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!
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.
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.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013
Thanks for all the help. I will sleep better now.