AC Unit conductor ampacity and over current protection.

[Dennis Alwon]

There has to be something missing it's not possible that the collective users of the NEC have been getting this wrong for many decades. I wonder what Mike Holt would say about this, after all many of us have learned from him how to properly size the AC conductors.

I have no doubt that what we are doing is safe and perhaps also the thought of the cmp. I see what Don is getting at but I am not certain that it matters, as I sorta mentioned above.

The fact that this writer in the mag stated his view makes me think that others see it as an issue also.

To be honest when I read the writers response I didn't think he was saying it was not compliant and the first part of his answer seems to not be applicable.

Here is his comments

Conductor protection
We are having an issue with the AHJ over a residential A/C compressor. The inspector is saying we cannot use a 40-amp (A) OCPD on No. 10 (NM) Romex. The compressor nameplate states the minimum circuit ampacity is 21.6A, with a maximum fuse or breaker size of 40A. We believe we are well within the NEC. Can you please help?
The electrical inspector is correct. In general, a 10 AWG copper conductor requires overcurrent protection that does not exceed 30A. As seen in your question, the nameplate on the compressor permits overcurrent protection not to exceed 40A. Using type NM cable, this would require an 8 AWG copper conductor that is rated at 40A. The general rule for the protection of conductors in Section 240.4 requires protection in accordance with the conductor ampacity in Section 310.15, unless otherwise permitted in 240.4(A) through (G). Section 240.4(D) provides requirements for small conductors. The maximum overcurrent protection for 10 AWG copper is 30A unless 240.4(E) or (G) apply. 240.4(E) provides a list of six tap conductor requirements that do not apply in this scenario. Section 240.4(G) provides a table for the Code user to reference requirements in other articles for specific conductor applications.

Requirements in Article 440 are referenced but do not apply in this scenario. Residential air conditioning compressors are located outdoors. Type NM cable is not permitted to be installed outdoors. Section 334.12(B)(4) prohibits type NM cable installation in damp or wet locations. See Section 300.9, which clarifies that raceways installed outdoors above grade are considered to be a wet location. This means that type NM cable cannot be installed in a raceway to a compressor installed outdoors.

 

[Michael15956]

240.4(G) permits you to protect a conductor for A/C or refrigeration equipment at other than its ampacity.

David, where are you seeing these words?

 

[Michael15956]

Part IV doesn't give any direction on overcurrent sizing.

440.6(A) Exception #1 rarely applies.

I think it does, it states the nameplate shall be used. As in the OP 21.6 amps, and max 40 amp breaker. #10 cu on a 40 amp breaker.

 

[david luchini]

David, where are you seeing these words?

240.4 Conductors...shall be protected against overcurrent in accordance with their ampacities specified in 310.15, unless otherwise permitted...in 240.4(G).

240.4(G) Overcurrent protection for the specific conductors shall be permitted to be provided as referenced in Table 240.4(G).

I think it does, it states the nameplate shall be used. As in the OP 21.6 amps, and max 40 amp breaker. #10 cu on a 40 amp breaker.

You will have to point out the part of Art 440 Part IV that references overcurrent protection. I've read the section several times...there is nothing in there about overcurrent protection.

 

[kwired]

240.4 Conductors...shall be protected against overcurrent in accordance with their ampacities specified in 310.15, unless otherwise permitted...in 240.4(G).

240.4(G) Overcurrent protection for the specific conductors shall be permitted to be provided as referenced in Table 240.4(G).



You will have to point out the part of Art 440 Part IV that references overcurrent protection. I've read the section several times...there is nothing in there about overcurrent protection.

Ultimately those conductors are still protected from overcurrent, but it is by the motor overload and not the branch circuit device. Same with art 430 applications. The branch circuit device is only providing short circuit and ground fault protection.

 

[Michael15956]

You will have to point out the part of Art 440 Part IV that references overcurrent protection. I've read the section several times...there is nothing in there about overcurrent protection.

440.6(A) "and the separate motor overload protection." 440.31 reference this section.

 

[david luchini]

440.6(A) "and the separate motor overload protection." 440.31 reference this section.

440.6(A) is in Part I of Article 440, not Part IV. But I'm still not sure what you are getting at. 440.6 is titled Ampacity and Rating. There is nothing in that section that tells you how to size the overcurrent protection for the conductors. That is in Parts III and VI.

440.6 provides direction of what current rating (the rated load current of the compressor) shall be used in determining the conductor sizing in Part IV and the overcurrent protection sizing in Parts III and VI.