Conductor Sizing Question

[vhemp30]

Hey all, I have a disagreement with a classmate on this question. I believe the answer is 10awg. I am told I am wrong and the answer is 8awg.

I see this as 10awg because Table 310.15(B)(16) in the 75 degree column says 10awg THHN/THWN is rated for 35 amps and the (**) informational note in 240.4(D) says "unless specified in (E) or (G), the overcurrent for 10awg copper has to be 30 amp rated overcurrent". So my wire can be 10awg copper and my over current can be 35 amps. There is overload protection in AC unit motors so the overcurrent only needs to protect against short-curcuit and ground-fault which a 35 amp OCPD will do.

The other side is saying, it has to be 8awg because 240.4(D) is saying 10awg copper can only be on a 30 amp breaker.

Am I missing something???? ***Question Below***

What size THHN/THWN conductor is required for an air-conditioning unit if the nameplate requires a conductor ampacity of 34A? The terminals of all the equipment and circuit breakers are rated 75 degrees C and the raceway is installed outdoors in a wet location?

A. 14awg B. 12awg C. 10awg D. 8awg

 

[infinity]

You're correct the answer is C. #10 AWG. There are exceptions to 240.4 (D).

Welcome to the forum.

 

[texie]

Hey all, I have a disagreement with a classmate on this question. I believe the answer is 10awg. I am told I am wrong and the answer is 8awg.

I see this as 10awg because Table 310.15(B)(16) in the 75 degree column says 10awg THHN/THWN is rated for 35 amps and the (**) informational note in 240.4(D) says "unless specified in (E) or (G), the overcurrent for 10awg copper has to be 30 amp rated overcurrent". So my wire can be 10awg copper and my over current can be 35 amps. There is overload protection in AC unit motors so the overcurrent only needs to protect against short-curcuit and ground-fault which a 35 amp OCPD will do.

The other side is saying, it has to be 8awg because 240.4(D) is saying 10awg copper can only be on a 30 amp breaker.

Am I missing something???? ***Question Below***

What size THHN/THWN conductor is required for an air-conditioning unit if the nameplate requires a conductor ampacity of 34A? The terminals of all the equipment and circuit breakers are rated 75 degrees C and the raceway is installed outdoors in a wet location?

A. 14awg B. 12awg C. 10awg D. 8awg

As infinity said, you are correct. But I get the impression that you may think the max OCPD would be 35 amp in this case?

 

[LarryFine]

Now, to clarify, you would not be able to use #10 NM, because of its temperature rating limitations.

As Texie touched on, the breaker is sized to the MOCP, which could be well over 35a.

 

[vhemp30]

Correct. #10 NM you cannot use. The question states the specific wire it is asking for THHN/THWN.

And yes, 440.22(A) says 175% of the load/current for allowable MOCP = 59.5

 

[LarryFine]

And yes, 440.22(A) says 175% of the load/current for allowable MOCP = 59.5

Unless specified by manufacturer, as most, if not all, now are.

 

[GoldDigger]

Now, to clarify, you would not be able to use #10 NM, because of its temperature rating limitations.

As Texie touched on, the breaker is sized to the MOCP, which could be well over 35a.

Right. Unlike THWN-2, THHN/THWN has a different maximum temperature rating for wet use. Note also that if the wire is outdoors in a raceway, the inside of that raceway is also by rule a wet location, no matter how well it is sealed.

 

[LarryFine]

Isn't it great the way we whittle down each others' posts until there's absolutely no doubt as to the answer.

 

[Beaches EE]

Back to the OP, when wiring air conditioning units and especially the condensing unit / heat pump it's necessary to follow the nameplate. The important points are minimum circuit ampacity (will depend on the wiring method used), maximum overcurrent protection and whether or not fuses are required or optional with a breaker for overcurrent protection.