 240/415 and in germany it's 220/380. (that's in most homes over there)

> a house you can run 12/2 to a 240 and your computer will use less amps and that means less your electric bill will be. I have wired buildings that have had over 25 panels in them. Guys in this they are no romex here you make your own circuits and you have to know what you are doing. When you see something that like 120/208 or 277/480 this means something a lot of motors have taps to <

I would like to see your calculation for power consumption if you believe 240V is cheaper than 120V to the same load. Power (watts) is P=IE (current times voltage). If you draw 10 amps at 120V, you will draw 5 amps at 240V so...

P=10x120V = 1200 watts
P=5x240V = 1200 watts

2 out of 2 members thought this post was helpful...

Tim... you probably have done some investigation of branch-circuit losses. Some advocate the use of Power Factor Correction (PFC) devices, for energy-use reduction.

My study, to the Florida EPA, showed that increasing branch-circuit wiring for major appliances, by one-size, will achieve the same savings at a fraction of the cost!

I am talking of new house wiring, of course!

Regards, Phil Corso

> you probably have done some investigation of branch-circuit losses. Some advocate the use of Power Factor Correction (PFC) devices, for energy-use reduction.
>
> My study, to the Florida EPA, showed that increasing branch-circuit wiring for major appliances, by one-size, will achieve the same savings at a fraction of the cost!
>
> I am talking of new house wiring, of course!

Since when is a personal computer considered a major appliance?

1 out of 1 members thought this post was helpful...

Powah... I certainly never claimed computers are major appliances.

Would you like a copy?

Regards, Phil Corso

> I would like to see your calculation for power consumption if you believe 240V is cheaper than
> 120V to the same load. Power (watts) is P=IE (current times voltage). If you draw 10 amps
> at 120V, you will draw 5 amps at 240V so...

> P=10x120V = 1200 watts
> P=5x240V = 1200 watts
Smaller gauge copper costs less...

> you can run 12/2 to a 240 and your computer will use less amps and that
> means less your electric bill will be.

Wrong.

Stuff uses less amps at higher voltage this is correct but it uses the same amount of power, which the power companies measure in kw/h, and it is exactly the same amount regardless of if you run your computers xwatt power supply at 120 or 240V.

but looses are higher when current is higher, so finally he can save some money if Voltage is higher and current smaller :-)

By edwin Valerio on 16 January, 2012 - 9:36 am

Only have 125 coming from one wire in electrical panel. Other wire is dead. Tried to check if there was a fuse box or something and did not find any.

What to do?

> Only have 125 coming from one wire in electrical panel. Other wire is dead.
> Tried to check if there was a fuse box or something and did not find any.

> What to do?

...Reset the circuit breaker, its possible you have a tie broken between your two breakers. Double check that both breakers are in the on position and hold there. If this does not work then denergize the circuit, ALL OF IT AND CONFIRM WITH A VOLTAGE METER THAT IT IS DE-ENERGIZED, at the device tie the phase to the ground then at the panel check to see if you have continuity between phase and ground, if not then your wire has been compromised and must be spliced or replaced.

Sir can you explain further?

The 208 VAC is the voltage between two legs of a Y connected xformer or alternator. so for each leg at 120 VAC to neutral, the leg-to-leg voltage is 240*cos(30 deg)~208 VAC. Same deal with the 277/480 VAC

it means that the related equipment can be energized under a rated voltage from 120V (ac supply i think) up to 208V.

THEO

By Bilal Janjua on 26 September, 2003 - 4:21 pm

I think they refer to primary and secondary voltages.

The number are for three phase systems - 120 -voltage from phase to neutral / 208 - voltage phase to phase 277 -voltage from phase to neutral / 480 - voltage phase to phase

Things that are rated are for these voltage suitable for service at these voltages.

Bill Mostia
==========================
William(Bill) L. Mostia, Jr. P.E.
Partner
exida.com
Worldwide Excellence in Dependable Automation
wmostia@exida.com (b) wlmostia@msn.com (h)
www.exida.com 281-334-3169
These opinions are my own and are offered on the basis of Caveat Emptor.

120/208VAC means that power distribution is 3 phase with neutral connection (Y System). 120V represents voltage of each phase to neutral point and 208V represents voltage between any two phases.

For balance systems, line to neutral voltage is line to line voltage divided by 1.73 (Sq. root of 3). i.e 120x1.73=208V and 277x1.73=480V

It seems that there are 3 numbers 120VAC or the eqipment can run on 208VAC

the first number is your L-N voltage, the second is your L-L voltage on a 3 phase system.

By Wasim Ullah Khan on 27 September, 2003 - 8:32 am

This is the Voltage rating commonly used in the world. Some countries have 120 V the others have approx. 240V at consumer end. These ratings are single phase. THe 277/480V is 3 Phase rating. So the appliances which bearing rating 120V/208V or 227/480V can be plugeed in both standards.

> This is the Voltage rating commonly used in the world. Some countries have 120 V the others have approx. 240V at consumer end. These ratings are single phase. THe 277/480V is 3 Phase rating. So the appliances which bearing rating 120V/208V or 227/480V can be plugeed in both standards.

Here in Canada alot of residential services are 120/240V systems which is a single phase 120V system but two poles seperated by 180 deg therefore giving the effective Voltage of 240V for large appliances (ie Stoves, Dryers etc..)

When it comes to three phase services here you will see 120/208V or 347/600V for larger application. The odd industrial site you will see 277/480V but those are only sites where it was engineered by an American company that doesn't realise Canada uses a 347/600V system.

Caveat emptor. This whole thread has been interesting because of the many different levels of responses. Some on the money, others just plain absolutely wrong.

Bottom line, learn something about your subject if you are asking a question and be prepared to weed out poor information from good. It's kind of like the internet in general I guess.

However, I do wish the people who like to hear themselves would be more careful about their answers. Hey, if you don't know something pretty well, skip feeling the need to respond.

Los Angeles, CA
USA
bob@mindspan.us

By David Adams on 5 October, 2003 - 5:12 pm

ditto

2 out of 2 members thought this post was helpful...

I am very interested in what you identified as ... "Some on the money, others... absolutely wrong."

This is, after all, a List that members use to elicit help. Others impart knowledge. I'm sure that those with wrong answers would like to know what is right!

ps: welcome back "Johan"

Regards, Phil Corso, PE Boca Raton, FL [tal-2@webtv.net] (Epsiconinc@aol.com) {pcorso@itt-tech.edu}

By robert trask on 16 October, 2003 - 10:07 pm

Not to sound nasty or anything but... I and others posted a valid response to the original question. I do not have the time to correct every 'bad' answer I see posted to the list. Not to mention that people tend to get REAL upset
when you 'correct' them, even privately. So, no, my experience has shown me that people posting crummy answers just get real defensive, so I don't even go there anymore.

As smart engineers (or whatever you consider yourself) we have to be able to weed out good information from poor. Period.

I'm getting some good chuckles out of the responses to the 'synchronous vs asynchronous motor' thread right now. There are some real hooters in there.

Los Angeles, CA USA
bob@mindspan.us

By Johan Bengtsson on 27 September, 2003 - 11:25 am

It is a 3-phase device (motor/heater/...)
Those devices can be connected in two different ways to the three phases.

D connection - connect each of the three parts of the device between two phases in such a way that each phase is connected to two of these devices. Feed it with a 3-phase voltage with the lower rating

----------------
| |
Phase 1 ------*---XXXX---- |
| |
------------ |
| |
Phase 2 ------*---XXXX---- |
| |
------------ |
| |
Phase 3 ------*---XXXX--------

Y connection - connect one end of each part of the device to one phase each and the remaining ends tyed torether, those ends can optionally be connected to neutral. Feed it with a 3-phase voltage witht he higher rating

Phase 1 ----------XXXX----
|
Phase 2 ----------XXXX---*
|
Phase 3 ----------XXXX---*
|
Neutral (optional) -------

Note that the higher voltage is approx equal to the lower multiplied with sqrt(3). In europe we usually have a 230/400V system by the way...

/Johan Bengtsson

Do you need education in the area of automation?
----------------------------------------
P&L, Innovation in training
Box 252, S-281 23 Hässleholm SWEDEN
Tel: +46 451 74 44 00, Fax: +46 451 898 33
E-mail: johan.bengtsson@pol.se
Internet: http://www.pol.se/

By Christian Albert P.Eng. on 27 September, 2003 - 5:27 pm

No Problem. It means that you have a Y configuration three phases circuit. 3 phases circuits are widely used for power transmission and distribution because it's the most economical.
You can have two configurations: Y and Delta. The voltage you indicated says to an electrician that you have a Y configuration. The center of the Y is grounded and the phase to phase voltage is 208 volts or 480 Volts. The voltage from ground to each of the three phases is always the phase to phase voltage divided by the square root of 3 which is equal to 1.73. So, 208 divided by 1.73 equals 120 Volts and 480 divided by 1.73 equal 277 Volts. In Canada, we use 600 volts and the phase to ground voltage is 377 Volts. The phasing between voltage sinusoidal curves ( we say phases A, B and C) is 120 degrees. If you draw a phasor diagram with three vectors of equal amplitude (voltage) with angles of 120 degrees apart and add these vectors, you will have a result of zero. So, for a current balanced three phases circuit, the neutral current is zero. In practice, the three phases are rarely balanced.
The Y configuration needs four wires, the Delta uses only three wires. Y and Delta are used for technical consideration which are more complicated to explain.

Hope this will help.

By Nuno Torrado on 28 September, 2003 - 2:21 pm

I don't know what's the standard voltage in your contry but:

Sqrt(3)*120 = 208
Sqrt(3)*277 = 480

I think the first number refers to voltage between live and neutral and the second number to voltage between two lives.
Why the 277? don't know

The most popular connection on the secondary side of a 3-phase transformer is a "Y" connection. This means that one of the transformer secondary terminals (usually marked X0) is connected to earth ground and the remaining 3 terminals (usually marked X1, X2, and X3) are each of the three ouput phases.

On a 120/208 transformer secondary, the voltage measurement from the grounded X0 terminal to any of the other 3 terminals would be 120 VAC. The voltage between X1-X2-X3 in any combination would give you a measurement of 208 VAC. This is the typical connection scheme for residential power. You may also see this specified as 230/133.

On a 277/480 transformer secondary, the voltage measurement from the grounded X0 terminal to any of the other 3 terminals would be 277 VAC. The voltage between X1-X2-X3 in any combination would give you a measurement of 480 VAC. This specification is usually found in industrial or commercial applications (such as isolation transformers for variable speed drives). You may also see this specified as 460/266.

I tried to keep the above descriptions as simple as possible. If you need further details, email me at rjstyer@adelphia.net.

It means it will work for these two voltages i.e., a motor can be wired for either voltage. A electrical panel can be one or the other. Between to phases you would measure 208vac then one phase to netural 120vac or 480vac between phases and 277vac one phase to netural.

hope you understand

By James Ingraham on 28 September, 2003 - 2:23 pm

277V would be a single phase of a 3-phase 480V system, so those numbers make some sense. 120 would be a single phase of 208V... but I've never heard of using 3-phase 208 volt. 220, 230, or 240 volt, sure, but 208?

Incidentally, to figure out the voltage level of a 3-phase system given the voltage of one leg, multiply by the square root of 3 (about 1.732). If you want to know the voltage level of one leg of a 3-phase system divide the 3-phase voltage by squart root of 3.

-James Ingraham
Sage Automation, Inc.

You may note that the higher number is the lower one multiplied by approx. 1.742 (sqrt of 3) - the lower number is the phase to neutral voltage, the higher is the phase-to-phase voltage. The rest is electrical theory or plane geometry.

Meir

2 out of 2 members thought this post was helpful...

Each designation refers to a 3-phase, 4-wire source of power. They are used to supply power to both 3-phase and single-phase loads. In other words in addtion to the 3-phase wires carrying current, the neutral wire can too.

For example, in the USA, the first is properly designated as 208Y/120. It describes a source having three phase-to-phase voltages (3 of the wires) that are each 208 V, while each phase-to-neutral (the 4th wire) voltage is 120 V. The second is designated 480Y/277, meaning that the phase-to-phase voltages are 480 V, and the phase-to-neutral voltage is 277 V.

By the way, the neutral wire must be connected to ground (earth) at only one location, i.e., the neutal terminal of the supply transformer.

Regards, Phil Corso, PE
Boca Raton, FL
[tal-2@webtv.net]
(Epsiconinc@aol.com)
{pcorso@itt-tech.edu}

Well, if you noticed that if you multiply the first number by 1.73 you get the second number. That should tell you right away that it is something to do with 'balanced three-phase' power.

Both those 'somethings' are a three phase, four wire electric distribution system. And, for example, 480V would be the phase-to-phase voltage potential (although not in phase, a whole other issue) and 277 would be the voltage between a phase conductor and the neutral, or 'grounded conductor', as the NEC likes to call it.

Encinitas, CA
USA

1. There are basically two standards for AC input power supply worldwide. That is 230V in some parts of the world and 110V in some other parts.
2. 120/208V is for a 110V system basically with 120V as L to N and 208V as L to L. Similarly, for 277/480V, 277 is L to N and 480V is L to L
(V L to L = V L to N).
3. Now your question means that the equipment is expected to work either on 110V AC basic or 230VAC basic.

Dear confused:

First of all are you talking about transformers or motors?

Transformers are typically rated to be wired by several input voltages on the primary and may provide either a fixed output or a selected output voltage (done by taps usually).

Motors will show ratings for a say 240 vac and 480 vac, because it would be possible to wire the input of the motor to accept either voltage, along with those nameplate ratings they show the respective current (FLA) But in sizing the starter, you should use the smaller voltage as that is the HP rating you are sizing for (if you sized for the higher voltage and thne had to go back to the lower voltage for some reason, you would need a new starter.)

Matt Hyatt

By Anonymous on 2 April, 2006 - 6:01 pm

120/208 is single phase 277/480 three phase

By Anonymous on 8 June, 2006 - 1:15 am

Maybe I can help. I'm also learning about that too. I don't fully understand also. I'm still learning. I went to a trade school for electrical (it was a 20wk. deal so i had to learn as much as I could). But what 120/208v means is that the transformer from the pole outside is changing 7200v down to 120v which most of our equipment uses. The 208 comes from multiplying 120& the square root of 3 (also the same for 277/480). I think that's why we have 3 phase system. Meaning 3 lines or phases that are hot. (1st phase is black, 2nd phase is red, 3rd phase is blue and this is call low voltage. For high voltage(277/480) 1st ph. is brown, 2nd ph. is orange, 3rd ph. is yellow.

But for residential our voltage is 120/240v which is 3 wires. 2 hots (black, red, white (neutral) and of course the ground wire (green or bare). So to get 240 from 120 you'd have to get a 2 pole breaker and hook up both wires black and red to it and going to whatever equipment utilizing it such as washer/dryer, A/C, etc...
(120v+120v=240v) 277/480v is a 3phase system. 277 is usually for luminaires and the 480 can be for motors and other equipment in commercial and industrial Co.

It gets a little complicated but i hope that this will help you. Here are some more...

118/236, 120/240v, 240/480v, 120/277v, 208Y/120v
277/480v, 208/277, 208/480, 230/460v and etc...

I'm still learning as I go. I was like you wondering what the hell 120/240v, 208Y/120v, 240/480v meant. But now I know. So be careful. Remember it's not the voltage that kills you, it's the amps(I). Laters.

By antonio perez on 28 June, 2011 - 8:28 am

120/208V means its rated for both 120v and 208v you'd need a electrical testing meter to check the voltage on the source in other words the electrical panel. 120V is mostly for lights, plugs,and certain equipment. you have to check and see if its single phase or three phase thou. its a huge difference. same thing with 277-480V. 277 is mainly for commercial electrical lights. no plugs. 480V is for certain plugs but that are for certain mechanical, electrical machine equipment. very dangerous voltages. be very careful always use gloves and protective wear. only a licensed and experienced electrician should handle anything like this. but any type of electrical source is dangerous. depending on the situation.

Heavier motors love 3-phase power and its dumb powering them on single-phase. Motors work better on 3-phase power because of their revolving magnetic field characteristics. 480volt is a typical 3-phase power source and because of that it is used to power heavy motors only, not plugs, not lights. Other 3-phase sources include 208volts and 600volts, both are also good for motors. It's good for them! This is why high-rises and industrial sites have 3-phase power because they have heavy motors and machinery and these loads need 3-phase.

Single-phase power examples are anything in a home whether it be 120 or 240 volts (furnace motor included, this motor is not powerful enough to warrant a 3-phase supply to homes). 208volts can also be single-phase as well as 3-phase.

In a commercial environment, which is now 3-phase source, the single-phase loads here are convenience receptacles or 'plugs' and lights. Convenience receptacles are almost always 120 volts, like in a home. Lights, on the other hand, may not be 120 volts, but rather 277 or 347 volts depending on your 3-phase source whether it be 480 or 600 volts.

Lights, heaters or other resisitive loads can benefit from the higher voltage associated with 3-phase power but they don't require it because they don't have a revolving magnetic field like motors so they are almost always single-phase power in a high-rise. This is all part of 'splitting out' your 3-phase supply.

120 (single phase aspect) of 208 (3-phase aspect) that's essentially what 120/208 means. But there's so much more to know really, and I haven't read all of these responses but a lot of people probably have answered a lot of this on here.

how to calculate phase to line voltage. r=200, y=220, then what is ry=?

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Vas... your two readings should be measured relative to a common reference... for example, like neutral or earth!

Regards, Phil Corso

I have ordered a piece of equipment (still) from Germany that is rated at 80 amp 3 phase 240 volts. We have 208 volt 3 phase power to our building with 80 amps @ 208 volts designated for this still. The electrician will not connect this still because he thinks that the still will draw more current than he allowed for.

Heating elements. 3 - 7.5 KW heating elements (like in a water tank) and 1 - 4.5 KW heating element. The diagram shows 3 x 230/240 50/60 Hz

What will the current draw be for the heating elements if connected to 208 3 phase power? Is it true that the elements will produce about 74% of the rated KW if run on the lower voltage. ie 7.5KW + 7.5KW + 7.5KW + 4.5KW = 27KW at 240 volts

Please help. My electrician also stated that the motors will run hot at higher current if rated for 240 volts and run on 208 volts. Is he correct?

Another question.Would it be difficult to increase the voltage to 240 volts?

Tony

By Steve Myres on 14 November, 2011 - 7:57 pm

OK, X kw of resistance heaters rated for 240V will generate 75% X kw when operated on 208. The areal watt density will be lower, amp consumption lower, the heaters may even last longer (but if too closely sized may not create the amount of heat you need).

Motors, different story. Most 3-phase induction motors currently sold for US use will accept either 230 or 460 volts, depending on how you connect them....but 208/3/60 is VERY common in the US, probably more so than 240/3/60, so the motors are designed to be operated satisfactorily on 208 and if you look at the nameplate, they typically list the voltage as "208-230/460". Note the dash between the 208 and 230 vs. the slash between the 230 and 460. You could read the label as 208 TO 230 OR 460 (plus tolerance). Correspondingly, the FLA (Full Load Amperage) will be listed as something in the format "8.6-8.2/4.1", so the electrician is correct that the motors will draw slightly more at 208 than at 230, but only fractionally, and hopefully within the same wire/contactor/overload sizing he would have used at 230, and unless the motors are loaded at exactly 100% of their nameplate HP, may even be below the amount he calculated for consumption at 230V. PLUS...if he did the load calcs before seeing the machine, he had to get his FLA figures from somewhere, very possibly the NEC, which tend to be high vs. real world motors, so once again you may be under whatever he calculated. Besides, as far as the overall feeders to the machine, the reduced load from the resistance heaters will probably more than compensate for the slight increase from the motors.

So hopefully the Germans, knowing these things about US power distribution, have supplied motors that will run happily on 208V.

Frankly, I'm a little concerned about the competence of your electrician. This voltage is VERY common in the US and the facts I laid out above are common knowledge and should definitely be second nature to any electrician qualified to do the installation you're discussing.

If I have a 208V 3-phase, 4 wire incoming power source for a facility that contains motors, lighting and a few other minor controls, and am looking to install an emergency diesel generator. is the generator required to be rated at 208V or would a 600V generator be compatible with the system as well?

There is a 600V panel in the facility.

Jim says,
Simply put, you will need an additional transformer designed to operate on your shop's primary high voltage to run 240vac equipment.

You cannot run 240 equipment on 208.

Your electrician has understated the consequences and in fact, any attempt to operate 240vac (2 or 3 wire config) will likely burn your new equipment up.

Remember that the operating voltage and amp draw is not the only consideration-the inrush current on start up of motors will exceed the amp rating of the start and run capacitors for motors because of the lower voltage, and if equipment is loaded down by the work it does, it will likely get "hot enough to fry chicken" and then just quit. And any warranty will be voided by operating the equipment on the incorrect voltage.

I've installed additional transformers for a dedicated machine or application. (they're usually small enough to hoist up in the air and mount on building steel with unistrut if floor space is at a premium) This is usually in older shops that were wired for 220/440 with a high leg (3 phases-2 phases are 110vac and one "high leg" is between 190vac and 210vac-wiring receptacles to high legs in panels usually results in burned equipment-if you see a pattern of two single pole breakers and an empty space repeated in the panel that tells you that it's an old, 220 high leg transformer-however, you can use the high leg with a 120 v phase for a two or three wire breaker and it will work).

Most new factories and shops are equipped with 277/460 transformers for the fluorescent lighting, sighting lighting (luminaires) and heavy shop equipment, and additional transformers 120/240 for receptacles, copiers, water heaters, fire alarm, security and additional control transformers low voltage apps such as a doorbell or other equipment that offers 120/240 wiring options. (Sight lighting ballasts for instance offer multi voltage wiring options and the higher the operating voltage the more economical it is to run) All of the step down and isolation transformers work with the same primary high voltage, which in my US STATE OF DELAWARE starts at 12,700 VAC 3Ø and may be as high as 22,000 VAC 3Ø primary voltage.

So, a small, dedicated transformer that steps down your shop's primary supply voltage to 240/480 3Ø with a minimum 100 amp secondary rating (allowing 20% headroom over the new equipment's, rating) should be on your "things to buy" for Christmas list.

By scott lofley on 16 October, 2011 - 9:43 pm

> what does it mean when something is rated 120/208V or 277/480V? What do the two numbers mean?

i would like to tell you in short that the voltages that you are talking about are definitely 3 phase voltages. these voltages are derived by using the first number and multiplying it by the square root of three or 1.73. hope this helps=120x1.73+=208.....277x1.73=480 both rounded up. I'm a journeyman line tech for withlacoochee river electric. be safe, Scott L.

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Tony... when connected to the 208V supply, line-currents Ia, Ib, and Ic will be 62A, 47A, and 62A, respectively.

The values obtained are based on the following:

o The resultant load is an Unbalanced-Delta.

o At 208V, the 7.5kW heating-elements (call R1, R2, R3) will each require 6.5kW.

o At 208V, the 4.5kW heating-element (Call R4) will require 3.9kW.

o Load distribution is: R1 (6.5kW) between Line A and Line B; R2 (6.5kW) between Line B and Line C; and R3 & R4, in parallel (10.kW), between Line C and Line A.

Anyone, except PTM's, may contact me off-forum if detail is needed!

Regards, Phil Corso (cepsicon[at]AOL[dot]com)

By Steve Myres on 15 November, 2011 - 7:14 pm
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Phil,

I don't think your kw derating by voltage is correct. The voltage and current are both reduced by 13.3%, giving an overall power decrease of 25%.

He didn't say his heaters were single phase, and immersion heaters in this range are commonly available in three phase. When I select them, the threshold is either 3kw or 4kw, can't remember which, and in a case like this, where only one heater would have ended up selected as single phase, I would have put that one on 3-phase as well to maintain a balanced delta.

So I'm going to say his total HEATER load, presuming 3-phase heaters, is (3 x 7.5 + 4.5) * 0.75 / 208 / sqrt(3) = 56.2 FLA. He still has to add in the motors, and one other wild card is that heaters of this type vary significantly in actual kw. The mfgr might build two nominal 7.5kw units and one might come out at 7.0 and the next at 8.2 or something.

By W.L. Mostia on 16 November, 2011 - 9:12 am

I think that it should be noted that with a 208 VAC supply, your heaters will supply roughly 13% lower heat capacity for your process which may be create operational problems. This may be fixed using buck-boost transformers or a different distribution transformer ( e.g. 480/240VAC).

I have not used the below company but they have an explanation of buck-boost transformers which are commonly used to boost voltage from 208 to 230 VAC.

http://www.acmepowerdist.com/pdf/Page_104-109.pdf

William (Bill) L. Mostia, Jr. PE
Sr. Consultant
SIS-TECH Solutions, LP

Any information is provided on Caveat Emptor basis.

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Steve, my calculation of power at 208V is incorrect. It is 75% of their 240V rating. But the calculations for amperes are not based on power. Instead, I used phase voltage, 208V, and element resistance. I believe you must agree that at rated design voltage of 240V, the resistance for a 7.5kW element is (I apologize to the PTMs for using formulas) Vab^2/Pab, or 7.68 Ohms. The corresponding resistance for the 4.5kW element is 12.8 Ohms.

Regarding your interpretation of configuration, i.e., multiple-elements per phase, vs my using one-element per phase (except for the 4.5kW unit) can only be settled by Tony! He did say 3-7.5kW elements. Also, I did not interpret Tony's post to include motors.

Regards, Phil

By Steve Myres on 17 November, 2011 - 1:33 am
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Steve, my calculation of power at 208V is incorrect. It is 75% of their 240V rating. But the calculations for amperes are not based on power. Instead, I used phase voltage, 208V, and element resistance. I believe you must agree that at rated design voltage of 240V, the resistance for a 7.5kW element is (I apologize to the PTMs for using formulas) Vab^2/Pab, or 7.68 Ohms. The corresponding resistance for the 4.5kW element is 12.8 Ohms.

But for a resistive load that's the same thing (assuming we neglect the change in resistance due to running at lower watt density -- most materials change in bulk conductivity with temperature). Whether we note that the voltage across the load and the current through it have both decreased 13%, giving a total power decrease of 25%, or whether we pretend to ignore current and go with P=E^2/R (which amounts to the same thing), we end up at the same place.

Regarding your interpretation of configuration, i.e., multiple-elements per phase, vs my using one-element per phase (except for the 4.5kW unit) can only be settled by Tony!

True enough.

Also, I did not interpret Tony's post to include motors.

He said "My electrician also stated that the motors will run hot at higher current if rated for 240 volts and run on 208 volts. Is he correct?"

Regards, Steve

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Steve, I apologize for not being clearer with my calc comment.

I meant that I didn't use "Power" in the Unbalance-Delta equations to determine phase-currents and line-currents. Thus, the values I originally presented are still valid for the Calculation-Basis shown!

An aside: the corresponding currents for the original design basis (240V), i.e., Ia, Ib, and Ic, are 71A, 54A, and 71A, respectively. Phase-sequence used is A-B-C

Regards, Phil

Oh, OK, I misunderstood.

Thanks,

By Jim Eylander, Jr. on 27 February, 2012 - 3:37 pm

120/208 power is derived with a "Y"-connected transformer, whereby the Neutral (which is then bonded to Ground) is common-tapped to the center of all three phases at 120-degrees of phase shift. 208-volt power only grants 75% of 240-Volt power, so motors will definitely run hotter on 208 than 240 if they are not specifically wound for 208, but most of them, particularly small or fractional horsepower motors that are multi-voltage rated, are able to handle the additional heat. The 120-Volt designation is for what each leg of the 3-phase power's potential is to Ground, or Neutral Center-tap. The 208-Volt designation is the potential between any two legs of power. Typical household current is 120-240 VAC Single-Phase, which is derived from a single transformer at 180-degrees of phase angle, the two poles of the power being center-tapped and bonded to Ground for the 120-Volt designation, 240-Volts between the output legs (Secondary winding of the Transformer.) 120-240 VAC 3-Phase is derived from a "Delta"-connected Secondary winding, whereby the Neutral only comes halfway between two of the windings, the third being "wild," which will have a fluctuating voltage to ground between around 140 to 208 Volts, hence the term "Wild Leg," designated by orange or red tape on the conductor. Any device or appliance requiring 120-volts will burn out if connected to the wild leg, so since most office buildings and non-industrial commercial buildings utilize a lot of 120-volt circuits, it's much more practical to serve 120-208 VAC to the structure, since the motors for HVAC systems usually are 208-240 VAC rated. I've been a Journeyman for nearly 40 years, and to my dismay the majority of Electricians, even in the Union, are just "installers," pipe-benders and wire-pullers, not being diligent enough to truly learn the basis of their trade. Consequently, far too much "knowledge" that is divulged by them is little more than "urban legends," and not based on truth. BTW, 120-VAC household current will KILL you just as fast as the industrial 277-480-VAC will if you are caught in the circuit's path across your heart. The 60-Hertz power will fibrillate your heart to nearly explode... "Don't try this at home" is a GOOD rule for those without proper training and supervision!!!

Thanks Jim Eylander, Jr.,

Finally, a good, clear explanation.

>Thanks Jim Eylander, Jr.,
>
>Finally, a good, clear explanation.

Hello friends, this is Agate, I am electrician in profession. I would like to say that plenty of great advice is available here actually.

A brief history: When Thomas Edison built the Pearl Street Station in New York City, electricity became a hot item - every city wanted some! It was wonderful to have electric street lights, and businesses started installing lights AND motors. It was all DC (Direct Current) - Edison was firmly against AC all his life. The severe drawback to DC was that after a few miles of wire, the lights wouldn't light any more - too much energy was lost in the power lines.

The city of Buffalo wanted to get electrical power out of Niagara Falls, which was about 20 miles away, but DC power could only go 3 to 5 miles at low voltages. George Westinghouse got the contract, and hired Nicola Tesla to make it work. They knew they had to use AC, because with AC you could transmit power at high voltage to reduce the line losses, and then use step-down transformers at the other end to get safer, user friendly voltages.

The severe drawback was this: the lights would light but the motors wouldn't run. Since the AC goes positive then negative, the motors would just vibrate in place.

Tesla figured out how to send five parallel power lines out from the generator at Niagara Falls, each line connected at even spaces around the generator. As the rotor was turned by the waterfalls, it passed all five power pickup points during each rotation, one after another.

The voltage in each line looks like each of the others - a sine wave of voltage going on continuously, but each one is phase shifted from each of the others. At the user end, they could make the motors turn by applying the five phases to a motor, hence creating a rotating magnetic field to pull the rotor around. This solved the whole problem - high voltage AC for long distance transmission and multiple phases to make the new AC motors turn.

Present Day Power: Now we use three phases instead of five. If you look at all three simultaneously, it looks like this:

Almost all high power electrical transmission is done using this kind of three phase power - three separate power lines, each identical to the others in voltage, but the sine wave is phase shifted from one to the other by exactly 120º, or 1/3 of a cycle. Almost all businesses receive power from the utility company as three phase power.

The final voltage step-down transformer at any business might be inside or outside of the building, and it's almost always a three phase Delta-Wye transformer. It produces three legs of 120 V. electricity, phase shifted from each other as shown above. When you measure the voltage between any two of the phases, it's 208 Volts AC. So you can get any of the following combinations out of any ordinary three phase circuit breaker panel anywhere in the country:

120 Volts single phase - use any one of the three available legs.
208 Volts single phase - use a two pole breaker, and any two of the three legs.
208 Volts three phase - use a three pole breaker, and all three legs.

Since the watts needed by the load is derived from Volts x Amps, some servers now use 208 Volts single phase instead of 120 Volts. You can get more watts at the higher voltage, without increasing the wire size for higher amps.

UPS's up to 3000 VA are nearly all 120 V. single phase units. 120 Volts goes in and 120 Volts comes out. External Bypass is easy, because the input voltage matches the output voltage.

Single Phase UPS's from 4 kVA to 25 kVA almost always take 208 Volts input, but the output might be any of the following: (Caution! This can be very confusing.)

a). 208 Volts single phase (no neutral wire, and no 120 V. output).

b). 120/240 Volts split phase, which is two legs of 120 volts, one going positive exactly when the other goes negative - a phase shift of 180º.

c). One leg of 120 V. and a second leg of 88 V., at 180º phase shift. This provides some 120 V. output and some 208 V. output: 88 + 120 = 208.

d). Two legs of 120 V. with a 120º phase shift, same as the input. This version is more recent. It requires two separate inverters in the UPS, but it's the easiest style to bypass externally because the input and the output match.

With single phase UPS, it's important to understand exactly how your UPS works and what additional transformers you need to supply the voltage you want, for output power or for the ability to install an external bypass switch.

Three phase UPS's start at about 10 kVA, and go as large as 1000 kVA. Most models below 75 kVA are 208 Volt input and output, while most larger models are 480 V. input and output. You can buy models with 480 V. input and 208 V. output, but we strongly recommend matching input and output voltages to allow for installation of an external maintenance bypass switch.

In the range of 10 kVA to 25 kVA, you can purchase either a single phase or a three phase UPS. The single phase units are typically less expensive than three phase (less electronics inside), but many facility managers don't like to put so much single phase load on their system - it's harder to keep the three phases balanced.

Those are the voltages the panel or device is rated to handle.

I take it you are talking about a service panel.

Did you know that in some parts of the mining industry they used 7 phase power. I was told that when running big motors with a lot of cable, the extra phases reduce the total amount of copper required.

Think motors 2-3 times the size of house and running miles of copper.

Martin

> b). 120/240 Volts split phase, which is two legs of 120 volts, one going positive exactly when the other goes negative - a phase shift of 180º.

This is correct. But why if you measured this with an oscilloscope would you get 240 volt sine wave? Shouldn't you get 0 volts if 120 - 120 = 0? Why does it double the voltage? I think this has to do with the fact that the pole pig transformer outside is center tapped neutral.

> c). One leg of 120 V. and a second leg of 88 V., at 180º phase shift. This provides some 120 V. output and some 208 V. output: 88 + 120 = 208.

I've never heard of one leg being 88 volts. I think you're referring to single-phase 208volt, this is 120volt each leg with respect to neutral, not 88 volts left over in leg 2. It's generally not uneven like that.

> This is correct. But why if you measured this with an oscilloscope would you get 240 volt sine wave?

Yes, a scope or meter (or load) will see 240V. If it weren't 240V, you couldn't hook larger household loads (range, water heater, dryer, AC condenser) across those two lines and have them do anything.

> Shouldn't you get 0 volts if 120 - 120 = 0? Why does it double the voltage?

Because when leg A is at +120, leg B is at -120 which is a difference of 240. Arithmetically, 120 - (-120) = 120 + 120 = 240

By Hugh Leyton on 30 June, 2012 - 8:15 pm

> what does it mean when something is rated 120/208V or 277/480V? What do the two numbers mean?

Electrical supplies can be Single phase, such as 120V or in Much of the world, 230V relative to Neutral. However in the States, supplies are usually 2-wire + Neutral, that will give you 120V to Neutral on two wires and 240V between the two Hot wires, as wired to most houses. However larger sites may well have 3-phase supplies, particularly for motors. 120/208V is obviously a 3-phase supply which usually has 3 Hot wires, each one 120V from Neutral, but only 208V between any two Hot wires. Higher power 3-phase supplies are typically 277V each Hot wire to Neutral with 480V between each of the 3 phase wires.

If you already have a 3-phase supply, you may well want some of that power to provide 120/240V for Office buildings etc. Look at http://dl.dropbox.com/u/88752751/Transformer%20for%20120-240V%20from%203-phase%20-%2030Jun2012.doc

I've always understood it to mean that 120/208V is the operating voltage that it can safely operate at eg. from 120V and up to 208V. This makes sense as it allows for voltage fluctuations whilst providing reliability.

Nope. First of all, that's too wide a range for about anything but a switch-mode power supply (and even there it's just for convenience -- normal supply systems and components don't need that much tolerance for voltage variation).

Also note that on a motor label for example, the voltage will be noted as "208-230/460". Read the dash functionally as "to" and the slash as "or". In this example, a slash was used.