Discuss Wire size for 15kW heat pump electric backup element in the DIY Electrical Advice area at ElectriciansForums.net

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In a few days our local HVAC company will replace our propane furnace with a Mitsubishi heat pump (PUZ-HA42NKA1) and ducted air handler ((PVA-A42AA7) with a 15kW backup electric unit (EH15-MPAS-LB). Submittals and installation instructions can be found on mylinkdrive.com. I'm doing the electrical work, which is allowed by the State of NH.

The distance from the main panel to the air handler is in the range of 30-40 feet, depending on where the installers place the air handler and how they want the electric feed for the backup element dressed.

The breaker and wire sizes for the outdoor compressor and and indoor air handler are listed in the Mitsubishi installation instructions (40A #8/2). But neither the installation instructions nor the submittals specify the breaker or wire size for the 15kW backup heat unit.

The backup heat unit is divided into two internal circuits with three elements each. Each of the two circuits draws 31.2A, for a total of 62.4A. 125% of that is 78A, so I'll need an 80A breaker in the main panel. This makes sense, as the MOP for each internal circuit is 40A and each circuit has a 40A breaker that's used for shutoff (not for overprotection.)

But what wire size should I use? I've looked at many tables, articles and forum posts on this subject, and found some that say 2/2 or 3/2, others that say 4/2. Not sure that 3/2 is even available locally.

The schematic in the installation manual shows one feed from the panel that connects to the two separate 40A breakers, the inputs of which are jumpered together. It doesn't indicate whether the two breakers can be fed separately (i.e., that the jumper can be removed.) If I can feed the circuits separately, then I could run two 8/2 wires and use 40A breakers in the panel. I haven't priced this out yet, but I think it may be less expensive than a single 2/2 run.

If the internal breakers require a single feed, should I use 2/2 or 4/2? If I can feed the circuits separately, is it OK to run the two #8 wires together through holes in the floor joists?

Thanks in advance for any help.
 

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