What Would Happen? Furnace Breaker... - Electrical - DIY Chatroom Home Improvement Forum

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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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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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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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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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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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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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