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Posted By: CTwireman A/C circuit sizing - 08/01/03 01:00 AM
I know this is a "dead horse" topic, but I'm still confused about it. This subject seems to be a source of mass confusion in the electrical trade.

So, as an example, here is an a/c unit nameplate (from an 2 ton Trane unit):

Min circuit ampacity: 15 amps

Min fuse/breaker size: 20 amps

Max fuse/breaker size: 25 amps

Line running amps: 11 amps


Does this mean that the unit can be fed by #14 protected by a 20 amp breaker?

Or #12 on a 20 amp breaker?


Do you size the conductors by the "minimum ampacity," and size the breaker by "minimum fuse/breaker"? [Linked Image]

One thing I do understand is that in this situation, the breaker only provides short-circuit protection, as the motor is protected by a thermal overload device.
-Peter
Posted By: resqcapt19 Re: A/C circuit sizing - 08/01/03 02:37 AM
In this case the code would permit #14 on a 25 amp breaker.
Don
Posted By: elecbob Re: A/C circuit sizing - 08/01/03 04:53 AM
A/C unit: 120/208 V 3 phase
Nameplate reads:
18.2 amps FLA.
Min. circuit: 31.5 amps
I had to replace the 30 amp disconnect and #10 wire with a 50 amp disconnect (40 amp disconnect not available) and run #8 wire.
Posted By: Redsy Re: A/C circuit sizing - 08/01/03 10:52 AM
Bob,
Were you using NM Cable?
If not, it seems you were in compliance with 310.16, which lists #10 as rated for 35 amps.
As far as having to upsize the disconnect, I think you need to know the Locked Rotor Current so you can back-calculate using Table 430.151(B).
Posted By: elecbob Re: A/C circuit sizing - 08/01/03 02:25 PM
Thanks Redsy. I was going to pull 4#10's but the inspector said I needed #8 for the 31.5 amps. I was a little suprised to say the least. He is coming out for an inspection on some other gear next week. I will ask him again before I do the pull.
Posted By: Redsy Re: A/C circuit sizing - 08/01/03 09:01 PM
Bob,
He may be thinking about the asterisk at the bottom of Table 310.16, which refers to 240.4(D).
However, 240.4(D) refers to 240.4(G), which permits particular Articles to specify conductor ampacity selections.
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