AC Unit conductor ampacity and over current protection.

[Greg1707]

This month's Electrical Contractor magazine has a question in the CODE FAQS that caught my eye.

AC unit with the following name plate:

MINIMUM CIRCUIT AMPACITY 21.6 AMPS
MAXIMUM OVERCURRENT PROTECTION 40 AMPS

The author states that #10 NMC cannot be used. Number #8 would be required.
Page 38 for those interested.

 

[Dennis Alwon]

The author is incorrect, IMO....assuming the ac unit has overload protection-- they all do at that size anyway. If it is in conduit @ 75C then #12 could be used.

 

[LarryFine]

The author is wrong. #10 is suitable in all cases, #12 in some.

 

[infinity]

I agree with Dennis and Larry. NM cable #10 minimum conductor size some other wiring methods like MC cable #12 is fine with the 40 amp OCPD.

 

[kec]

Interesting, I also read the same article this morning and had to take a second look at what he was stating. I agree #10 is fine in this application and have been doing so for manys years.

 

[texie]

Well, I read the piece in question and can't seem to understand what he is saying. On one hand he is saying that #10 is undersized and needs to be #8 which I think we all agree is wrong. But the he is saying that NM can't outside which we also probably all agree with. But he seems to be implying that if it is #8 NM it is OK.
I think this is probably a misunderstanding/loss in translation situation as I have met Mr. Dollard and seen him present on various topics and he is very skilled.
To me the correct answer to the questioner would be that #10 NM is correct for the indoor portion but the outdoor portion would have to be not NM of any size.

 

[LarryFine]

To me the correct answer to the questioner would be that #10 NM is correct for the indoor portion but the outdoor portion would have to be not NM of any size.

You are indeed correct. #10 NM inside can supply the disco, and #12 W-rated single conductors can be in the whip.

 

[texie]

You are indeed correct. #10 NM inside can supply the disco, and #12 W-rated single conductors can be in the whip.

Imagine trying to tell the inspector that said you needed #8 and now you are telling him that in fact you could use #12 for the outside portion. Unfortunately we see way too much of this nonsense and in my view this is a growing problem of uninformed/poorly trained inspectors and tradesmen in general.
And as for the trade mags this is true as well. I've seen too many contributed articles and code Q and A that have clearly wrong info or need clarification. A handful of times I have over the years, when thought some were egregious enough, I have sent email to the authors for comment and have never even received a reply from any, let alone a correction or discussion.

 

[MAC702]

Whew. I've been doing it wrong all these years, too.

 

[LarryFine]

Start with Art. 240.4, then 240.4(D) followed by 240.4(G), which takes you to Art. 440, in particular, Part IV.

Tell them the MCA applies to the wires*, and the MOC applies to the breaker.

* The standard #14=15a, #12=20a, #10=30a limitations do not apply here.

 

[ActionDave]

Start with Art. 240.4, then 240.4(D) followed by 240.4(G), which takes you to Art. 440, in particular, Part IV.

Tell them the MCA applies to the wires*, and the MOC applies to the breaker.

* The standard #14=15a, #12=20a, #10=30a limitations do not apply here.

And then you get a moment of silence followed up with, "No, that's not right"

 

[ActionDave]

Imagine trying to tell the inspector that said you needed #8 and now you are telling him that in fact you could use #12 for the outside portion. Unfortunately we see way too much of this nonsense and in my view this is a growing problem of uninformed/poorly trained inspectors and tradesmen in general......

Drives me nuts. The cost to get a license has gone up radically, we've got all this mandatory continuing education and I'm still stuck arguing with inspectors and other electricians who don't know how to do their job.

 

[don_resqcapt19]

Start with Art. 240.4, then 240.4(D) followed by 240.4(G), which takes you to Art. 440, in particular, Part IV.

Tell them the MCA applies to the wires*, and the MOC applies to the breaker.

* The standard #14=15a, #12=20a, #10=30a limitations do not apply here.

Table 240.4(G) only refers you to Parts III and VI of Article 440. Without a reference in Table 240.4(G) to Part IV, the answer in the Electrical Contractor magazine is technically correct. However that is not how the rule is normally applied.

 

[LarryFine]

Table 240.4(G) only refers you to Parts III and VI of Article 440. Without a reference in Table 240.4(G) to Part IV, the answer in the Electrical Contractor magazine is technically correct. However that is not how the rule is normally applied.

I didn't feel the need to go into detail once getting him to 440.

 

[don_resqcapt19]

I didn't feel the need to go into detail once getting him to 440.

But the code language in 240.4(G) does not mention Part IV, Branch Circuit Conductors, of Article 440, and without that reference, we can't do what we think we can do. Mr. Dollard is correct in his code answer based on the actual code language. I don't think that the actual code language matches the panel's intent for this iissue. I have put a PI to add a reference to Part IV of Article 400 in Table 240.4(G) on my list of 2023 PIs.

 

[Greg1707]

But the code language in 240.4(G) does not mention Part IV, Branch Circuit Conductors, of Article 440, and without that reference, we can't do what we think we can do. Mr. Dollard is correct in his code answer based on the actual code language. I don't think that the actual code language matches the panel's intent for this iissue. I have put a PI to add a reference to Part IV of Article 400 in Table 240.4(G) on my list of 2023 PIs.

Yes, but the name plate states MCA of 21.6. Why would a conductor more than 21.6 amps be required?

 

[Dennis Alwon]

Yes, but the name plate states MCA of 21.6. Why would a conductor more than 21.6 amps be required?

Because 240.4(D) doesn't allow a #10 on a 40 amp breaker

 

[LarryFine]

Because 240.4(D) doesn't allow a #10 on a 40 amp breaker

It does if you follow the exceptions for HVAC equipment.

 

[Michael15956]

But the code language in 240.4(G) does not mention Part IV, Branch Circuit Conductors, of Article 440,

That section, 240.4(G) reference Table 240.4(G) and the first listed item in that table is an air-conditioning unit and that item reference Article 440 part 3&4.

 

[kwired]

That section, 240.4(G) reference Table 240.4(G) and the first listed item in that table is an air-conditioning unit and that item reference Article 440 part 3&4.

That is what I am seeing as well, I thought maybe there was possibly a change that prompted that, but I see same thing in 2014, 2017 and 2020.