Fault Current From Utility?

[IllinoisElectrician]

I went in to pull a permit to do a small 9kw generator today, and ran into the inspector. He said to contact the utility to find the peak fault current for the area or something like that? I don't know what that means, I do small residential work. This is a 200A 120/240 1P 3W service, I am feeding the ATS/Loadcenter from a 50A breaker off the main panel. He said it needs to be protected from this fault current. I called a few contractors in the area and they said they haven't encountered this before.

Please explain this to me? What does he want me to do?

Please spare the harassment, this is one of the first permits I've had to pull.

 

[Buck Parrish]

Will your neutral be switced?
Im in NC right now, I have never heard of it either. but chances are he is talking about the utilities ground.
However I would call and ask to speak to a utilities engineer.
This is not how to size your ground or neutral. Since the utilities does not use the NEC and you need to be NEC compliant. some times the neutral is not swtched . And in some of these rual counties the pc has thier own inspections.

 

[wireguru]

the utility will tell you the available fault current at the service (if its residential, i would be suprised if it was more than 10ka unless large mdu with one big transformer), then make sure all your gear has an appropriate fault current rating (AIC) to match, or do the necessary calculations. Basically if the utility says its 10,000 amps or less then you pretty much dont need to do anything. I am sure others who are better versed in this will chime in.

 

[templdl]

It will probably end up to be around 22-25ka.

Since the transfer switch has to switch the normal side it must be able to handle the available fault current.
If you are using a breaker type transfere switch they are often available with a service entrance label. Since breakers do have an interupting rating it is often easy to get one that is suitably rated.
The way that I understand a contactor type transfer switch they most likely will have a withstand rating but are not designed to interupt a fault as a breaker would be able to do. The main breaker must be able to protect the contactor.

 

[curt swartz]

It will probably end up to be around 22-25ka

That would surprise me for a typical residential service. In my area the 3 POCO's I work with state that residential services 200 amps or less will not exceed 10K. Even most 400 amp services are less than 10K.

 

[jim dungar]

I went in to pull a permit to do a small 9kw generator today, and ran into the inspector. He said to contact the utility to find the peak fault current for the area or something like that? I don't know what that means, I do small residential work. This is a 200A 120/240 1P 3W service, I am feeding the ATS/Loadcenter from a 50A breaker off the main panel. He said it needs to be protected from this fault current. I called a few contractors in the area and they said they haven't encountered this before.

Code sections 110.9 and 110.10 require protective devices and equipment to be rated for the amount of 'short circuit amps' on their line side terminals.

You may have seen residential load centers with numbers like 22kAIC or 10kAIC. These numbers tell you the manufacturers ratings, you then compare these to the "peak" fault current from the utility to see if you meet section 110.9.

Likewise, your transfer switch must have an SCCR (short circuit current rating) that is greater than the utility's fault current in order to meet section 110.10.

 

[kingpb]

Jim, good answer. Also make sure that any breakers you put in the rated panel also have at least the same rating. i.e. don't put in a 10kA rated breaker in a 22kA panel, if you need over 10kA in short circuit capability.

 

[hillbilly1]

Jim, good answer. Also make sure that any breakers you put in the rated panel also have at least the same rating. i.e. don't put in a 10kA rated breaker in a 22kA panel, if you need over 10kA in short circuit capability.

.....unless it is a series rated system, which most residential panels now are with a main rated 22k with 10k branches

 

[jim dungar]

.....unless it is a series rated system, which most residential panels now are with a main rated 22k with 10k branches

Hey, lets really split hairs.
The panel itself will never have a series rating. Combination ratings, allowed in section 110.22(B), are between protective devices (i.e. between the main device and its branches). Although, the panel will have a label showing the allowed ratings.

 

[jghrist]

.....unless it is a series rated system, which most residential panels now are with a main rated 22k with 10k branches

What good is a 22kA main with 10kA branches? A fault just on the load side of a branch will have the same fault current as a fault in the panel. So the fault at the panel can't be more than 10kA without exceeding the rating of the branch.

 

[raider1]

What good is a 22kA main with 10kA branches? A fault just on the load side of a branch will have the same fault current as a fault in the panel. So the fault at the panel can't be more than 10kA without exceeding the rating of the branch.

You can have a series rated system. See 240.86.

Chris

 

[rcwilson]

What good is a 22kA main with 10kA branches? A fault just on the load side of a branch will have the same fault current as a fault in the panel. So the fault at the panel can't be more than 10kA without exceeding the rating of the branch.

If the breakers are series rated, they have been tested together to succesfully clear a 22 kA fault at the terminals of the 10 kA breaker without blowing up. (Well not quite the terminals, a short length of cable is used to make up the short circuit.)

If the fault is above 10KA, the 22KAIC rated breaker also trips. The two breakers share the arc energy and work together to clear the fault.

A disadvantage is the main trips for a close in fault solid fault, but those faults don't happen very often. A big advantage is lower cost.

 

[jim dungar]

If the fault is above 10KA, the 22KAIC rated breaker also trips.

This is not true of all series rated combinations. It is entirely possible for the downstream device to clear faults all by itself.

Say that a 120V breaker can successfully interrupt an 18kA fault, but UL does not have a listing for that AIC at that voltage, so the breaker goes through life labeled as only 10k AIC. Now say, it also has a series rating of 22k AIC with a specific main breaker. See how, it is entirely possible for a downstream breaker to clear a fault >10kA while the upstream breaker remains closed?

 

[jghrist]

If the breakers are series rated, they have been tested together to succesfully clear a 22 kA fault at the terminals of the 10 kA breaker without blowing up. (Well not quite the terminals, a short length of cable is used to make up the short circuit.)

If the fault is above 10KA, the 22KAIC rated breaker also trips. The two breakers share the arc energy and work together to clear the fault.

A disadvantage is the main trips for a close in fault solid fault, but those faults don't happen very often. A big advantage is lower cost.

I should have realized that. Thanks.