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Posted by Bud-- on March 18, 2007, 2:41 am
zeb7k@hotmail.com wrote:
> I understand that AIR stands for Ampere Interrupting Rating.
>
> I'm wiring a 200A service for a residence with a Q0 load center.
> Where in the NEC (pun) does it show what AIR rating I should use when
> selecting breakers? When should I use 10,000 AIR, when should I use
> 22,000 AIR or higher?
>
NEC 110.9 requires the overcurrent devices to have an interruput rating
high enough to interruupt the available fault current. Get the available
fault current from the utility. It depends on supply wire size and
length, transformer % impedance, fault current available to the supply
transformer. For a large installation with multiple panels, the
available fault current at panels is calculated by the designer,
commonly an engineer. The designer also needs to know the fault current
avilable from the utility. I would guess 10,000 would be high enough.
--
bud--
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