Breaker/wiring for gas furnace


Old 10-05-17, 10:53 AM
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Breaker/wiring for gas furnace

I was just checking around my gas furnace (converted from Oil years ago), and was surprised to learn that it's using both 14-2 and 12-2 wires. It's on a dedicated circuit, on a 20a breaker, running 12-2 wire to the shut off junction box, where it then is a short 14-2 wire to the furnace.
I assumed it would be 12-2 all the way as it should be. At this point, should I be replacing that short 14-2 wire with 12-2, or leaving everything as is, and correcting the breaker with a 15a than the 20?
The furnace runs great and never had issues, just concerned that a smaller wire size is on a 20A circuit.
Just replacing the breaker with 15a seems easiest route but Im more comfortable with 20a dedicated circuit for my furnace..
The plate on the furnace says "Min. circuit ampacity is 12.0 AMPS, max fuse 15 AMPS"
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Old 10-05-17, 11:09 AM
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Most furnaces use around 4-6 amps so a 15A circuit has plenty of spare capacity.

Your furnace is spec'ing a 15A circuit size.Technically that means there should be a 15A fuse where the furnace connects.

The only problem with putting a 15A breaker on #12 wire is that someone can increase it back to 20A based on the wire size. You could put a label on the wire inside the panel to keep the wire on a 15A breaker.
Old 10-05-17, 11:33 AM
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Since the furnace calls for 15 amp max you need to change the breaker .
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