I had an inspector question me about the interupt rating on a 200 amp main breaker today on a job I am finishing that another electrician started. Yes, it is a pain in the neck finishing somone elses work.
Anyway, I have a 300 amp underground feed to the meter pan, and 2 raceways out to a pair of 200 amp main breaker panels. The interupt rating on the MB is 22,000.
Is this sufficient?
Remember, I did not install the service, nor do I know enough about the formula for figuring the interupt rating. I have to assume that the rating is based on the largest ungrounded conducter from the POCO.
The AHJ also noted that he would prefer that I also strip back the insulation on all of my GEC's where they enter the panel for heat protection. Is he serious?
Shock: AIC (fault current) is available from PSE&G. Resi should be 'fine' with a 22K breaker. Most areas are 10K, a few are 15K.
Calculation requires impedance of transformer, length/size of drop, length/size of your service conductors, for starters. "Bussman" has a fairly simple point-to-point method that works; but you have to like numbers.
I'm working on preparing a AIC course, hopefully I'll have it ready by fall/winter.
If you want further info, send me an e-mail, and I can get you the literature.
As to the 'stripped ground conductors & heat???? What Article did he cite??
Re: Article 110.9#98559 06/02/0608:59 AM06/02/0608:59 AM
I have talked to some different people that I know throughout the country on this, and the consensus is that most power companies won't provide more than 10K available to dwellings, unless the dwelling unit is quite large.
Ryan Jackson, Salt Lake City
Re: Article 110.9#98560 06/02/0609:38 AM06/02/0609:38 AM
The major POCOs, in Wisconsin have required 22K AIC rated equipment for almost 20 years now. They typically feed (4) houses from a single padmount, so they could easily exceed the 10K fault level if they buy 2%Z transformers or put in an oversized one. This is one reason they give such high "design" fault values, to prevent future problems as they upgrade their equipment.
If you need 22K rated equipment make sure the branch circuits are either fully or series rated also. Do not just look at the AIC rating of the main.
Thanks for all the replies. I never had an inspector ask me about interupt ratings for a residential service before. Thought it was a bit strange. I understand these issues become much more important when working with higher voltages so I thought I'd take the time to try and find out more.
The AHJ didn't cite any code referrence for stripping back the GEC at the enclosure. I sure as heck don't see one, that's for sure. Is the AHJ out of line for requesting this to be done in order to get a final? AND the service has already been inspected, and passed, but additional work has been done (finished bsmt) since the original inspection.