Wiring for heat pump

[wanderer]

Quick question - am wiring a feed for a heat pump in 3/4" conduit with THHN wire. FLA of the load is 29.6 amps. I see that the maximum ampacity for #10 THHN is 40A, which would work fine since I happen to have a roll of it already.

Problem is I see several notes about maximum amperage of #10 wire is 30 amps, but I'm not sure if it is referring to the load or to the breaker size. If it's the load I'm within that number, otherwise I'm not.

https://www.lapptannehill.com/resources/technical-information/ampacity-chart

So, the load is 29.6 amps and it needs a 40A breaker to allow for motor starting. Do I need a #8 wire or a #10?

 

[MattT]

You shouldn't be using FLA for sizing a heat pump circuit. The units nameplate should have minimum circuit ampacity, used for sizing the wire, and a maximum fuse or breaker size.

 

[wyliesdiesels]

Youre going about this all wrong.

The notes youre reading about ampacity do not apply to your situation

As said above, need to size based on nameplate ratings, which already has the ampacity that you need to base the wire and fuse size off of.

Post a pic of the nameplate then we can help you

 

[wanderer]

The unit has a bunch of motors on it...nameplate info is incomplete (tandem units with 15kw aux heat, three separate power feeds). There is a circulating pump for the hot water generator, compressor, blower, and two circulating pumps for the water. Total unit FLA is 29.6, minimum circuit AMPS is 34.6, maximum is 50.

I'm disabling the aux heat since the units are capable of heating the house without it. This way I can run one unit on my generator.

 

[wyliesdiesels]

Ok if the plate says MCA of 34.6 then you use wire rated for 35a which means #10 THWN.

If the 50a is the MOCP rating then you use 50a breaker.

Does it say anything about fuses?

 

[wanderer]

Ok if the plate says MCA of 34.6 then you use wire rated for 35a which means #10 THWN.

If the 50a is the MOCP rating then you use 50a breaker.

Does it say anything about fuses?

Nothing about fuses. This is what I have. It's the 049 model without the asterick.

I already have #10 THHN wire, would like to use it if possible. Otherwise I'll buy a roll of something else.

 

[Bert_]

Total unit FLA is 29.6. We don't need to do anything with this number, the equipment manufacturer has already done the math for us and listed it below.

Minimum circuit AMPS is 34.6, Actually reads "Minimum Circuit Ampacity" MCA for short.
This is how we size the wire. #10 THHN/THWN can use the 75* ampacity which is 35A, perfect size. #10 romex is limited to 60* ampacity of 30A so it would be to small.

maximum is 50. Actually reads "Maximum Overcurrent Protection", MOP. The overcurrent protection is the breaker so it can be up to 50A. Note that this is sized independently of the wire. Normally #10 would not be large enough for a 50A breaker. Because this is basically a motor circuit the NEC lets us use a larger breaker for the startup current that is typically very high for a few seconds.

Wylie already said it but I wouldn't mind a bit of detail.

 

[wanderer]

Wylie already said it but I wouldn't mind a bit of detail.

Thanks! I can handle this!