What is My Fault Current?

[sparkyrick]

Anyone care to figure the fault current for me. I'm just making sure the inspector isn't messing with me.

208/120 75KVA 3 phase POCO transformer, 3.6% impedance.
3 parallel runs of 350Kcmil Al, 68' tansformer to tap box on service.

 

[ron]

If you show your work, we can check it.

http://www.cooperindustries.com/con...ch_Lib_Short_Circuit_Current_Calculations.pdf

 

[raider1]

I ran the numbers and got 5,784 amps at the utility transformer and 5,430 at the tap box.

I used the available fault current calculator that you can download here at Mike's site.

What is the inspector telling you?

Chris

 

[sparkyrick]

He's telling me I need to add a 65K main breaker inside my SqD panel, trying to tell my the available fault current is 65K.

 

[rcwilson]

He's only off by a factor of 10.

 

[don_resqcapt19]

Why do you have 750 amps of wire for a circuit with a current of 260 amps? Something doesn't look right to me.

 

[sparkyrick]

Why do you have 750 amps of wire for a circuit with a current of 260 amps? Something doesn't look right to me.

Multi meter stack. One 400 serving a resturaunt, two 200A for future build-outs and a 100A house panel.

 

[steve66]

Multi meter stack. One 400 serving a resturaunt, two 200A for future build-outs and a 100A house panel.

Utility companies often size transformers a lot smaller than the service conductor rating. It's a good idea to add some extra AIC rating just in case they replace the transformer in the future. Still, 65K would be a real stretch.

Are you sure the inspector had all the same info. you have? If he doesn't know what size tranformer the utility is installing, or what the impedence is, he is probably figuring everything on a worst case basis.

 

[robbietan]

He's telling me I need to add a 65K main breaker inside my SqD panel, trying to tell my the available fault current is 65K.

probably got the figure using the utility transformer at the substation

 

[kwired]

I come up with with 5781 line to line and 8672 line to neutral at the transformer. At those values I wouldn't even worry about further calculations for conductors if you have 10K equipment.

 

[JoeStillman]

He's telling me I need to add a 65K main breaker inside my SqD panel, trying to tell my the available fault current is 65K.

Just divide the transformer secondary FLA by the impedance to get a first approximation - 75 kVA @ 208V = 208A. 208A/0.036 = 5,787. If the 400A service feeds a 400A motor, the motor contribution would be about 400A x 4 or 1,600A. The most conservative estimate I can make for this service is 5,787 + 1,600 = 7,387 A. 65 kAIC makes no sense at all.

 

[jim dungar]

Utility companies often size transformers a lot smaller than the service conductor rating. It's a good idea to add some extra AIC rating just in case they replace the transformer in the future. Still, 65K would be a real stretch.

The utility serving the OP uses 'design level' fault currents for determining the SCCR rating of service entrance equipment 9I live in their coverage area). They do not care about what is installed today, they are worried what their transformer may be sized as in the future. The OP said (1) 400A +(2) 200A meters

Assuming 800A service 208V:
If the utility is WPS, they would say the SCA could be 42,100, which would require 65kAIC equipment.
If the utility is We Energies, it is hard to guess their design level, but I have seen 29,700A.
Xcel Energies would list it as 52,100A

So what is the utility, and how big of a service did you order?

 

[sparkyrick]

The utility serving the OP uses 'design level' fault currents for determining the SCCR rating of service entrance equipment 9I live in their coverage area). They do not care about what is installed today, they are worried what their transformer may be sized as in the future. The OP said (1) 400A +(2) 200A meters

Assuming 800A service 208V:
If the utility is WPS, they would say the SCA could be 42,100, which would require 65kAIC equipment.
If the utility is We Energies, it is hard to guess their design level, but I have seen 29,700A.
Xcel Energies would list it as 52,100A

So what is the utility, and how big of a service did you order?

I found out the fault current of the service equipment is specified by the power company, Alliant Energy. I talked with a rep today and he said 3 phase commercial services up to 800 amps require AIC rating of 42,000 and services over 800 amps require AIC ratings of 65,000. The inspector was stating their requirements, but did so in such a way that made it sound like his requirement.

Thank you everyone for the input. I'm changing the panel tomorrow to a main breaker panel with a 65K main, plus a few more breakers will need to be 22K rated (sub-panel, and roof-tops).

 

[jim dungar]

I'm changing the panel tomorrow ....

You are not the first electrician that has had to do this in WI.

 

[kwired]

I found out the fault current of the service equipment is specified by the power company, Alliant Energy. I talked with a rep today and he said 3 phase commercial services up to 800 amps require AIC rating of 42,000 and services over 800 amps require AIC ratings of 65,000. The inspector was stating their requirements, but did so in such a way that made it sound like his requirement.

Thank you everyone for the input. I'm changing the panel tomorrow to a main breaker panel with a 65K main, plus a few more breakers will need to be 22K rated (sub-panel, and roof-tops).

But that number should only be what is available at the transformer terminals. Length, size and type of conductors after the transformer are still going to drop what is available at the load end of those conductors

In your case there was not a really long run but it was long enough that 42K rated equipment would likely still be ok if they ever did install a transformer with a full load rating approximately same as service equipment capacity.

 

[GoldDigger]

Length, size and type of conductors after the transformer are still going to drop what is available at the load end of those conductors:

Can that reduction use the actual distance from the service transformer, or would you have to only use the minimum distance from the POCO right of way to the
panel, in case POCO decided to move the transformer or put in a new one? :huh:

 

[sparkyrick]

But that number should only be what is available at the transformer terminals. Length, size and type of conductors after the transformer are still going to drop what is available at the load end of those conductors

In your case there was not a really long run but it was long enough that 42K rated equipment would likely still be ok if they ever did install a transformer with a full load rating approximately same as service equipment capacity.

I agree that 42K should be just fine, but my hands are tied because of their requirements. Maybe it's for when that 75KVA poco transformer smokes and they have to install a bigger one

 

[jim dungar]

But that number should only be what is available at the transformer terminals.

Be careful, many utilities provide fault currents which are based on some amount of secondary conductors. Make sure you understand their reference point. The WPS value in my example was for 50' of their sized conductor, if the transformer were closer the fault current would be higher, while the Xcel example was at the terminals.

 

[kwired]

Be careful, many utilities provide fault currents which are based on some amount of secondary conductors. Make sure you understand their reference point. The WPS value in my example was for 50' of their sized conductor, if the transformer were closer the fault current would be higher, while the Xcel example was at the terminals.

Good point. If asking POCO for a value, you need to know just what point that value is valid for. If you have conductors between that point and equipment that needs to be rated for interrupting capacity then you will have a lower available fault current at the load end of those conductors.

I guess I can understand there may be local rules that may say you need equipment rated at a higher level like in the OP's install where the service capacity is much higher than the output rating of the supplying transformer. Or it may even be a reasonable design practice to figure you may need higher rating some time down the road and install proper rated equipment now. NEC has no such rule though.

 

[jim dungar]

NEC has no such rule though.

You are correct.

However, many electricians need to remember, the NEC is not the one 'allowing' equipment to be connected to its grid. The NEC and the AHJ don't come into play until there is an have an approved service point.