Thread: Size of circuit breaker required

1. Junior Member
Join Date
Aug 2007
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4

Size of circuit breaker required

Can some one tell me what size circuit breaker is required for a 15 kw furnace, and where to find this in the code bock?

2. Senior Member
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Sep 2004
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1,497
See NEC 424.3. This is the chapter on fixed electric space heating. Newer units may have an MCA rating for the resistance heating elements. If no MCA value, the convert KW to amps at the supplied voltage and phase. If this is 240V single phase, then the amps would be 15000/240 = 62.5A. You are required to treat this as a continuous load, so ampacity requirement is 62.5*1.25 = 78.1A which means an 80A circuit breaker.

3. More typically, we supply 15Kw furnaces with a 60a circuit and a 30a circuit. If the unit has externally-operable built-in breakers, they will suffice as the local disconnects.

However, I suggest waiting for the unit, or at least detailed specs. Some can accept a single 80 or 90a supply, internally divided, or you can run a single 80/90a feeder to a small sub-panel.

I did the latter in a warehouse with the panel in the back and the office space up front. I opted for a single #2 al SE cable to a 4-space 125a panel, supplied by a 90a breaker.

I installed the 10Kw furnace/air handler's 60a breaker and the exterior AC compressor unit's 30a breaker in this panel. Since there were no 120v loads, I used SE cable, and not SER.

4. Senior Member
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Jul 2003
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Originally Posted by Pnkstephens
Can some one tell me what size circuit breaker is required for a 15 kw furnace, and where to find this in the code bock?
suemarkp told you how to calculate the amperage.

Any appliance with resistance heating elements rated over 48A must have the heating elements sub-divided (with few exceptions). 422.11(F)
This is what LarryFine was referencing (I think).

steve

5. Originally Posted by LarryFine
More typically, we supply 15Kw furnaces with a 60a circuit and a 30a circuit. If the unit has externally-operable built-in breakers, they will suffice as the local disconnects.

However, I suggest waiting for the unit, or at least detailed specs. Some can accept a single 80 or 90a supply, internally divided, or you can run a single 80/90a feeder to a small sub-panel....
Good advice to wait, since we got burned on one job:

HVAC guys told us in advance we needed 90 Amps for a 15 kW unit. I asked if we could install a 60 + 30 to do the job, and they said fine.

THEN, when they came out and installed the actual unit, they threw a curve. The unit required a 50 +40, NOT a 60 + 30! Grrrrr .... :mad:

We had to change out that #10 for a #8 after the fact, something that I did not like having to eat.

6. Originally Posted by kbsparky
We had to change out that #10 for a #8 after the fact, something that I did not like having to eat.
In my opinion, and despite my previous advice, I would not have eaten this cost. At least, I would have fought it vigorously by presenting the bill to the HVAC guy.

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