Conductor rating

[bdktoys]

I'm a little confused with 240.4 D.
Can a number 12 THHN copper conductor be fused at 30 amps for a motor/HVAC load? The way I'm reading that in conjunction with the table in 310.5, I believe the max overcurrent device has to be rated at 20 amps max.

 

[augie47]

It takes some careful reading, but if you note the 1st paragraph of 240.4(D) has an "except for" clause with excludes the(D)(5) 20 amp requirement for certain applications.
As far as the conductor, you still need to keep 310.16 and 110.14 in mind including Art 430 and 440.
As far as the short circuit device, 430.52 guides you.

 

[tom baker]

Motor connections are rated at 75 deg C, so you can typically use the 75 deg C rating of 25 amperes for 12 THHN/W.
The motor branch circuit, short circuit and ground fault protective device (fuse) is sized larger than above to allow the motor to start. The wire protects the motor, the overload protects the motor