1000-watt appliance requires 6 AWG?!

[michaelbluejay]

If I'm reading NEC correctly, my mere 1000-watt mini splits will require whopping 6 AWG wire, for the legs from the attic junction boxes through the exterior conduit to the panel.

MCA (Minimum Circuit Ampacity) per condenser nameplate: 20A

Wire Ampacity (Table 310.16) for THWN/THWN-2:

12 AWG: 25 / 30A
10 AWG: 35 / 40A
8 AWG: 50 / 55A

Three 115V units, two 230V units = 12 conductors, 15 wires. Table 310.15(C)(1) says wire capacity is reduced by 50% for 10-20 conductors in raceway. So after adjustment for number of conductors:

12 AWG: 12.5 / 15A
10 AWG: 17.5 / 20A
8 AWG: 25 / 27.5A

Only about 2' of the wire will be in the a attic, but I don't see any exception in NEC for that, my reading is that if *any* portion of the wire is in the hot environment, I have to reduce the wire ampacity. So as per Table 310.15(B)(1)(1) at 134°F (Texas attic), reduce THHN by 0.58 factor, THWN-2 by 0.71 factor.

12 AWG: 7.250 / 10.65A
10 AWG: 10.15 / 14.2A
8 AWG: 14.50 / 19.53A


If that's true even 8 AWG doesn't get me up to 20A wire ampacity, unless I'm missing something. Am I missing something?

 

[Dennis Alwon]

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