I need to see if you guys understand the allowable ampacities for air conditioners the way I'm reading it.

Here's the situation. Several moths ago we wired 2 A/C units on a roughin. The job wasn't huge, about 1500 sq ft per unit. So since nobody informed us diferent the units were prewired with 10awg CU. Today we go back to trim out and find a monster unit sitting by 1 disco. As it turns out they decided to use a large heat pump (the inside unit needs 100 amps by itself). The nameplate says min circuit ampacity 31.2, rlc 24, maximum OCP 55.

art 310.16 rates 10 awg thhn at 35 amps, you are allowed to go over 30 amp OCP for equipment such as AC units. According to art 440 I can put a breaker up to 175% of the rlc (24) or the branch circuit capacity (35) whichever is greater.

If I read and calculate correctly, The #10 wire is OK and we can use a 55 amp breaker on it if we need to, although I think a 35 or 40 will do the job. Correct?

Just tell me if this is safe and legal, not how you would want to do it. Trust me, we would have run a larger wire if we knew if was going to be this close. Thanks.

As long as the terminations are rated for at least 75C on both ends you can use #10 wire and protect it with a 55 amp breaker. The breaker will only be providing short circuit protection for the wire. The overload protection in the compressor will protect the wire from overloads. If either the circuit breaker or compressor are only rated for 60C conductors you will need to use #8 since #10 is only rated for 30 amps. If these branch circuits are very long you might want to calculate the voltage drop to make sure it meets the manufactures requirements.

Curt

One thing I forgot to mention in my post is that I believe the 175% figure you are reading applies to single motor compressors. The typical outdoor unit on split A/C systems is not a single motor compressor load since it also has a fan to blow air through the coils. These units are multi-motor or combination loads and are required to have a nameplate that includes the minimum circuit size and maximum overcurrent protection. I don’t have my copy of the NEC in front of me right now to give you the specific references in 440.

Curt

Thanks Curt. The distance isn't very long so VD won't be a problem. I'm really more concerned with tripping the main breaker with this new equip. If anyone had told me it would be a heat pump, I would've objected at the rough, but this builder never has used a heat pump in the past so there was no reason to expect it. Thanks again.