Electric Furnance Branch ciruit conductors

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Sonny Boy

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Just started my own contracting business and have a question for you 'experienced contractors.'

Connecting a residential 10 KW/240 Volt furnance and would like to know what type of wiring method you guys use. I was going to use 2 runs of 6/2 w ground NM-B. NEC ampacity table shows 55 amps for #6 NM-B. The branch circuit conductors need to be 60 amps based on 48 amps X 1.25. Can I still use the #6 NM-B?

The linear distance from the service panel to the breakers inside the furnace is 40 feet. I was going to staple the cable to the lower edges of the floor joists in the unfinished basement.

Thanks,
Mike
 
Are you supplying a 240 volt component with a 208 volt source? If not, then 10000 watts divided by 240 volts is 41.7 amps. 41.7 times 1.25 is 52. So a #6 should not be a problem.

Why ?two runs? of 6/2?
 
My Bad

My Bad

(Quote: Are you supplying a 240 volt component with a 208 volt source? If not, then 10000 watts divided by 240 volts is 41.7 amps. 41.7 times 1.25 is 52. So a #6 should not be a problem.Quote.)

Guess I better check my numbers before posting. Not supplying 208 to a 240 component.

The furnance contains 2 sets of 240 Volt/60 amp double-pole circuit breakers. So I need to install 2 runs of cable. I'm going to verify the KW rating before I buy the material.

If the adjustment factor is 55 amps or below, I can use 6/2 MN-B with the 60 amp branch circuit breakers at the service panel and the furnace circuit breakers? Based on Article 240.4 Protection of Conductors (B) Devices Rated 800 Amperes of Less (2).

Bear with me. I've only been a contractor for 2 months now. I'll be posting here to get help, insight, etc., from you guys.

Thanks,
Mike
 
Sonny Boy said:
.... The furnance contains 2 sets of 240 Volt/60 amp double-pole circuit breakers. So I need to install 2 runs of cable. I'm going to verify the KW rating before I buy the material....
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Be sure to verify those kW ratings. Many manufacturers install 60 Amp Square D QO"U" breakers, since those are readily available. Often times, there will be a 60 Amp unit breaker serving only a 5kW load, where a 30 Amp breaker would have been sufficient. You can install a #10 feeder to that portion of the unit if configured that way.

The other thing to look for is a common-feed lug kit, which may or may not be present on those electric furnaces. If there is one, you can use a single feeder cable for both unit breakers, although that option may be more expensive than running dual feeders from your source panel. Simply remove the lug kit if you want to use dual feeders.

In my experiences, most 10 kW units only require one 60 Amp feeder, a 15 kW unit will need a 60 and a 30 Amp feeder, and a 20 kW unit will need two 60 Amp circuits.
 
1-Check the heat strip label for model number.
2-Then reference model number label front of furnace for minimum circuit ampacity and maximum over-current.
3- Just because the furnace has to 60 amp breakers does not always mean that is the correct size for load served
 
Also be aware that the 60 amp "breakers" in the furnace unit may acually be 60 amp disconnects and not have the current interupting rating of a breaker. In the 5kw example of pulling one of these out and replacing it with a 30 amp breaker you'ld want to make sure you didn't put this in your truck stock as a 60 amp breaker.
 
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