While doing an inspection last night I came across something that I have seen in the past and wanted an opinion on from you guys. I found a 40 amp double pole breaker connected to 10 gauge wire. While I know this is incorrect per say however the circuit is for an AC compressor unit. In the past I have been told that this is allowed because the AC unit startup surge may cause a 30 amp breaker to trip and the running draw is not more than 30 amps. Thoughts???
You need to compare the conductor size and the OCPD with the information on the nameplate of the unit. Typically the OCPD can be larger than the ampacity rating of the conductor. Do you have the nameplate info?
The nameplate of the AC unit will tell you “Minimum Circuit Ampacity (MCA)” and Maximum Overcurrent Protection Device (fuse or breaker). These are the values that you need to rely on. Do you have a picture of the nameplate?
Min fuse or cir breaker 30/30 max fuse of cir breaker 40/40. Sorry cant upload pic now I am out of office.
There will be an MCA number which is used to size the conductor, the MaxOCPD is used to size the circuit breaker.
I dont see an MCA number.
Cropped and rotated picture attached.
Min Supply Circuit Ampacity: 25/25. The conductor is good for 30A so you are covered.
Max Cir Brkr. Size 40/40
Min Cir Brkr. Size 30/30
You have a 40A breaker so you are ok.
I see your point however what happens if for some reason the unit draws more that 30 amps like some sort of failure or short then the wire can overheat no?
There are two components that you’re dealing with for an AC unit one is for the protection from ground faults or short circuits, the other is overload. Ground fault and short circuit protection is provided by the OCPD ahead of the branch circuit. It’s only function is protect the conductors in the case of a ground fault or short circuit condition therefore the OCPD size can be much higher than the ampacity of the conductor connected to it (this can allow for a large starting current).
The other component is overload protection, the overload device is there to protect the AC equipment and is built into the compressor and fan motors. So if the compressor overheats due to say worn bearings or too low voltage the overloads within the compressor will open.
The MCA (minimum circuit ampacity) is a determined by the manufacturer and tells you what the smallest conductor you can use to feed the AC unit. It has no bearing on the size of the OCPD used ahead of it.