4

What is a safe and economical way to get two 60 amp circuits to air handlers with electric heat kits?

There are two air handlers (Goodman AVPTC25B14) each with an electric heat kit (Goodman HKSC10XC) in a utility closet. They each require a 60 amp service. It would require a little under 50' of wire to go from the panel to where the air handlers are, taking into account going up the wall, across the attic, and then back down the wall on the other side. I'm using 50' for estimating purposes. About 25' of the run goes through the attic which could get very hot

I'm currently stumped and going back and forth on the code tables and wire types, and also wire prices, trying to determine if I need #4 or #6 wire for these runs, and if I should be using NM or THHN in conduit. I don't think these units need a neutral so it would be two hots and a ground.

But we have different ampacity allowances for NM vs THHN, and potentially an attic heat derate (but I don't know the exact temperature of the attic so the chart isn't useful). And if the wire size has to go to #4 for NM, but #6 is OK for THHN, then it might be better to use #6 THHN, even with the extra cost of conduit. I also have some extra 1-1/2” PVC conduit sticks left over, which I think would be OK for running all four hots and two grounds (#6 THHN) inside of the 1-1/2” PVC so I wouldn't have to buy conduit.

So I'm looking for a wire type/size recommendation/strategy on what is safe (and then most economical), to get the two 60 amp 240v circuits to the air handlers, which need to go through an attic, and which are about a 50' run, each.

(Of course, if the wire pricing makes more sense, it might also be possible to put a sub-panel in the closet.)

6
  • 1
    is a heatpump not an option? That will save you some money in the long run being cheaper to run for the heat output. Commented Jul 19, 2023 at 10:59
  • 1
    If you are adding two extra 60 amp circuit you will need to check if your panel can even allow that. You will need to do a load calculation for the panel first. Unless you have 320 to 400 amp service, quite likely they will over load your panel.
    – crip659
    Commented Jul 19, 2023 at 10:59
  • Unless your attic is exceeding 194F then you don't need to worry about derating for heat.
    – APP
    Commented Jul 19, 2023 at 11:48
  • Can you post the square footage of your house, and photos of your main panel(s) as well please? Also, have you looked into envelope upgrades for your house, including but not limited to converting to an unvented compact roof with a conditioned attic underneath? Commented Jul 19, 2023 at 12:39
  • 1
    @Andrew The ambient air temp for conductors listed in Table 310.16 is footnoted to be 30°C (86°F). The temp rating on the wire is conductor operating at current Commented Jul 19, 2023 at 15:51

1 Answer 1

10

Are you paying the electric bill? Modern heat pumps. (Not the clunky old ones that need emergency heat).

Is cost a factor? Aluminum wire is a must-do, provided the terminals on the air handlers are rated for aluminum wire. If not, install two disconnects there, which are merely there to serve as cheap splices from Al to Cu wire. Though they also cross off any disconnect requirements.

Wire size, given your thermal concerns and its commodity status, I would say #2 Al.

3-wire SE cable should get it done. With SEU you can use the bare mesh neutral as a ground.

2/0 aluminum to a subpanel is another option. 1/0 aluminum would suffice if it was not for your thermal concerns.

And you have the 400A service to power all this, right? Because 120A of heat would bode ill for a house's ability to fit on a 200A service. Run that NEC Article 220 Load Calculation carefully. (best to use 220.82 alternate method).

3
  • Definitely #2 Al, SEU. At very least 60A needed with #4 gives you 0.8 of 75A rating, I think that gives you only 122°F. If he needs two 60A AC units that seems like that could be pretty questionable. If you use #2 the Inspector likely won't even question it. Commented Jul 19, 2023 at 16:35
  • I forgot to mention it in the post, but this is a heat pump system. Code requires an emergency backup system for when the heat pump is too cold to operate. The backup choices are 10KW toaster coils (60 amps required to the air handler) or gas, and there is no gas service here. So the wiring in this question is for the backup electric coils, not regular heating use.
    – Nick
    Commented Jul 20, 2023 at 8:06
  • @NickLockheart -- note that emergency/auxiliary heat on heatpumps is not a Code requirement, just one dictated by the way North American split system heatpumps defrost. If you're willing to pony up the extra cash (and returns and effort) to go with minisplits + matching ducted handlers, you can get a system that works at low ambients without auxiliary heat, as mini-splits stop their indoor fan during defrost Commented Jul 22, 2023 at 1:22

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.