wiring electric furnace

[Chuck52]

SIL is in HVAC and is installing a 5 ton 16 SEER Goodman heat pump and electric furnace for me. This is replacing an older central air and electric furnace combo, so we should see some savings on our electric bill. The new furnace has two 60A breakers in the unit and we're not sure if 6AWG is OK for the short run of four feet from the box to the furnace or if we should go to 4AWG. Got 6AWG, would have to go get some 4AWG....no big deal if it makes sense. Also, do you run separate wires from the disconnect to each breaker in the furnace, or use jumpers between the breakers?

I think my main advantage with the new system will be better duct work. The old duct work was duct board, and the returns were restricted so much the basement door would jump every time the heat kicked on. SIL does nice duct work and is putting in more returns and opening up the existing returns, which were mainly made by just drilling holes through the sub-floor and sill plate. If the old duct work hadn't been so leaky the old air conditioner would probably have frozen up from lack of return air.

Chuck

 

[gclark]

6 AWG will be fine for each breaker, run a seperate circuit from the main panel to each breaker in you new unit. Hope this helps.

 

[Chuck52]

Thanks. I found a couple sites that seemed to be saying that, but not as clear as I needed. The 6AGW is supposed to be good for 60A, and our test says it both stages pull about 80A total, so wired seprately the 6AGW should be good.

Turns out SIL has usually had an electrician doing the wiring.

Chuck

 

[rjkobbeman]

You should look on the tag of the unit. There should be something there that says something like "Minimum Circuit Amps". This tells you what your circuit (wire) must be rated at (or higher).

Often times an HVAC unit will have a smaller "Minimum Circuit Amps" rating than the "Maximum Breaker Amps". This is due to the compressor having its own built-in thermal breaker to protect the circuit. This is one (of a few) situation that the NEC allows a breaker rated higher than the wire (again, because the compressor itself has circuit protection built in).

The reason I say this is because your 6AWG wire may be overkill already. Overkill is OK, just wanted to let you know. Check the tag on your unit. It will tell you what the wire needs to be rated for based on the "Minimum Circuit Amps" rating. In the case of a compressor, the breaker is usually big for the surge amps as startup... and is usually bigger than the wire needs to be.

For example, I have a 6 ton geothermal heat pump in my home. It has a Max Breaker size of 70 AMPS... which is the breaker I have installed. The Minimum Circuit Amps is under 50 (it is like 48 or so). I ran 6AWG wire to my unit. The surge at startup is over 150 amps... but humms along at 27 amps after startup (1st stage).

You said you had two 60 amp breakers? What is the other breaker for, electric resistant heat? There should be a "Minimum Circuit Amps" tag for each circuit.

 

[Chuck52]

RJ,

Thanks for the reply. The two breakers are for two stages of heating coils. We first hooked it up with 6AGW going to one breaker and 6AGW jumps from that breaker to the other. A clamp on meter showed about 80A at full power with both stages on. From what I see on various wire gauge/Amp rating sites, 6 AGW is good for 60A, so either I can use 4AGW to the first breaker and jump with at least 6AGW to the second, or wire both with separate runs using 6AGW and be pretty safe. Since I already have the 6AGW, I'll go with the second option.

I'm hoping I don't need the full draw often, with the heat pump providing most of my heating, but I'm sure we'll have some really cold days this winter.

I would have gone with a ground source, even at the higher cost of equipment, but if I had gone that route I would have been paying for labor, too, since my SIL has no experience with those units. I get his labor free for this system....at least if you don't count in-kind stuff I have and will do more of for him. I'm helping as much as I can on this work, and finding all kinds of muscle groups I must not use much.

Chuck