AIC and Available fault current

[Sharpie]

I have had a study done on a distribution system including:
-Arc Flash
-Coordination
-AIC

There are a number of breaker panels with a much higher available fault current than the the AIC of the breakers.

My question is in the difference between the 1,2, & 3 pole breakers.

For example: If a panel has an available fault current of 26,000A @ 480V 3-ph, do I need to size the single pole breakers that high as well as the 3-p? Or can I reduce the amount of AIC needed for the 1-P breakers?

The engineer that did the study said that the AIC needed to be the same whether 1,2 or 3 pole. But it seems to me that the fault current on one leg would be significantly less than a 3-ph bolted fault.

Thanks,

If more info is needed, let me know.

 

[mcclary's electrical]

The single pole breakers would not have to match the fault currenet of the equipment, but they will need to meet or exceed the available fault current in your fault current letter. You can't find it in your panel



Ex. 10,000 AIC breakers in a 22,000 AIC panel connected to a service with 8,500 available amps will be fine

 

[Sharpie]

Thanks Mcclary.

Good catch. I didn't mean any AIC ratings found in the panel. I meant what the study showed the available fault current to be at that location.

 

[mcclary's electrical]

Thanks Mcclary.

Good catch. I didn't mean any AIC ratings found in the panel. I meant what the study showed the available fault current to be at that location.

As long as all breakers and equipment meet or exceed that number, it's compliant

 

[ron]

There will be a calculated 3ph fault and L-G fault value.
1pole breakers need to be rated equal to or higher then the L-G fault result.

 

[jghrist]

The current in each pole will be the same for a 3? fault. It is possible, but not necessary, that the 1? fault level is lower than the 3? fault level, but you have no assurance that the load side of the 1-pole breakers will not experience a 3? fault.

 

[steve66]

Series ratings are also common. For example, a panel that has 20 K of available fault current may have a main breaker rated for 22 KAIC, and branch breakers rated for only 10 KAIC, if the main breaker and branch breakers are a listed combination.

So having a 10 KAIC rated branch breaker in a panel that has 20 K of available fault current is not necessarily a violation.

Upstream breakers in other panels can also be part of a series rated combination protecting breakers in downstream panels.

Steve

 

[bob]

The engineer that did the study said that the AIC needed to be the same whether 1,2 or 3 pole. But it seems to me that the fault current on one leg would be significantly less than a 3-ph bolted fault.

It is possible that for close in faults the phase to ground fault will be equal to or greater than the 3 phase fault.

 

[bob]

The current in each pole will be the same for a 3? fault. It is possible, but not necessary, that the 1? fault level is lower than the 3? fault level, but you have no assurance that the load side of the 1-pole breakers will not experience a 3? fault.

Could you expand you thinking on this possibiity?

 

[mcclary's electrical]

Could you expand you thinking on this possibiity?

phase to phase short. I think is his mindset.

 

[Sharpie]

I appreciate the replies.
It seems there are differing opinions on what to expect.
I don't think it will be a waste of money to bring everything up to the rating that the engineer is suggesting. Thanks for the input everybody.

 

[jghrist]

Could you expand you thinking on this possibiity?

Inside the panel, three phases are present and could experience a 3? fault. If three phase conductors from separate 1-pole breakers are routed in a conduit to another box, then a 3? fault could occur elsewhere.