Residential wiring and available fault current ratings

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Dsg319

Dsg319

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West Virginia
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Wv Master “lectrician”
I do mostly industrial and light commercial electrical work. Very limited residential.

But lord willing in the future I’d like move more that direction. And just a curious thought in my head was do people actually do calculations to figure available fault current at there service. To have properly rated breakers and equipment? Seems like all’s I ever see is 10kaic( at least on branch circuits). I know you don’t have to label it on the service or anything but still do you do it to install properly rated equipment?

Also(doesn’t have to be residential). Say you have available fault current of 20ka. Obviously the mainbreaker is rated for that but also does all the following branch circuits after that need to be rated for same or can be less?
 
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Of course, a residential apartment house may be a different issue and perhaps Bill Gates house would need to be checked...LOL
 
Dennis Alwon said:
I have never seen anyone do a calculation for fault current on a residence. We always used standard breakers which I believe are 10k
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That’s what I was thinking. I imagine you’d have to have quite the circumstances to be over the 10ka range in a residential setting.

But say for instance in an commercial or industrial setting perform the calculations to for the service and come up with higher than that and need to go up to 20ka rated equipment. Does the individual branch circuit breakers have to be rated 20ka instead of a lower 10ka rating?
 
I don't know a lot about fault current but my understanding is that the further down the line you go the lower the aic rating is needed. So you can start with 20ka but the branch circuit may very well be 10ka breakers if those branch circuit are at a sub panel further down stream.
 
Dennis Alwon said:
I don't know a lot about fault current but my understanding is that the further down the line you go the lower the aic rating is needed. So you can start with 20ka but the branch circuit may very well be 10ka breakers if those branch circuit are at a sub panel further down stream.
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Thanks. I don’t know a lot about it either. Generally calculations and all that is already done for us and gear selected for the jobs. But still like to learn for my own sake. I did however download an app for short circuit calculations. Seems pretty nice, Anybody ever use it and have experience with it?
 

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The main breakers are rated higher, like 25KAIC for CH 150 and 200 amp service panels. The branch breakers are 10 KAIC.

I've never done a residential KAIC calculation either. You need the transformer impedance and size of drop from transformer, all specified by the power company (or what was on the truck when it showed up that day!), not you. If something was out of normal, the power company should inform you, in my opinion.
 
Often AIC calculations are omitted in residential, and frankly, seldom necessary. But, they are also extremely easy. The Bussman app is great, I use it a lot, but you don’t even need that for residential. Most utilities have a fault current chart in their service manuals that will give you the fault current at the transformer secondary, and with various combinations of service drop sizes/lengths. All you normally have to do is take a quick peek at the utilities chart to verify that you are under 10k, and you’re done. In a few rare circumstances you may need to make a quick call to the utility.
 
Frank DuVal said:
The main breakers are rated higher, like 25KAIC for CH 150 and 200 amp service panels. The branch breakers are 10 KAIC.
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Right and the branch breakers will have a series rating with the 25 or 22 k main.

Note there are a few cheap main breaker panels that have 10k mains (some Siemens es series and some br series. All SQ D are 22k).

The place you have to be careful is using a 10k breaker backfed as a main, or meter centers that have regular breakers for the mains. Even then 10k may be fine, but I have had inspectors comment on that if they look up and see a larger transformer serving it like a 50 or 100.
 
In the areas I work, residential AIC on services 400 amps or less is pretty well accepted as less than 10k based on small transformers and fairly long service runs. "
As far as branch breakers they are often "series rated" with mains. So a service might have a AIC of >10 but <22 and the main have a 22k rating but series rated beakers could be 10k.
 
electrofelon said:
Right and the branch breakers will have a series rating with the 25 or 22 k main.

Note there are a few cheap main breaker panels that have 10k mains (some Siemens es series and some br series. All SQ D are 22k).

The place you have to be careful is using a 10k breaker backfed as a main, or meter centers that have regular breakers for the mains. Even then 10k may be fine, but I have had inspectors comment on that if they look up and see a larger transformer serving it like a 50 or 100.
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How does the series rating work?
 
Also doing a short circuit calculation for a sub panel. Would it be done the same way? Or anything different.
 
Jolted said:
Often AIC calculations are omitted in residential, and frankly, seldom necessary.
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More specifically, available Short Circuit Amps (SCA) calculations are rarely necessary in residential....

AIC values are determined only by the manufacturer. An electrician selects equipment with an AIC rating higher than the available fault current SCA.
Sometimes specifications dictate a minimum level of AIC required. For instance, most of the majaor POCOs, in Wisconsin, require a minimum of 22k AIC equipment for residential services, however series rated 22/10K equipment is usually acceptable and pretty much what is found on the suppliers shelves.
 
When doing these calculations. Which short circuit rating do you need to use. It has one for line-line and than also one for line-neutral.
 

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Dsg319 said:
I assume line-line on mains and double poles and line-neutral for singles?
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You also need to consider Line-Neutral for multi-pole devices as they could also be called on to clear these faults.

Luckily the L-L values typically exceed the L-N ones. The area to be concerned is usually when you are close to the secondary of transformers, such as close coupled substations larger than 300kVA.
 
jim dungar said:
Luckily the L-L values typically exceed the L-N ones. The area to be concerned is usually when you are close to the secondary of transformers, such as close coupled substations larger than 300kVA.
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My understanding is due to typical single phase transformer construction, the L-N is typically higher. There is a somewhat common rule of thumb to use 1.5 times the L-L for the L-N for single phase
 
electrofelon said:
My understanding is due to typical single phase transformer construction, the L-N is typically higher.
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My discussion should have included the caveat that it was for 3 phase systems. There the L-N value falls off real fast as you move away from the transformer.

Center tapped single phase systems behave a little differently.
 
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