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Short Circuit Current Calculation
How can I calculate the short circuit current for an electrical panel?

How can I calculate the short circuit current for an electrical panel? In some applications it is required that an electrical panel shows in a tag their specifications and one of them is the short circuit current, but I dont know how to calculate it.

I appreciate any help.

Regards,
Arevalo

By Michael Griffin on 2 February, 2004 - 10:08 am

I don't know where you are located, but if the "short circuit current" you are referring to is similar to that required where I am, then you need the short circuit withstand rating of the components in your panel. This is *not* the same thing as maximum load current. This is the short circuit current the switch, fuse block, etc. are rated to withstand for the time before the short circuit protection interrupts the current, and also the short circuit current the protective devices (fuses or circuit breakers) are rated to be able to interrupt. The rating of the panel will be the rating of the lowest device.

The available short circuit current will not be the same throughout the panel. For example, a small control transformer will not pass through the full short circuit current, so the devices on the secondary side will see a much lower available short circuit current. Usually (although not necessarily), it is only the devices on the primary side which are the problem.

The purpose of the rating is so the electricians hooking up the machine will know if it is safe to connect it to the bus in their plant. If the panel is only rating to interrupt a 10,000 amp short circuit, they can't hook it up to a bus that has 100,000 amps available in a short circuit.

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Michael Griffin
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By Bouchard, James \[CPCCA\] on 2 February, 2004 - 11:27 am

The requirement for panels is to show the amount of short circuit current they can withstand ( without being damaged ) the way you do that is to examine the short circuit withstand ratings of all the equipment installed in the panel and find the weakest link. You have to be careful about the device ratings since many are voltage specific ( ie 10,000 amps at 240 volts but only 5,000 amps at 600 volts ). In some cases you can improve the short circuit rating by the use of specific types and ratings of fuses but that is another subject.

James Bouchard

So I have to check the short circuit capacity of every component in the panel and look for the weakest, right? Does that refer only to vircuit interruptive decives (fuses, circuit breakers, etc.)?

By Michael Griffin on 5 February, 2004 - 1:07 am

It also includes the disconnect switch, motor overloads, etc. These devices have to survive long enough for the fuse or circuit breaker to interrupt the short.

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Michael Griffin
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The panel and equip AIC ratings should have been selected in the original design to meet or exceed the available fault current at that particular location in the electrical distribution system. So you can assume available fault current is at or below these values. To do a concervative estimate look at %impedance of upstream transformer and use to calc. fault current on the secondary side. This, of course, ignores downstream cable impedances, insulation type. if raceway is mettalic or not, etc; and will give you a higher value than at panel location.

By Bouchard, James \[CPCCA\] on 19 February, 2004 - 6:06 pm

Panel builders rarely know the fault current available when designing their panel. In addition most motor control equipment has relatively low withstand ratings ( 5,000 to 10,000 amps ) so one approach is to specify current limiting fuses on the input of the panel that will keep the let thru current low enough for just about any equipment in the panel. Since these fuses are more expensive you sometimes see panel builders not including them or not including any fuses at all. A judicious choice of main fuse type and allow the smart panel builder to provide a substantial rating at a very reasonable price.

For example a 30 CSA class J current limiting fuse will limit a 100,000 amp fault current to 4,800 amps. 100,000 amp fault current would probably require a 10,000 kVA transformer so it is a rather big installation. The panel designer would still have to make sure everything in the panel would handle the 4,800 amp fault level but that is a lot more manageable problem. You do have to check the fuse ratings though as some manufacturers provide more current limiting than the standard requires. This means you have to specify the fuse manufacturer in your fuse rating (or use the higher rating in the standard to be safe).

James Bouchard

By Matthew Hyatt on 21 February, 2004 - 3:00 pm

Arevalo
Checkout www.mikeholt.com

great knowledge based site with lots of good information and links. Also most panels I have installed usually only have the rated voltage and maximum operating current listed -

480vac 3 phase at 120 amps. It is the designers job to insure that the fuses, disconnects, CB's ect are suitably rated for the proper short circuit current which could be several thousands or 10's of thousands of amps.

MJH

By Michael Griffin on 23 February, 2004 - 1:45 pm

Here where I am, we are required to also have the short circuit current rating on the name plate together with the other data in order to pass electrical inspection.

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Michael Griffin
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I know is kind of late, but here is a comprehensive guide offered by Bussmann. The point-by-point short circuit calculation is explained in part 2:
http://www.bussmann.com/library/docs/EPR_Booklet.pdf

If this is not easy enough, the same company offers a freeware software (Short circuit calculation):
http://www.bussmann.com/apen/software/index.asp

Regards,
Ramona

By Oscar Rodriguez Mendoza on 3 October, 2004 - 12:58 am

Dear Sir:

We´re an Electrical Engineering Firm located at Chihuahua, Mex.

We´ve developed an Excell application to calculate this and other values using REAL short circuit contribution of electrical motors, which are main contribuitors on this kind of failure, that consideres specific impedance values and voltages to calculate final short circuit current of an specific panel.

This tool is a dedicated chart for power calculations, and I think is perfect for your calculation needs.