Palm Beach attic derating

[tonyi]

(3) #10 + (4 maybe 5) #12 (current carrying) through a 20' 3/4" smurf in a Palm Beach attic.

My calculations said it doesn't work even at a 77 degree ambient, but the foreman was insistent derating wasn't going to be an issue...

What am I missing here? an envelope that some inspector is going to get maybe?

 

[charlie b]

It depends on the load.

You have to derate the #12 to 21 amps and the #10 to 24.5 amps, just for the number of current carrying conductors. So with no other considerations, you can use a 20 amp breaker for the #12, but no more than a 25 amp breaker for the #10. IF your calculated loads are within those limits, you are OK so far.

But you will need to assign an ambient temperature to the location of the installation. If, for example, you use 110F (temperature derating of 87%), then you become further limited to a 15 amp breaker for the #12, but a 25 amp breaker is still acceptable for the #10. Once again, whether or not those will adequately serve the installation depends on the amount of load.

 

[tonyi]

The 10's were for a 208V cloths dryer of some sort, two of the other 12's are washer, the rest unknown receptical loads (one for maybe a station for an ironing board). Physical design of that area is a bit unsettled at this point with architect and GC moving things around.

I couple of years ago I measured the temps in my attic some time in October got 120F at the floor, over 140 at the peak. Smurf in a south FL attic seemed kinda marginal too given the summer temps.