Page 1 of 3 123 LastLast
Results 1 to 10 of 30

Thread: conductor sizing vs. breaker sizing

  1. #1

    conductor sizing vs. breaker sizing

    I've always been taught that my conductor size couldn't be smaller than what the breaker is sized for. We have an HVAC distributer who is telling my employees differant.

    Example: Heat pump outdoor condensor section
    min. brch. cir. ampacity 24
    min. and max circuit breaker protection 40 amps. per nameplate.

    They say we can install the 40 amp breaker on existing circuit that had a #10 NM-B romex wire. I was always thought you couldn't put larger than a 30 amp breaker on this wire.

    Which way is correct?

  2. #2
    Join Date
    Feb 2006
    Location
    Lake of the Ozarks
    Posts
    2,875
    starting load for A/C those sneaky devils
    Last edited by Rewire; 10-26-07 at 04:59 PM.

  3. #3
    Join Date
    Aug 2007
    Location
    Broward Co.
    Posts
    361
    He is correct. 240.4(G). The #10 on a 30A breaker rule 240.4(D) applies to branch circuits, and other circuits not listed in 240.4(G). In Air Conditioning and Motors etc etc. the 'standard' sizing rules don't apply.
    See Art. 440 for more specifics on conductor and OCPD for A/C.

  4. #4
    Join Date
    Feb 2005
    Location
    NJ
    Posts
    9,324
    Quote Originally Posted by Donald Sherman
    I've always been taught that my conductor size couldn't be smaller than what the breaker is sized for. We have an HVAC distributer who is telling my employees differant.

    Example: Heat pump outdoor condensor section
    min. brch. cir. ampacity 24
    min. and max circuit breaker protection 40 amps. per nameplate.

    They say we can install the 40 amp breaker on existing circuit that had a #10 NM-B romex wire. I was always thought you couldn't put larger than a 30 amp breaker on this wire.

    Which way is correct?
    The HVAC guy.

    210.2 directs the user to 440.6, 440.31, 440.32

    Give those 3 sections an eyeballing....after you've read them, we can discuss this further.
    ĦĦĦĦĦĦĦĦĦĦĦĦ
    ««« CELTIC »»»
    ĦĦĦĦĦĦĦĦĦĦĦĦ
    An error on your part does not constitute an emergency on my part.8-)


  5. #5
    Join Date
    Mar 2006
    Posts
    4,016
    He is right, the breaker is providing short circuit and ground fault protection.

    Test your knowledge with this question: What size conductor and protection device must you use for a 24A motor-compressor on a 240V circuit (Fig. 3)


    Step 1: Size the branch-circuit conductor per Table 310.16 and 440.32.
    24A x 1.25 = 30A Per 110.14(C)(1)(a) and Table 310.16, you would select a 10 AWG conductor, rated 30A at 60°C.


    Step 2: Size the branch-circuit protection device per 240.6(A) and 440.22(A).
    24A x 1.75 = 42A Selecting the next size down protection device yields a 40A device.

    If a 40A protection device isn’t capable of carrying the starting current, you can size the protection device up to 225% of the equipment load current rating (24A x 2.25 = 54A, next size down 50A).

    Conductor sizing. Size these branch-circuit conductors no smaller than the spec on the equipment nameplate. If the equipment doesn’t have a nameplate that specifies the branch-circuit conductors, size the conductors per 440.32.
    http://ecmweb.com/ar/keep_your_cool/
    "America will never be destroyed from the outside.
    If we falter and lose our freedoms,
    it will be because we destroyed ourselves."

    Abraham Lincoln

  6. #6
    Join Date
    Dec 2006
    Location
    Chapel Hill, NC
    Posts
    26,576
    Quote Originally Posted by Donald Sherman
    I've always been taught that my conductor size couldn't be smaller than what the breaker is sized for. We have an HVAC distributer who is telling my employees differant.
    The HVAC guy is correct. You can have a number 10 wire and still use a 40 amp breaker on it.

    If you look on the nameplate of the heat pump it will tell you what the minimum circuit ampacity can be. It also states the max overcurrect protection. Usually it is about 175% (I believe) of the minimum circuit ampacity.

    Thus 24 amps times 175% will give you 42 amps. So you can put a 40 amp breaker on it.

  7. #7
    Join Date
    Sep 2004
    Posts
    1,479
    And he could really scare you by using #12 wire. Even at 60C, #12 copper has an ampacity of 25 which is higher than the compressor 24MCA value. The 20A breaker on a #12 goes out the window on motor, HVAC, welder, and a few other types of circuits.
    Mark
    Kent, WA

  8. #8
    With the Articles you all have had me look up I see where breaker can be sized differently than the conductor for the condensor. I see why I'm just an hvac guy. One question though. The table refered to for conductor sizing it was stated to use the 60deg C. column. The table says this is based on 30deg C. with a correction table below it for other temperatures. Alot of our applications have this cable running across an attic to get to where the condensor is. In southside Virginia where we are these attics can get upward of 120deg F. How much bearing does this have on us being able to use this same #10 conductor in this application?
    I think all of you for the help here. I don't want to arm my employees with bad information.

  9. #9
    Join Date
    May 2005
    Location
    Richmond, Virginia
    Posts
    21,838
    Quote Originally Posted by M. D.
    24A x 1.25 = 30A
    Quote Originally Posted by suemarkp
    And he could really scare you by using #12 wire. Even at 60C, #12 copper has an ampacity of 25 which is higher than the compressor 24MCA value.
    I agree with Sue. From what I understand, you needn't add the 25%. #12 would suffice for a 24a MCA. The 20a number doesn't apply in Art. 440 at all.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  10. #10
    Join Date
    Mar 2006
    Posts
    4,016
    Larry , I think in the example I posted it was ,if the nameplate did not contain the circuit information.
    You are correct when the info is there they have already done the calculation , no need to do it again.
    Here is a visual from the link.



    Last edited by M. D.; 10-27-07 at 06:53 AM.
    "America will never be destroyed from the outside.
    If we falter and lose our freedoms,
    it will be because we destroyed ourselves."

    Abraham Lincoln

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •