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Thread: proper breaker and wire size for gas furnace

  1. #1
    DIY Member estone's Avatar
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    Default proper breaker and wire size for gas furnace

    I had a Heil gas furnace installed that will be primarily used for a/c but it does have a heat too, Model N8MSN0701716A1. I installed the electrical with a 20 amp breaker/ 12-2 wire as per HVAC installer instructions. After searching online electrical data it calls for a 15amp breaker/14-2 wire, max. unit amps 9.5. It also says max. wire length 29ft. I have the furnace installed in my attic so I'm running close to 40 ft. of wire from the basement panel to attic. I'm really sweating this and was hoping I'm ok with the 20amp breaker/ 12-2 wire. If what I went with is really a no no I guess I can rip it out and start from scratch but I was hoping I would be ok. I would appreciate your expertise. Thanks in advance!

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    Jack of all trades DonL's Avatar
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    Quote Originally Posted by estone View Post
    I had a Heil gas furnace installed that will be primarily used for a/c but it does have a heat too, Model N8MSN0701716A1. I installed the electrical with a 20 amp breaker/ 12-2 wire as per HVAC installer instructions. After searching online electrical data it calls for a 15amp breaker/14-2 wire, max. unit amps 9.5. It also says max. wire length 29ft. I have the furnace installed in my attic so I'm running close to 40 ft. of wire from the basement panel to attic. I'm really sweating this and was hoping I'm ok with the 20amp breaker/ 12-2 wire. If what I went with is really a no no I guess I can rip it out and start from scratch but I was hoping I would be ok. I would appreciate your expertise. Thanks in advance!

    The Wire is fine, And the breaker is fine also.

    Do not sweat it, You can install a 15 amp breaker if it would make you feel better.
    Last edited by DonL; 04-14-2013 at 07:57 PM.
    Theory only works perfect in a vacuum.

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    DIY Member estone's Avatar
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    Installing the 15amp breaker with the 12-2 is ok? I thought a 15amp breaker had to be 14-2 wire, 20amp/12-2, 30amp/10-2? Or is that just good practice?

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    Jack of all trades DonL's Avatar
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    Quote Originally Posted by estone View Post
    Installing the 15amp breaker with the 12-2 is ok? I thought a 15amp breaker had to be 14-2 wire, 20amp/12-2, 30amp/10-2? Or is that just good practice?

    The wire is larger and can carry more current / power then needed for your unit, and is better than 14 gauge.

    A 15 amp breaker would trip quicker if your unit had a fault.

    The breaker is to protect the wire from over heating, and your wire is better than needed for 15 amps.
    Theory only works perfect in a vacuum.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    12g is the MINIMUM wire size for a 20A CB, but 12g, because it IS bigger, will have less resistance when supplying a lower current. So, based on the max length for 14g wire in the installation manual, the use of 12g is called for, and is a GOOD thing. I'd use the recommended CB size of 15A with it. CB is designed to primarily protect the wiring, not the device, so ANY CB 20A or less is good. Using a 15A is closer to the device's rating, and would also help protect the device in addition to the wiring.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Member estone's Avatar
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    Thanks for all the help. I will sleep better now.

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