HVAC min. circuit amp

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acrwc10

acrwc10

Master Code Professional
Location
CA
Occupation
Building inspector
So here is a interesting one IMO.
AC unit nameplate says "minimum circuit amp. 20.5" could you use a #12 or is #10 wire size the min. to feed the unit? I see the 1/2 amp as requiring the jump to the larger size wire, is there any code section I may be over looking that would allow the #12 ?
 
You can use a #12 for this application.

The limitations of 240.4(D) do not apply here, since the first sentence in that section states: "Unless specifically permitted in ... 240.4(G) ....

SO you go to 240.4(G) and find that it refers you to article 440, parts III and IV.

Part IV of article 440 applies to branch circuit conductors, which refers you to section 440.6(A). This section tells you to use table 310.16 through 310.19 to select your circuit conductors.

From table 310.16 a #12 conductor is allowed to carry 25 Amps, which is sufficient for a nameplate rating of 20.5 Amps. :grin:
 
I agree, the 60 and 75 degree C rating for #12 is 25 amps. The limiting factor may be the type of wiring method used. Is this case it wouldn't matter but in other cases the use of SE or NM would limit you to the 60 degree C condcutor ampacity which may require a larger sized cable.
 
continuing along arcwc10's line of thought....
what would you do if the MCA on the nameplate was 25.5 ? how about 25.1 ??
 
augie47 said:
continuing along arcwc10's line of thought....
what would you do if the MCA on the nameplate was 25.5 ? how about 25.1 ??
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I believe that there is a provision to eliminate the fraction portion of the resultant of a calculation (searching for section #) but I'm not sure that would apply here since the MCA is not calculated by the end user.
 
Without looking at a code book someone please tell me what ?minimum? means?

Then if the minimum is .1 then we can?t go below this .1 and must round up.
 
jwelectric said:
Without looking at a code book someone please tell me what ?minimum? means?

Then if the minimum is .1 then we can?t go below this .1 and must round up.
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I disagree, look at article 220.5(B)
(B) Fractions of an Ampere. Where calculations result in a fraction of an ampere that is less than 0.5, such fractions shall be permitted to be dropped.

I am interested in hearing your oppinion on this.
 
I was interested in opinons, also. For what it's worth mine is that 220 is for "calculations". In actually sizing the circuit, I don't think you should drop any fration..but again..my OPINION.
 
augie47 said:
continuing along arcwc10's line of thought....
what would you do if the MCA on the nameplate was 25.5 ? how about 25.1 ??
Click to expand...



For calculation of loads, as per 220.5(B) one can follow this section number.

For protection of conductors, one must follow 240.4


Augie
To answer your question, which is a good question, I would determine that the conductor that is terminated in a circuit that has any or all of the components rated at a 75C rating, 25 amps would be the maximum permitted ampacity for the 12AWG in regards to the A/C question. 25.1 amps is the deal breaker.
 
220.5(B) Fractions of an Ampere. Where calculations result in a fraction of an ampere that is less than 0.5, such fractions shall be permitted to be dropped.
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That's the one I was thinking of, but I agree with others since this isn't a calculation you can't disregard the fractional portion of the ampacity. therefore 25.1 amp MCA can't use a 25 amp conductor.
 
kbsparky said:
You can use a #12 for this application.

The limitations of 240.4(D) do not apply here, since the first sentence in that section states: "Unless specifically permitted in ... 240.4(G) ....

SO you go to 240.4(G) and find that it refers you to article 440, parts III and IV.

Part IV of article 440 applies to branch circuit conductors, which refers you to section 440.6(A). This section tells you to use table 310.16 through 310.19 to select your circuit conductors.

From table 310.16 a #12 conductor is allowed to carry 25 Amps, which is sufficient for a nameplate rating of 20.5 Amps. :grin:
Click to expand...

Maybe I missed it but where does it have an exception to ( 240.4(D)(5) 12 AWG Copper. 20 amperes )
 
Well I just did the math, and the .5 makes the difference. #12 has a max rating of 25 amps in table 310.16. When you multiply the name plate 20.5 amp by 125% you get 25.625 amps , with that you have to go to a #10 min. conductor. So even though the OCP could be rated much higher the conductor has to be able to carry the 125% continuous load. Or can I disregard the 125% because the name plate gives me a min. circuit amp. ?
 
jxofaltrds said:
Please go to the link that I provided above. AND read it.
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If you are referencing me it would be helpful to quote who you are addressing. I have read the link and I was not sure where you were going with that and I didn't think it answered the question stated in the post above the link. I gave a code reference. Do you disagree with this. We are not talking about OCP but rather the load on a #12
 
acrwc10 said:
Well I just did the math, and the .5 makes the difference. #12 has a max rating of 25 amps in table 310.16. When you multiply the name plate 20.5 amp by 125% you get 25.625 amps , with that you have to go to a #10 min. conductor. So even though the OCP could be rated much higher the conductor has to be able to carry the 125% continuous load. Or can I disregard the 125% because the name plate gives me a min. circuit amp. ?
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Why are you multiplying by 125%? This factor is already included in the MCA value. Now "nameplate" is a bad term when parts of the code book tell you to multiply it by 125%, because there can be many terms on a nameplate -- LRA, RLA, FLA, MCA.
 
suemarkp said:
Why are you multiplying by 125%? This factor is already included in the MCA value. Now "nameplate" is a bad term when parts of the code book tell you to multiply it by 125%, because there can be many terms on a nameplate -- LRA, RLA, FLA, MCA.
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Yup, the MCA already continuous the additional 25% so no further calculation is required. Take the MCA, then look up the mimimum conductor size in 310.16 and you're good to go. The Max OCPD can be any size between the minimum and the Max OCPD listed on the nameplate.
 
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