A/C MCA sizing with NM cable

[dnbob]

I have read many posts on the topic and have seen both conclusions.

It seems that if using NM cable to size an A/C MCA, you are limited to the 60 degree column, regardless of 240.G, 440 etc.

Thoughts?

Thanks,

Bob

 

[Buck Parrish]

No, you use the 75 degree column. That allows a 25 Amp breaker. See here http://forums.mikeholt.com/showthread.php?t=190070

 

[david luchini]

I have read many posts on the topic and have seen both conclusions.

It seems that if using NM cable to size an A/C MCA, you are limited to the 60 degree column, regardless of 240.G, 440 etc.

Thoughts?

Thanks,

Bob

334.80 limits the allowable ampacity of NM to the 60 degree column.

 

[augie47]

334.80 limits the allowable ampacity of NM to the 60 degree column.

agree:
"334.80 Ampacity. The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15.The allowable ampacity shall not exceed that of a 60°C rated conductor"

 

[infinity]

334.80 limits the allowable ampacity of NM to the 60 degree column.

I agree also. If you want to use the 75 degree ampacity use MC cable.

 

[kwired]

No, you use the 75 degree column. That allows a 25 Amp breaker. See here http://forums.mikeholt.com/showthread.php?t=190070

Bigger issue that is in that thread is the small conductor rule in 240.4

With NM cable you must use 60C conductor ampacity whether small conductor rule applies to your application or not.

 

[don_resqcapt19]

Typically the AC is outside so you can't run NM to it.

 

[DrSparks]

Typically the AC is outside so you can't run NM to it.

You can run a nm to the disconnect that is attached to the side of the building.

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[DrSparks]

No, you use the 75 degree column. That allows a 25 Amp breaker. See here http://forums.mikeholt.com/showthread.php?t=190070

You need to run 10 gauge NM. Also you wouldn't use a circuit breaker as the nameplate calls for a Max fuse size of 30 amps not a Max overcurrent protection device or circuit breaker.

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[DrSparks]

Bigger issue that is in that thread is the small conductor rule in 240.4

With NM cable you must use 60C conductor ampacity whether small conductor rule applies to your application or not.

You can use the 90-degree column for applying adjustment factors but the final ampacity cannot succeed that of a 60-degree conductor

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