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Thread: HVAC min. circuit amp

  1. #1
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    HVAC min. circuit amp

    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 ?

  2. #2
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    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:

  3. #3
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    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.
    Rob

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    All responses based on the 2011 NEC unless otherwise noted

  4. #4
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    continuing along arcwc10's line of thought....
    what would you do if the MCA on the nameplate was 25.5 ? how about 25.1 ??
    At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

  5. #5
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    Quote Originally Posted by augie47 View Post
    continuing along arcwc10's line of thought....
    what would you do if the MCA on the nameplate was 25.5 ? how about 25.1 ??

    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.
    Rob

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    All responses based on the 2011 NEC unless otherwise noted

  6. #6
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    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.
    Mike Whitt
    God answers Knee-Mail.

  7. #7
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    Quote Originally Posted by jwelectric View Post
    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.
    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.

  8. #8
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    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.
    At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

  9. #9
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    Quote Originally Posted by augie47 View Post
    continuing along arcwc10's line of thought....
    what would you do if the MCA on the nameplate was 25.5 ? how about 25.1 ??


    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.
    Instructor, Industry Advocate

  10. #10
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    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.
    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.
    Rob

    Chief Moderator

    All responses based on the 2011 NEC unless otherwise noted

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