I have been finding multiple homes with #10 solid wire on 40,45 and 50 amp breakers for a/c units in the main panel, these are all on one year warranty inspections. Is there something that allows an electrician to use this size wire with these breakers… thanks
Yep…Look at the nameplate on the AC Unit and it will tell you the minimum circuit ampacity and the maximum OCPD and it’s just that simple.
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As Paul stated it’s pretty straightforward if the circuit is fed with NM cable. You would use the MCA to size the conductors and then any size OCPD from the MCA to the MaxOCPD listed on the nameplate. The conductors would be sized using the standard #14=15 amps, #12=20 amps #10=30 amps etc.
If the circuit is run in MC cable or conduit and wire then it gets more complicated because you may be able to utilize the 75° C conductor ampacity.
Yes, in motor circuits the breaker only provides short circuit protection, overload is done via the motor’s/unit’s internal overloads. Code lets you size the breaker by 250% for general motor circuits, and with AC units the sticker usually lists min/max breaker value.
MCA would be your max current draw which lets you size the wire, and then the max breaker rating tells you what size breaker to use.
So… I have an MCA of 22amps and a max breaker size of 50amps. I would then select #10 NM which is good for 30amps at 60*C rating, and then install a 50amp breaker.
This of course is motor/AC/refrigeration circuits only. Welders can do something similar. Other than that its business as usual.
Just to expand on what Martin stated the MCA is a calculation of the unit which adds 25% to the largest load, usually the compressor. For example a unit with an MCA of 22 amps may have a compressor with a current of 16 amps and a fan current of 2 amps.
So you end up with:
16*125% = 20 amps + 2 amps = 22 amp MCA. This additional 25% factor will ensure that the circuit conductors will not be overloaded when they’re protected by the motor overload device.
The nice thing about AC units is that the manufacturer does all of the calculations for you (the information is on the nameplate) so you only need to ensure that the circuit conductor ampacity is greater than or equal to the MCA and that the OCPD does not exceed the MaxOCPD listed on the nameplate.
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Which brings you back to my post # 2…Just follow the nameplate…