I am looking at installing a sub panel for an existing heat pump. The HVAC contractor just extended an existing AC circuit but didn't noticed that the circuit was also used for the range.They never looked in the panel to see the 30 amp range circuit piggy backed with the AC circuit.
The heat pump specs say a min 21.5 amps and max breaker size is 35 amp. It looks like the feeder is #12 to me. I couldn't get the arc shield off the HP disconnect to confirm as I have to reposition the shield to get it off.
The red jacketed loomex is available in mutliple gages, so it could be 14, 12, 10 etc - you would need to confirm.
As far as the subpanel size, you may be able to get away with 30 amp, but the specs say max overcurrent of 35 amps, so if a 30 amp overcurrent did'nt hold on start up, you may have to consider time delay overcurrent or non time delay overcurrent and be prepared to go up to 35 amps, which means the sub panel would have to be rated accordingly.
Check out Table D16 (app D) under 'single phase all types'. Under circuit breaker of 35A, it shows an FLA of either 14 or 15A (if these match the heat pump) Picking the bigger one...15A X 1.25 is the max conductor ampacity. Which is 18.75A T2 says #12.
The table also gives fuse sizes. If needed. I'd use a 30A to feed it (cause it worked before)
For some reason sometime in the early 70's some house's where wired with 10/3 for the range and on a 40 amp breaker. If we do a service upgrade then we need to change the feeder out to a 8/3 if that is the case. Same with a mobile home that may need a CSA re certification if it is a 10/3 feeder