Derating

[Tmark84]

I have (4)-20 amp double pole breakers feeding lights that doesn’t have a neutral. Can I use the 8 #12 current carrying conductors in a 3/4 conduit or do need to go to #10s to derate?

 

[infinity]

Yes derating is required because you have more than 3 CCC in a single raceway. Good news is that you can still use #12 AWG conductors.

#12 THHN = 30 amps @90 degrees C
30 amps*70% = 21 amps adjusted ampacity.

Welcome to the forum.

 

[roger]

To follow Robs reasoning, depending on the insulation type of the conductors see article sections 110.14(C), 310.15(B)(3)(a), and 310.15(B)(16).

Roger

 

[infinity]

Yes, I did make some assumptions with the 90 degree C conductor. A 75 degree C conductor might also work if you can apply the next size up rule.