A/C breaker size

[sryan]

Installing a 120V ac unit. It has a max nominal current of 10.1A and a starring current of 39.2A. Do I size my wire for the 10.1A and my breaker size would be a 50A breaker? Please let me know your thoughts, thanks

 

[GoldDigger]

Installing a 120V ac unit. It has a max nominal current of 10.1A and a starring current of 39.2A. Do I size my wire for the 10.1A and my breaker size would be a 50A breaker? Please let me know your thoughts, thanks

The equipment label should have values for the Minimum Circuit Ampacity (MCA) and the Maximum OverCurrent Protection Device (MOCPD). Use those numbers (without any additional 125% multiplier) to determine wire size and breaker size respectively.
You can use a breaker smaller than MOCPD, all the way down to MCA, but doing that increases the chance of nuisance trips. Generally MOCPD is used. But if MOCPD is between breaker sizes you cannot, AFAIK, go to the next larger standard breaker size.

 

[jeremy.zinkofsky]

The equipment label should have values for the Minimum Circuit Ampacity (MCA) and the Maximum OverCurrent Protection Device (MOCPD). Use those numbers (without any additional 125% multiplier) to determine wire size and breaker size respectively.
You can use a breaker smaller than MOCPD, all the way down to MCA, but doing that increases the chance of nuisance trips. Generally MOCPD is used. But if MOCPD is between breaker sizes you cannot, AFAIK, go to the next larger standard breaker size.

The conductors need to be protected from Overcurrent and are sized according to their rated ampacity using table 310.15(B)(16).
The Motor needs to be protected from Short-Circuits and Ground-Faults. The feeder breaker accomplishes this task. The breaker is sized according to Table 430.52.
You can take the nameplate numbers at face value, as was previously stated but don't get confused by the nomenclature. Just think of it this way; for motors, the breaker is not intended to protect it's feeder cables. It is intended to protect the motor. That is why you may see a 50A breaker with #14AWG wire landed on it.

 

[jumper]

You can use a breaker smaller than MOCPD, all the way down to MCA, but doing that increases the chance of nuisance trips. Generally MOCPD is used. But if MOCPD is between breaker sizes you cannot, AFAIK, go to the next larger standard breaker size.

That is what I was taught and have always done. I did not look up and see if there is a code section for this, prolly is.

 

[OldSparks]

The conductors need to be protected from Overcurrent and are sized according to their rated ampacity using table 310.15(B)(16).
The Motor needs to be protected from Short-Circuits and Ground-Faults. The feeder breaker accomplishes this task. The breaker is sized according to Table 430.52.
You can take the nameplate numbers at face value, as was previously stated but don't get confused by the nomenclature. Just think of it this way; for motors, the breaker is not intended to protect it's feeder cables. It is intended to protect the motor. That is why you may see a 50A breaker with #14AWG wire landed on it.

Are you sure about that? Since when does the NEC protect equipment? Is it not up to the equipment manufacturer to (usually internally) protect his equipment? If an AHJ allows #14 wire on a 50 amp breaker, and then the circuit gets used for other than original intent (say a 30 or 40 amp load), some wire is gonna cook. Distinct fire hazard. That goes against all I ever learned about the rationale behind the NEC.

 

[jeremy.zinkofsky]

That is what I was taught and have always done. I did not look up and see if there is a code section for this, prolly is.

430.52(C)(1)

 

[infinity]

If an AHJ allows #14 wire on a 50 amp breaker, and then the circuit gets used for other than original intent (say a 30 or 40 amp load), some wire is gonna cook. Distinct fire hazard. That goes against all I ever learned about the rationale behind the NEC.

What someone might do in the future is not always the concern of the NEC. You could certainly have a 200 amp switch with 110 amp conductors and 110 amp fuses but that would stop someone from installing 200 amp fuses.

 

[jumper]

430.52(C)(1)

Thanks, but 440.22(C) seems more likely to fit what I was referring to.

 

[jeremy.zinkofsky]

Are you sure about that? Since when does the NEC protect equipment? Is it not up to the equipment manufacturer to (usually internally) protect his equipment? If an AHJ allows #14 wire on a 50 amp breaker, and then the circuit gets used for other than original intent (say a 30 or 40 amp load), some wire is gonna cook. Distinct fire hazard. That goes against all I ever learned about the rationale behind the NEC.

Just my opinion here, but what the manufacturer does to protect its equipment is outside the scope of what the NEC is meant to provide. In the case of Motors, it may be more acceptable to burn up the wire before the motor windings. Otherwise the NEC would require motor feeder cables to be sized the same way the feeder breaker is.

 

[iwire]

Are you sure about that? Since when does the NEC protect equipment?

Every time the NEC limits the maximum over current protection for equipment and that happens for most if not all equipment.

It would be a code violation to supply a 30 amp water heater with a 60 amp breaker even if you used 60 amp wire.

 

[ActionDave]

Are you sure about that? Since when does the NEC protect equipment? Is it not up to the equipment manufacturer to (usually internally) protect his equipment? If an AHJ allows #14 wire on a 50 amp breaker, and then the circuit gets used for other than original intent (say a 30 or 40 amp load), some wire is gonna cook. Distinct fire hazard. That goes against all I ever learned about the rationale behind the NEC.

What about this?

90.1 Purpose

(C) Intention. This Code is not intended as a design speci-
fication or an instruction manual for untrained persons.

If you don't know what you are doing and you take a perfectly safe installation and make it unsafe that's a you problem not a code problem.