Derating Ampacities in FMC

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John Lombardo

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Location
Laguna Beach, CA
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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

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