# Electric Furnace conductors



## mikeg_05 (Jan 1, 2009)

subelect said:


> We roughed in the wiring for a residential electric furnace today and the furnace nameplate data has me wondering. We brought in a single phase, 240 volt service.
> 
> The nameplate indicated the following for the 2 circuits:
> Min. ckt. ampacity: 28/24 and 42/37 (the label starts out 208/230 Volt, but I am pretty sure they have the amp values backward, so it should be 208/230 Volt).
> ...


well #8 is only good for 40 amps (8/2 NM-B). We were running 6/2 NM-B, because well thats what we were told to run even though its only good for 55 amps, the boss gives you the "just run it, we have always done it that way". But as always we get nailed. So we started running 6/2 USE cable it is good for 65 amps, but then I just went to a class and the inspector said that USE cable is now going to fall under the 60 degree column so were F'ed. Now we gotta jam a #4 into a 60 amp breaker. Anyways Since your under sizing the wire for the over current protection device, I would say your in violation.


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## electricista (Jan 11, 2009)

subelect said:


> The nameplate indicated the following for the 2 circuits:
> Min. ckt. ampacity: 28/24 and 42/37 (the label starts out 208/230 Volt, but I am pretty sure they have the amp values backward, so it should be 208/230 Volt).
> 
> KW: 11.3 and 15 KW
> ...


First off the KW's you list do not match the min cir. amp. A 15kw unit should have a min cir. amp of at least 78 amps. Are you sure the kw isn't something like 5kw and 10kw? Your numbers don't compute.


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## subelect (Nov 25, 2007)

No, I have not crunched the KW numbers yet. I had not brought any 6-2 or larger wire with us on this trip. I looked at the nameplate and wrote down the figures and got the 1" conduit run down from the rafters for the wiring.

Thanks for your input. I'll talk to the boss and figure out if he wants to run #4s.

Rick


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## chenley (Feb 20, 2007)

Wouldn't 10AWG on the 30A and 6AWG on the 60A amp work. Wired in a GeoThermal Unit a few months ago and inspector said it was fine. 

Also, maybe I'm forgetting something don't have my codebook with me, are HVAC units supposed to be sized for 125%? I've never seen one run continuously for three hours. I have always run conductors sized for the maximum breaker/fuse size on the nameplate.


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## MechanicalDVR (Dec 29, 2007)

chenley said:


> Wouldn't 10AWG on the 30A and 6AWG on the 60A amp work. Wired in a GeoThermal Unit a few months ago and inspector said it was fine.
> 
> Also, maybe I'm forgetting something don't have my codebook with me, are HVAC units supposed to be sized for 125%? I've never seen one run continuously for three hours. I have always run conductors sized for the maximum breaker/fuse size on the nameplate.


 
On the coldest design day an hvac unit would run continusly and due to lower outlet temps electric furnaces tend to run way more then fossil fuel units. I would say 10 awg for 30A and 4awg for 60A.


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## electricista (Jan 11, 2009)

chenley said:


> Wouldn't 10AWG on the 30A and 6AWG on the 60A amp work. Wired in a GeoThermal Unit a few months ago and inspector said it was fine.
> 
> Also, maybe I'm forgetting something don't have my codebook with me, are HVAC units supposed to be sized for 125%? I've never seen one run continuously for three hours. I have always run conductors sized for the maximum breaker/fuse size on the nameplate.


This could work but I have had some 10KW furnace that require a 57 amp min. cir. amp. Unfortunately if you run #6 SE or nm cable you have to use the 60C table and that's only good for 55 amps.


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## mikeg_05 (Jan 1, 2009)

electricista said:


> This could work but I have had some 10KW furnace that require a 57 amp min. cir. amp. Unfortunately if you run #6 SE or nm cable you have to use the 60C table and that's only good for 55 amps.


:yes: Does the furnace manufacture take the 125% continuous load into account when figuring out wire size and over current protection?


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## electricista (Jan 11, 2009)

mikeg_05 said:


> :yes: Does the furnace manufacture take the 125% continuous load into account when figuring out wire size and over current protection?



Yes. The 125% is figured in the min. cir. amp.


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## chenley (Feb 20, 2007)

electricista said:


> Yes. The 125% is figured in the min. cir. amp.


Was wondering that also after I left today. Also had realized that my heat may have run 3+ hours yesterday, considering we were in the middle of the ice storm and it was 37 in the house when we got power back. I definitely could go for quite awhile without hearing the word generator and seeing a meterbase and service mast ripped off a house.

Whelp off to bed have four more services to replace tomorrow.


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## randomkiller (Sep 28, 2007)

chenley said:


> Was wondering that also after I left today. Also had realized that my heat may have run 3+ hours yesterday, considering we were in the middle of the ice storm and it was 37 in the house when we got power back. I definitely could go for quite awhile without hearing the word generator and seeing a meter base and service mast ripped off a house.
> 
> Whelp off to bed have four more services to replace tomorrow.


There are a few factors in the length of time a furnace will run. But, I know what mechdvr was referring too. When doing a heat loss/gain spread sheet in our area most guys use 0F as the cold design day, when the unit would run full time to maintain 70F inside and 95F for heat gain conversely. I can't speak for your area, according to the class we had most southern places size by heat gain and take heat loss as a secondary thought.


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