wire size for 30.5 MCA heat pump

[KarlD]

Hi All - I am having a contractor install a new heat pump system with a label that shows a 40A recommended breaker size, and 30.5A MCA. They are running the circuit with 10 gauge wire. I have always thought of 10ga as having a 30A max. Does this meet code in this application, or should they be upsizing to 8ga? Thanks in advance for your advice.

 

[Little Bill]

I'm allowing this for discussion purposes only of code.

 

[tom baker]

See if your contractor can explain section 240.4 (G) to you. Many electricians believe (wrongly) the ampacity of 12 AWG THHW is 20 amps. Its not. Same with 10 AWG THHW.

 

[curt swartz]

What type of wire/cable are they running? Romex (NM), MC, raceway with individual conductors?

 

[texie]

What type of wire/cable are they running? Romex (NM), MC, raceway with individual conductors?

Yep, this is one of those cases where it will matter.

 

[KarlD]

What type of wire/cable are they running? Romex (NM), MC, raceway with individual conductors?

It is Romex. About 20 ft from the breaker box.

 

[david luchini]

It is Romex. About 20 ft from the breaker box.

#10 romex has an ampacity of 30. It is too small for the 30.5mca.

 

[Rock86]

#10 romex has an ampacity of 30. It is too small for the 30.5mca.

I was about to jump in on this convo, but you have it right. If it had been some THWN in conduit, they could have gotten away with using #10.

 

[Dennis Alwon]

(B) Fractions of an Ampere. Calculations shall be permitted
to be rounded to the nearest whole ampere, with decimal fractions
smaller than 0.5 dropped.

You missed the cutoff.

If the wiring is not nm cable then #10 could work... rated 75C wiring and 240.4(G) would allow it.

 

[infinity]

I agree the #10 NM cable is too small. Use #8 NM or run #10 MC cable for the 20'.

 

[KarlD]

Thanks All! I appreciate the help!