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Old 02-20-2012, 12:07 PM   #1
blackdart66
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Default wire size for electric furnace

Hi all...

Little confused on what size wire to run for a 10KW electric furnace I am installing in the shop.

I have a 200 amp service in the shop.

HVAC guys who is doing the install said he needed a 60 amp breaker and #8 wire with 2 conductors and a ground. When I went to Home Depot, I see that wire is only rated for like 55 amps. Or maybe it was 50. anyway I know it was less than 60. I see I can get the THHN and it was like 65 amps. I would have to run conduit for the THHN which is easy to do as the interor is not finished yet but the romex style is a lot easier.

Advice ?
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Old 02-20-2012, 12:15 PM   #2
50cal
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Default Re: wire size for electric furnace

An HVAC guy is not a Electrician. I have seen them clamp a meter on a conductor and it only reads X draw so wire is big enough. Use the proper size wire for the rating of the unit your insurance company will appreciate it.
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Old 02-20-2012, 12:33 PM   #3
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Default Re: wire size for electric furnace

Here are a couple of links.
http://ecmweb.com/mag/electric_comme..._calculations/
http://www.rowand.net/Shop/Tech/WireCapacityChart.htm
http://www.altestore.com/howto/Solar...ur-System/a62/
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Old 02-20-2012, 12:37 PM   #4
PRH44
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Default Re: wire size for electric furnace

A 10 KW load at 240 volt will result in 41.67 amp load.
Your wire and breaker should be sized at 1.25% of 41.67A = 52A .
This would require a 60 amp breaker.
The following is based on copper conductors only.
this would require #6 wire, #8 is not large enough.
You could use a # 6 romex or SE cable this will net you 55 amps at 60 degree C.
Code allows you to round up to the next standard size breaker in this instance, which would be a 60 Amp.
A #6 THHN single conductors in a conduit will be 65 amps at 75 degree C.
Bottom line is you need # 6 wire on a 60 amp breaker.
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Old 02-20-2012, 12:38 PM   #5
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Default Re: wire size for electric furnace

When I did my electric heat I went to a local electric supply house to get my wire. They will have colors Red, Blue, Black, Green. Helps to avoid the confusion.
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Old 02-20-2012, 12:50 PM   #6
PRH44
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Default Re: wire size for electric furnace

Blackdart66. I based my information a 240 volt single phase feed. Is what you have?
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Old 02-20-2012, 03:33 PM   #7
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Default Re: wire size for electric furnace

yes....240 volt single phase.........

Thanks for the info !
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Old 02-20-2012, 05:21 PM   #8
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Default Re: wire size for electric furnace

What I do is look for a plate on the side of the unit that says:

MCA(minimum circuit ampacity=wire size)
MOCP(maximum overcurrent protective device=breaker size)

If for some reason I can't find that plate, then I'll use the breaker size in the unit to determine wire ampacity and breaker size.
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