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Old 01-23-2013, 12:22 AM   #1
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What would happen? Furnace Breaker...


Hey All...

Doing my calculations for my transfer switch, figuring out what circuits I want to use on generator power. I came across something that Im not sure if its an issue or not in my existing wiring.

My house was new construction back in 2010, and everything inspected.

My furnace, a small 2 tons system. The brochure I found for it online, states that its a minimum 8.1 amps, and a max fuse of 15 amps.

My questions is... the electrician wired the furnace circuit with 12ga wire and a 20amp CB. Since the furnace called for a max of 15amp, will this be a problem. Its been like this for about 2 and a half years with no problems.

I was thinking of just changing the existing 20 amp CB to a 15 and see what happens, since its 12ga wire, using a 15amp CB shouldnt be any issue, I mean I can always go back.

Thanks !

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Old 01-23-2013, 12:24 AM   #2
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What would happen? Furnace Breaker...


Oh.. this is a natural gas furnace, in case that matters.

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Old 01-23-2013, 05:09 AM   #3
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What would happen? Furnace Breaker...


Change it to a 15 amp breaker. The circuit breaker size is determined by the manufacturer specs,
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Old 01-23-2013, 01:59 PM   #4
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What would happen? Furnace Breaker...


It is best to use manufacturer recommended breaker sizes for dedicated equipment circuits and not larger. Then if there is an equipment malfunction, the smaller breaker would trip sooner - possibly preventing damage to the equipment.

So say the squirrel cage bearings started to go out and the motor had a difficult time turning the fan - used more amperage. A 15 amp breaker would trip sooner than a 20 amp breaker and possibly keep the motor from burning up.
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Old 01-23-2013, 10:17 PM   #5
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What would happen? Furnace Breaker...


From what I understand, to be UL listed appliances need to have their own internal protection -- the breaker is to protect the wire, not the appliance. The blower motor, for example, should have an internal thermal cutoff, and the furnace control board typically has a little 3A automotive style blade fuse.

That said, the NEC requires you to follow manufacture's instructions. So if the furnace says 15A, you use 15A. I'd swap out the breaker.

I wouldn't necessarily blame the electrician -- he probably roughed in the circuit before the furnace arrived, so he had no way to know. If anything he did you a favor, in case you install a 20A furnace at some point in the future.
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Old 01-24-2013, 07:44 AM   #6
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What would happen? Furnace Breaker...


Thanks guys.... When I go to move the furnace circuit over to the transfer switch I will use a 15amp instead.

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