How to wire this electric furnace

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Old 11-06-20, 02:53 PM
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How to wire this electric furnace

My brother in law has ditched his gas forced air furnace for electric. He got a massive solar array, and wants to get rid of all his gas appliances.

Anyway, he picked something out, and I'm a little confused how to wire it. He had told me it as a 14.4kw device, so I was going to run 4 gauge on a 60a breaker through conduit. When I got there, the heating element has TWO built-in breakers. One DP 50a and one DP 60a. I'm confused by this. Then I looked up the model which is a 6HK16501806. Looking it up here, it's listed as 18kw.

Reading the manual, it looks like there is a single point wiring bar (maybe then internally branches off to the two breakers?). But why two sets of breakers? And do I wire this with #2 gauge and a 100amp breaker? He does have a relatively sparse brand new 200a service. Is this heater overload? His house is 1300 sq ft, in the SF Bay Area where it almost never goes below freezing in the winter. Seems excessive.

Are the two breakers because of two elements? Can we "cap off" one element and just use either the smaller or larger one?

Thanks
 
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Old 11-06-20, 04:09 PM
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With that 17.3kw unit..... there are two breakers requiring two feeds.
It is too big to use the breaker shorting bars.
That unit requires roughly 72A @ 240v.

You could run #2 wiring and set a 100A panel with two breakers feeding the two internal breakers.
If you just want to run one element.... it would be the larger one as that is usually tied with the blower. You would require some low voltage wiring changes to lock out the second element.
 
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Old 11-08-20, 04:17 AM
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The NEC requires large heating loads to be broken down into smaller loads of no more than 48 amps.
 
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Old 11-08-20, 07:55 AM
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The NEC requires large heating loads to be broken down into smaller loads of no more than 48 amps.
I am not sure when this change to the NEC happened, but back in the day a 15 KW furnace might be either a 2 circuit (1-30A & 1-60A) or a 1 circuit (90 or 100A). A 20KW furnace was either a 2 circuit (2-60A) or a 1 circuit (1-125A) and a 25 KW was generally always a 3 circuit (2-60A & 1-30A). I am a little surprised to see electric furnaces listed in the odd KW ratings, the common typical ratings used to be 10, 15, 20 and 25 KW from all manufacturers.

I seem to recall that Carrier generally produced single circuit furnaces and Lennox furnaces were generally 2 circuits. Been a very very long time since I wired an electric furnace.
 
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Old 11-08-20, 12:29 PM
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Been in the NEC as long as I can remember.
 
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Old 11-08-20, 02:46 PM
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That doesn't eliminate a sub panel.....

 
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Old 11-10-20, 11:56 AM
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Ok, thanks of the help. So I use a 100amp breaker, 2 gauge to a subpanel and then do I put a 50a and 60a breaker in that subpanel to match the same on the furnace? The furnace doesn't require a neutral. Can I just run red and black to the sub, or should I have a neutral on the sub? Also, I was surprised that there isn't a neutral buss bar in the main. Apparently the conduit is the ground which I have never seen before. All of this recently got inspected. Do I run a ground bonded to the sub and maybe a locknut bond ring at the main? I will be using conduit, but wouldn't mind using NM.
 
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Old 11-14-20, 06:05 PM
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So I use a 100amp breaker, 2 gauge to a subpanel and then do I put a 50a and 60a breaker in that subpanel to match the same on the furnace?
Yes, feed a subpanel with a 100 amp breaker with #3s, but ONLY IF the furnace is 14.4 KW OR not over 15 KW. I'd wait till I could inspect the furnace personally before making a final plan and purchasing materials. Typically for estimating purposes you can figure 30 amps for each 5 KW. I am puzzled why a 14.4 KW furnace would need a 60 amp and a 50 amp circuit. That sounds more like an 18 KW unit.
 
 

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