40A Circuit Through Attic

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PWR73

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I ran an 40 A Circuit through an attic and for some reason I installed 60-2 NM cable. I can't remember exactly why but at the time I thought it was a code issue. Anyone remember offhand why I might have had to do this. I have been searching through my code book.
 
Might not have been necessary. Under the '08 Code #8 NM has an (interior) ampacity of 40 amps. The 90? rating is 55 amps which would derate to 41 amps at 131?F
 
I don't think A/C units are typically considered continuous loads by defintion, but the circuit must be sized to the MCA which is not less than 125% of the rated -load current (440.32) so it is the same
The circuit ampacity would normally be determined by the MCA listed on the nameplate.
If a load is 40 amps and is continuous then you would need a conductor rated at 50 amps, or a #6 NM.
There is no problem in your installing a #6, but I think most inspectors would look at the A/C unit MCA and assure the cable installed was adequate for that number.
 
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60/2 must be one very small cable . :grin:

Perhaps it was the ambient temp or you added 125% to the a/c unit where the mca already has 125% added to it.
 
zappy said:
But if he ran 6-2 nm cable, whats the problem?
Click to expand...

ActionDave said:
250.122(B) says that any time you oversize your conductors your EGC has to be increased in size proportionally. 6-2 would only have a #10 EGC.
Click to expand...

Exactly. If the circuit could have been installed with #8, then we effectively increased the size, so using circular mils, #6/#8 = 1.59 times bigger than needed. So the EGC would also need to be 1.59 times bigger than the #10 we could have used, so 10380 cmils x 1.59 = 16504, so a #8 Equipment Grounding Conductor would be required.

Or, raise the temperature of the attic until a #8 wouldn't be big enough.

Or, increase the size of the OCPD and/or load until a #8 wouldn't be big enough.
 
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