Derating Ampacities in FMC

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John Lombardo

New User
Laguna Beach, CA
I will be running circuits for new Air Conditioning. One for the compressor and one to the air handler.

The calculate as such:

Compressor: #12 wire 75 degrees with 25A breaker MCA 22.7 MOP 25A
Air handler: #14 wire with 15A breaker MCA 6.5 MOP 15A

My knockouts in the main electrical panel are limited. I would like to run all the conductors out the panel in FMC to a junction box and then split out the two circuits. There will be 4 current-carrying conductors in the ¾’ FMC (plus two ground conductors) from the panel to the junction box(10-12 feet). Do I need to derate the ampacities of the conductors to do this?

If the #12 need to be derated, the #12 wire could carry: 25 * .8 = 20A The MCA is 22.7 A. Do I need to up the wire to #10?

The #14 can carry 20A. Derated 20*.8 = 16 amps MCA is 6.5A OK?

After the first junction box (+- 10 feet), there will only be 2 current carrying conductors in a single conduit. Should I keep the smae wire size from the outcome above. From the junction box to the equipment is only another 15' max.

Thanks for your help.

Dennis Alwon

Staff member
Chapel Hill, NC
Electrical Contractor
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