60-amp electric furnace wire size?

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Old 06-03-09, 04:04 AM
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60-amp electric furnace wire size?

Hi,
I'm installing an electric forced-air furnace in my basement. The furnace is less than ten feet from the panel. The furnace requires a 60-amp breaker, and in fact it came with one pre-installed on the face of the unit to serve as the within-arm's-reach disconnect. I have a couple questions:

1. What size wire do I need to run for this unit? Most everything I read places 60 amps at about #6 but some sources claim #4 depending on a couple of variables. Which is it?

2. What size PVC conduit will this wire, plus a #12 neutral (for the condensate pump) fit in?

3. Do I need to install a second 60-amp breaker in the panel to make the termination or will the breaker mounted in the unit suffice, and I can hard-wire to the panel?

Thank you very much!
Darel Matthews
 
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Old 06-03-09, 08:23 AM
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You really need to look at the nameplate on the unit, find the corresponding model # and find the MCA rating(minimum circuit ampacity) and the MOCP rating(max over current protection).

This will determine your circuit size. The 60 amp "breaker" is just a means of disconnect and has no bearing on the size circuit to be run.

The circuit for the condensate pump will need to be a separate 120 volt circuit than the AHU. You can't tap one leg of the AHU and pull a neutral for the 120 volt.

Also, there needs to be a GFI receptacle within 20 or 25' (can't remember exactly) of the AHU.
 
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Old 06-03-09, 09:57 AM
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Originally Posted by Darel View Post
Most everything I read places 60 amps at about #6 but some sources claim #4 depending on a couple of variables. Which is it?
If you use conduit for the entire run and THHN conductors then #6 is okay. If you use NM-B cable, then #4 is required.

What size PVC conduit will this wire, plus a #12 neutral (for the condensate pump) fit in?
3/4" minimum will fit (2) #6 hots, (1) #10 ground, (1) #12 neutral.

However, you can't just run the #12 as a neutral unless the manufacturer's instructions on the furnace specifically call for that.

Do I need to install a second 60-amp breaker in the panel to make the termination
Yes, however the size of the breaker isn't necessarily 60A. That depends on the max OCPD rating on the furnace nameplate. Knowing the min ckt ampacity would help too if you have that information available.
 
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Old 06-03-09, 04:12 PM
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Thanks for the replies so far guys. MCA is 55A and MOCP is 60A.

Why can't I tap one leg for 120v? Just curious.

Also, the GFI - are you saying the GFI needs to be protecting the condensate pump circuit? Or there just has to be one? I have one on the board the panel is mounted on, I can pull the 120v for the condensate pump off of it.

I am going to use 3/4 EMT since that's what I'm most used to. Therefore, I can use #6 THHN for the 60A circuit, right? Plus #12 hot and neutral and #10 ground.

Thanks a lot, guys!
Darel
 

Last edited by Darel; 06-03-09 at 04:50 PM. Reason: typo
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Old 06-03-09, 06:06 PM
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In your case, #6 NM will be ok because your MCA is 55 and you can use a 60 amp breaker.

Reason you can't tap one leg of this circuit is because your going to have a 15-20 amp circuit/device protected with a 60 amp breaker.

As far as the GFI, no it's not to protect the condensate pump. It's just required as a general purpose receptacle for a technician. I would do as you say and branch off the load side GFI and let that serve 2 purposes

3/4 is only good for 3-6's so your fill of 2-6's,2-12's and a 10 may be overfill without doing all the cross sectionals for all that wire. I'll say just go to 1" or do 2 separate circuits.
 
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Old 06-03-09, 07:26 PM
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Great, thank you very much!

(He says smacking himself in the forehead for wanting to protect a 15A device with a 60A breaker).
 
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