#10 Cu on 40 amp breaker?

[Rob Dols]

IS it acceptable to use a 40 amp breaker with #10 copper wire?
The load is a 3 phase 30 amp, 480 volt RTU, with a 3 pole 30 amp FSS at the unit. But the panel breaker (part of a gear assembly quote) is the 3 pole 40 amp breaker. So the unit is actually protected at the correct ampacity of the Fused Safety Switch at the unit, but I'm concerned the breaker is too large for the #10 branch circuit.

I think it's OK, right?

 

[kwired]

IS it acceptable to use a 40 amp breaker with #10 copper wire?
The load is a 3 phase 30 amp, 480 volt RTU, with a 3 pole 30 amp FSS at the unit. But the panel breaker (part of a gear assembly quote) is the 3 pole 40 amp breaker. So the unit is actually protected at the correct ampacity of the Fused Safety Switch at the unit, but I'm concerned the breaker is too large for the #10 branch circuit.

I think it's OK, right?

For motors an air conditioning equipment it happens frequently. For most other applications it is a violation though.

Motors (including the compressors in AC equipment) need conductors sized for the load, but need short circuit and ground fault protection at a level that doesn't nuisance trip during starting. The motors have separate overload protection that protects not only the motor from overload but also the conductors.

There are times you might even see 12 AWG on a 40 or even 50 amp breaker for a motor load and is acceptable per NEC.

If dealing with instantaneous trip breakers the allowable trip setting goes even higher, but this wouldn't be something you would run into often if ever when dealing with air conditioning or refrigeration units.

 

[Rob Dols]

Thanks very much!

 

[tom baker]

Check the nameplate, it will tell you the min ckt ampacity and max size OCPD, and if it can be a CB or fuse. Newer CBs are HACR rated, and few HVAC units require fuse protection

For example if the min ckt ampacity is 25 amperes, then a 12 THHN is OK

 

[Dennis Alwon]

I have notice that the overcurrent protective device on many of the resi units are now much closer to the min. cir. amp. Years ago I saw #12 on a dp 40 but I have seen some that have a min. cir. amp of 26 amps and a max overcurrent protective device of 30