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I have a subpanel that feeds two pieces of HVAC equipment - a minisplit and a heatpump compressor. Their MCA (minimum circuit ampacity) figures on the name plate are 16 amps and 24.1 amps, respectively. These, of course, add up to 40.1 amps.

Is it code-compliant and safe to feed this subpanel with 8awg NM-B protected by a 40-amp breaker in the main panel ?

Purely based on the numbers given, the answer would be no. But I'm aware that the NEC has cases where some wiggle room is given, such as upsizing a breaker (e.g. 60-amp breaker on a circuit wired with 6awg). Is this one of these cases ?

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  • the 80% rule says your wires and breaker have to be 20% higher then actual consumption
    – DIY75
    Commented Apr 19 at 6:50
  • I believe wires(not cable) in conduit can be on a larger breaker, than what is allowed for cable. When instructions give a range or say minimum/maximum that gives some wiggle room. You are already looking at a 20 and a 30 amp breakers/50 amp to have the minimum.
    – crip659
    Commented Apr 19 at 11:31
  • That's not wiggle room. That's "there are no 55A breakers made" and you are not permitted to put a calculated load of more than 55A on that particular case. A calculated load of 40.001A is 45A (because those are made) before any other factors are applied.
    – Ecnerwal
    Commented Apr 19 at 12:08
  • @Traveler I believe ti's 25% higher for continuous loads, but I also believe the MCA has that 125% factor baked in.
    – RustyShackleford
    Commented Apr 19 at 15:55
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    That's not wiggle room. youtube.com/watch?v=Gp4SyoB5D5s
    – Harper - Reinstate Monica
    Commented Apr 19 at 20:45

1 Answer 1

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Why not just feed it with #6? Cost?

If it's cost, then feed it with #6 aluminum SER. Because it's SER and not NM, it doesn't have the 60C curse.

Unlike NM it's also allowed outdoors (but not underground).

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  • Somewhat cost, mostly just what a PITA #6 conductors are to work with. I mean, it's off by less than half a % ... and that 16 amp number has all sorts of margins baked in (the max operating current is like 5.5 or so). But, I want to do the right thing.
    – RustyShackleford
    Commented Apr 19 at 21:56
  • On the aluminum possibility, the aluminum curse cancels out the absent 60-degree curse, so I still have to use #6. But cheaper and easier to work with, I suppose. It's not clear the main panel breaker and the subpanel lugs allow aluminum is it ?
    – RustyShackleford
    Commented Apr 19 at 22:05
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    @RustyShackleford -- you'd have to have a very strange panel to have copper-only lugs on it
    – ThreePhaseEel
    Commented Apr 20 at 1:24
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    @Rusty yes, all modern breakers and subpanel lugs are rated for aluminum, in fact you'll probably find the lugs are made of aluminum (tin plated). Aluminum is the universal donor since it plays well with copper wire. (thermal expansion differences now work favorably).
    – Harper - Reinstate Monica
    Commented Apr 20 at 5:55
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    None in large wires. However it performed poorly in small 15-20A solid-core wires, partly due to using a powerline alloy that was never meant for solid wire, but mainly because a) terminals were not properly rated for aluminum, and b) nobody used a torque screwdriver on small stuff back then, so torques were all over the map. Whereas with the big stuff, people obeyed rules a) and b) as a matter of course.
    – Harper - Reinstate Monica
    Commented Apr 22 at 19:21

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