Sizing wire to Electric Furnace

[Electrical Unlimited]

I have what is probably a stupid question. I typically do commercial wiring and I currently am roughing in a House. It has a furnace with dual 60A breakers. I looked up the NM cable which I know is 60deg. and it looks like I would have to use a 4/2 NM cable or off the 90deg. Al for SE cable I would have to use a 4/4/8. The whole sale houses do not stock either of these which makes me think that I am possibly doing something wrong. What do you guys do for a install like this, any tips? Thanks for your help.

 

[iwire]

If the circuit load is under 55 amps you can use 6 AWG with a 60 amp breaker because of 240.4(B)

 

[Electrical Unlimited]

Thank you very much!!

 

[pete m.]

First, check the nameplate of the furnace for the minimum circuit ampacity. If the minimum ampacity is 55 or less per circuit 6awg NM cable will probably serve your needs (as long as no correction or adjustments have to be made to the ampacity of the conductors). If you choose to utilize aluminum SE or SER cable be aware of 338.10(B)(4)(a). This section, by way of 334.80, will limit you to the 60 degree celcius ampacity for the cable.

Pete

BTW... welcome to the forum

 

[Cavie]

I have what is probably a stupid question. I typically do commercial wiring and I currently am roughing in a House. It has a furnace with dual 60A breakers. I looked up the NM cable which I know is 60deg. and it looks like I would have to use a 4/2 NM cable or off the 90deg. Al for SE cable I would have to use a 4/4/8. The whole sale houses do not stock either of these which makes me think that I am possibly doing something wrong. What do you guys do for a install like this, any tips? Thanks for your help.

Sounds like you have 2 heat strips. 20 kw. Most likely two 9.6kw strips. Run one wire from the panel to the area of the air handler to handle the total load and hit a 4 ckt panel then feed the two strips.

 

[KWH]

Wire Size

Wire Size

Check the nameplate as stated some of these units are sent with 60amp breakers whether that is the correct size or not usually with a label stating for disconnecting means only.

 

[augie47]

As KWH states, don't let the 60 amp breakers fool you. All the the units installed locally which use two circuits come with (2) 60 amp breakers regardless of the size of the heat strips.
I've not run the numbers, but 99% of the units I inspect are fed by two separate home runs so it makes me think that may be more economical than a single circuit.
Up until the '08 change to 60? for SE, the majority of the units I saw were suppled by a #6 and a #10 (they were 15 heat).
That still works for some as the MCA is 55 or less, but the ones with the bigger blowers ofter exceed 55 MCA and a #4 is needed.