advertisement
from the Automation List department...
High leg on 3 phase power
Information resources, documentation. topic
Posted by mkiger on 1 August, 2004 - 12:04 pm
I have 3 phase power that has a high leg of 211 volts to ground. The other two legs are 120 volt to ground. Is it fine to run a three phase machine onthis voltage?


Posted by Steve Myres on 2 August, 2004 - 2:06 pm
This arrangement is known as "bitch leg" or "stinger leg" delta. The middle of one leg of the delta is the ground point, rather than the center point of the star, as in Y-secondary transformers. The source will appear as a 240V three phase delta to the load.
--
Steve Myres, PE
Automation Solutions
(480) 813-1145


Posted by Instructor HVAC on 23 October, 2011 - 7:02 pm
I was checking voltage on a 3 phase compressor, upon checking across all legs I was reading 230 volts, however when I went to round on one leg, I was reading 0 Volts to ground. Please explain this condition The building has its own transformer


1 out of 1 members thought this post was helpful...
Posted by Phil Corso on 23 October, 2011 - 9:24 pm
Instructor HVAC... the arrangement you observed is called a Corner-Grounded-Delta. It is a 3-phase, 3-wire system.

I suggest you post a new thread because this thread is the Center-Tapped High-Leg or Red-Leg, or Bitch-Leg, etc. It is a 3-phase, 4-wire system.

Regards, Phil Corso


Posted by Jim Zupich on 2 August, 2004 - 3:22 pm
I'm guessing this is a 3 phase Y connected 208/120 system. In this system, you get 120 volts between neutral and a leg. If you want 208 volts, you read Line-to-Line. (Neutral is connected where the three lines that form the letter "Y" meet)

Are you sure you're looking at ground?
The only other possibilities are major 3 phase imbalance or a funky/unusual transformer design with taps as you'd listed. I'm not familar with systems other than the most common ones (120/208Y, 480V Delta, 480V/277V Y). I guess it could be a delta system with the neutral tapped halfway between two phases (the two you see 120V on).

If the machine you're connecting is truely three phase (doesn't use a neutral, just Phase A, B, C, and ground), all that should matter is the line-to-line voltages. You need to measure them to be sure it matches nameplate of the machine.

Hope this helps. Good luck and don't blow anything up!!

Jim Zupich
jzupich@riverconsulting.com


Posted by Chad Sims on 18 October, 2004 - 3:45 pm
I am having the same issue in a way. My building has a high leg and we are hooking up a machine that needs 208Y. I am having to buy a converter to get it from 240 down to 208. If it reads 212 with a ground, shouldn't there be some way to use that for 208? It may sound stupid but I thought it made since rather than buying a transformer. The machine installation tech is used to seeing all three legs being 120 and says if they are not then you have to buy a transformer to convert or "step down" to 208Y.

chad@primaatlanta.com
Chad


Posted by ajaynejr on 28 January, 2011 - 9:20 am
First order of business is to convert from 240 volts to 208 volts. Your machine needs 208 volts from A to B, 208 volts from B to C and 208 volts from C to A. So you need the transformer or converter.

One allotment of 208 volts from B to N is not good enough.

Also check to see if your machine has a neutral (white) fourth wire. If so you need three more voltages, 120 volts from A to N, 120 volts B to N, and 120 volts C to N. Here the transformer needs to provide a neutral off of its secondary with those voltage relationships. Do not connect the machine to the neutral you already have.


Posted by John Conniff on 4 November, 2004 - 12:16 pm
I'm having trouble following all that. can you say it differently?


Posted by Curt Wuollet on 5 August, 2004 - 9:30 pm
I'd hesitate to say yes to "machine" as some may rely on balanced leg voltages. But 3 phase motors and anything without a neutral connection should never know the difference. Here in the US. many factories have this setup.

Regards

cww


Posted by D.L. on 17 August, 2004 - 4:32 pm
I have a similar situation and can't say with any degree of certainty that that a "high leg" won't burn up my motors. I feel that the best solution is to install a "Y" transformer in order to achieve balanced power.

D.L.


Posted by Curt Wuollet on 19 August, 2004 - 4:19 pm
Without a neutral reference, it _is_ balanced power. But there might be an issue with insulation as there is higher voltage to ground. But this has been done in many installations as an economy measure. I don't know if it's common any more as it does unbalance loads from the power
company's perspective. Lots of small shops have it.

Regards

cww


Posted by Steve Myres, PE on 20 August, 2004 - 5:48 pm
Curt,

The wire used in buildings (THHN, etc.) or for power wiring in machines (MTW, TEW) is all rated for 600 volts, so a measly 208V to ground is no problem at all.

Steve


Posted by Curt Wuollet on 24 August, 2004 - 4:07 pm
Hi Steve

I was more concerned with the motor insulation. Wire insulation has a much higher safety factor.

Regards

cww


Posted by Doug McHatton on 3 September, 2004 - 8:48 am
It's called a "B-High" leg and is not uncommon in rural areas.


Posted by Rich Thienel on 18 May, 2010 - 11:50 am
Doug be careful about calling it a "B-high" leg I don't know if there are any other places in the country like Long Island, but in Nassau and Suffolk County's the original power authority LILCO brought there high-legs into buildings on the C-Phase. I learned the hard way when a generator made in Ohio came wired with the high-leg wired on the B-Phase while I was accustomed to it entering the building on the C-Phase.


Posted by Anonymous on 15 September, 2004 - 6:40 pm
yes, this is a good source for 3 phase 240 volt loads. you have a 3 phase 240 volt delta tapped winding. be careful with any single phase loads you supply with this as you can get that hi leg onto a 120 volt system and burn up equipment.


Posted by Anonymous on 22 October, 2004 - 12:33 am
This is normal for Created 3 phase power. The Hi leg is a lighting leg and should be about 277v.
That's were 277 volt lighting come from.
Hook the line between the three phases, not ground or neutral.
All should be well.


Posted by Anonymous on 11 January, 2005 - 11:43 pm
So the high side is 277v to ground and the other legs are 120v. so from leg to leg it should be? 208v or 220?


Posted by McConnell, David P on 13 January, 2005 - 11:07 pm
Excuse my saying so, but some of the information offered in this thread is not only completely (and dangerously) wrong, it also gives a not too rare insight into the incredible level misunderstanding of a subject capable of
killing the person taking the advice as dead as a hammer!

I know this is an old thread, but I can't stay silent any longer.

In an area like this, if you don't know what you are doing, don't ask for help here. Get some education or hire a credentialed professional.

My opinion and my opinion only. Not that of my employer.

David McConnell


1 out of 1 members thought this post was helpful...
Posted by Phil Corso on 14 January, 2005 - 12:56 am
Responding to Anonymous' query:

(1) The 'high-leg' 3-phase system is more properly referred to as a center-tapped (high-leg or red-leg) 4-wire, delta system. It is important to note that this system finds favor if the 120 V 1-ph, loads are small compared to the 240 V, 3-ph, loads.

(2) Visualize the delta-connected secondary of a 3-phase transformer. The terminals are A, B. & C. Conductors connected to the three terminals are labeled ph-A, ph-B, and ph-C, respectively.

(3) Although a misnomer considering the IEEE definition of the term "neutral," the center-tap (midpoint) of the A to C phase winding is labeled N. It's conductor is also labeled N. (A more correct label is the groundED-conductor.) At this point the N terminal is connected to ground or earth. Phase B is now considered the 'high-leg' or 'red-leg'.

(4) If the rated phase-to-phase voltage of the transformer secondary is 240 Volts, then the corresponding terminal-to-terminal voltages follow:

Phase-to-Phase
A - B, 240 V.
B - C, 240 V.
C - A, 240 V,

Phase-to-Neutral.
A - N, 120 V.
B - N, 208 V. [Sqrt(3)/2]x[Vph-ph]=the 'high-leg')
C - N, 120 V.

Please note that the above system is not to be confused with the corner-grounded delta system. That's another topic!

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[tal-2@webtv.net] (Cepsicon@aol.com)


Posted by Steve Myres, PE on 20 January, 2005 - 12:58 am
Phil,

You can't tell it because the threads aren't indented anymore, but my reply about the trig was for Anonymous' Jan 11 post, not yours of Jan 14. Sorry if there was any confusion.

Steve


1 out of 1 members thought this post was helpful...
Posted by Phil Corso on 21 January, 2005 - 10:40 pm
Steve, an apology is unnecessary.

I thought, based on David McConnell's post, that a more complete response was in order. Hopefully, it will clear up some obvious (and dangerous) interpretations by one or some of the anonymi.

An aside... I'm not sure about the plural of anonymous... is it "mi", "mous'", or "mouses". Same dilemma for caboose, goose, or moose?

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[tal-2@webtv.net] (Cepsicon@aol.com)


Posted by chris on 1 July, 2011 - 11:33 pm
doing a job on a building that has a 3 phase 4 wire delta coming in. all the service entrance conductors were ripped out. at first look thought it was a 4 wire Y system 120/208, all phases were used. in the panel inside, after looking into it more, there was a jumper wire inside the panel, going from phase a-c. I know I can't put anything 120 on the high leg, but unsure if I can put any 240 circuits on it. as it is right now, I think I have to install a sub panel, and move a lot of circuits, because that one is maxed out, because it was rigged.


Posted by Steve Myres, PE on 14 January, 2005 - 2:59 pm
You've got a problem in your trig. Try drawing it to scale on paper.


Posted by Anonymous on 15 February, 2006 - 11:16 am
I think this person has a three phase 120/240 with a high leg. it's not 480 which would give you 277 on a single phase. Your question is yes, you can hook up a 240 v three phase device up if it indeed is a 240v not a 480 volt device.


Posted by Anonymous on 30 March, 2006 - 1:09 am
It is a red leg delta system. Take your measurement between each phase and see what your reading that a to b, a to c, and b to c all read within a few volts of each other.

The greatest risk you have with your current situation is the possiblity of burning up your single phase 120 Volt equipment if you mistakenly connect it the 211 leg to ground.


Posted by LD on 7 June, 2006 - 10:43 pm
WOW! I haven't seen this in 20 years. Didn't even know it was still used. Most electricians don't know it ever was. Must be an old building with a lot of 220 double poll breakers. Seems like (I said seems like) it was also referred to as a low voltage 3 Phase system or 240 3 phase. Your machine may be a 480 3 phase. If you get an electrician, I'd make it an old timer. LD4GP@earthlink.net


Posted by Jackie Roberts on 15 May, 2010 - 3:37 pm
Obviously you weren't involved heavily in the industrial field if you haven't seen a delta system in 20 years. Any industrial journeyman should be able to tackle open and closed delta systems and yes they are still in common use especially in plants that have mostly 3 phase loads such as motors and welding machines.

Your use of this site is subject to the terms and conditions set forth under Legal Notices and the Privacy Policy. Please read those terms and conditions carefully. Subject to the rights expressly reserved to others under Legal Notices, the content of this site and the compilation thereof is © 1999-2014 Nerds in Control, LLC. All rights reserved.

Users of this site are benefiting from open source technologies, including PHP, MySQL and Apache. Be happy.


Fortune
Q: What's a light-year?
A: One-third less calories than a regular year.
Advertise here
Advertisement
our advertisers
Help keep our servers running...
Patronize our advertisers!
Visit our Post Archive