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  1. #1
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    minimum circuit ampacity

    I know that you can only load a wire 80 %. For example #14 wire @ 12 amps
    Does the minimum circuit ampacity take into account the 80 %

    Can you use a #14 for a mca of 14.2?

  2. #2
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    Yes.
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  3. #3
    No you cannot...You can load up a wire to 100% but the CIRCUIT BREAKER can only take 80%. So for a MCA of 14.2 you will need a 20A circuit breaker which requires a #12 AWG conductor.

  4. #4
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    What are the conditions of use? Branch circuit? Free air? Bundled Cables? Conduit? Ambient temperature? Insulation rating? Under 600 volts?

    For example, NEC 210.19 says for branch circuits not more than 600 volts that minimum conductor size before any adjustments shall have ampacity not less than the noncontinuous load, plus 125 percent of the continuous load.

    Article 310 gives the ampacities for conductors of different insulations and temperatures. Tables for free air, conduit, buried,

    Article 440 sets the requirements for HVACR equipment.

    The original question needs to be conditioned.

  5. #5
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    Quote Originally Posted by Walleye82 View Post
    No you cannot...You can load up a wire to 100% but the CIRCUIT BREAKER can only take 80%. So for a MCA of 14.2 you will need a 20A circuit breaker which requires a #12 AWG conductor.
    Your wrong.
    MCA, is not the running load of the condenser. So yes you can have a 14 gauge wire and 15amp breaker on an MCA of 14.2amps.
    MCA is the min circuit allowed. Not the load on the circuit.

    MCA is calculated by, compressor RLA times 1.25 plus condenser fan motor RLA.

    The RLA of a compressor in a condenser with a MCA of 14amps, is about 8 or 9 amps.
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  6. #6
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    I agree that the mca for an outside unit may be calculated as mca=(compressor RLA X 1.25) + condenser fan motor RLA, or condenser fan motor FLA.

    The labeled branch circuit selection current could also be used.

    The original question was "Can you use a #14 for a mca of 14.2?" The answer is, it depends.

    Assuming that we are talking HVAC, then Art 440 is our starting point. It tells us to go to Art 310.15. Because the conditions were not stated, let's assume that the conductor is copper, is in conduit, not more than three conductors, with an ambient outside temperature of 100*F maximum, and not in direct sunlight.

    Since most building wire is rated for 90*C, we could use the 90*C insulation column of Table 310.16. However, the breakers in use today are only rated for 75*C. If we keep the breakers satisfied, then we will use the 75*C column. For Copper wire, 14 AWG, allowable current is 20A before derating. Derating to ambient 100*F gives an allowable current of 17.6A.

    If this is a branch circuit, then derating 125% moves the allowable current to 14.1A. If the circuit is a hermetic compressor, from Art 440 the derating is 115%, and the allowable current is 15.3A.

    The next condition is that this is a small conductor, assumed at less than 600V, which means that we go to Art 240.4(D), which says that the appropriate breaker for 14 AWG copper is fixed at 15 A.

    Under these conditions, a branch circuit would not carry 14.2A. A hermetic compressor would carry 14.2A. But in any case, the breaker is fixed at 15A.

    The technique here is that each link has to be checked, and the weakest link controls the ampacity. Otherwise, the conductor insulation or the breaker gets fried.

    In places like Phoenix, the breaker could need to be derated for high temperatures, dropping it below 15A.

    I have had breakers crumble in my hands from summer heat damage.
    Last edited by sparksandfan; 06-19-2008 at 11:03 PM. Reason: Correct cranial malfunction over temperature.

  7. #7
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    how about for a 80ft circuit?

    how about for July 20th when a/c running all day, or Jan20 when heat strip on all day [ at least over 3h ]?

    with cost of elec increasing, you are using 90C rating?
    -- [ C, not F ] at the SURFACE of the metal conductor.

    20*0.91= 18.2
    harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!

  8. #8
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    Right you are.The tables are presented in degrees C with degrees F in parentheses. I did manage to look at the right side of the correction factors table to get the degree F column.

    As you picked up on 3 hours, that is the defined duration for continuous. So a July 20th day should be covered by derating the continuous loads 125% and derating temperature at the peak temperature of the year. Heat strips and crankcase heaters also must be added to load.

    90*C rating for insulation on building conductors, unless it is old NM, then 60*C. NM-B is rated at 90*C. Of course if they are UF or THHW, or non-90*C, then those ratings would be used. The hiccup happens when the conductor goes into a breaker with 75*C rating. The breaker now becomes the weak link. The current then has to be adjusted to lower the heating at the connection. The correction factor for temperature is taken from the 75*C column for .88 and not .91 from the 90*C column in that case.

    The 80 foot circuit is a voltage drop consideration. The conductor may be sized larger to prevent excessive drop.

  9. #9
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    Quote Originally Posted by cem-bsee View Post
    how about for a 80ft circuit?

    The question was for a MCA rating of 14.2. Length calculation was not asked. If every fine detail was gone over for every question asked. CB would need more servers and storage.

    how about for July 20th when a/c running all day, or Jan20 when heat strip on all day [ at least over 3h ]?

    Explain why on July 20th that 8RLA compressor suddenly draws more amps. Or why the amp draw of the indoor unit has any bearing on the outdoor units amp draw.

    with cost of elec increasing, you are using 90C rating?
    -- [ C, not F ] at the SURFACE of the metal conductor.

    20*0.91= 18.2
    Lets not get too carried away.
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  10. #10
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    what I attempted to indicate is that this installation needs someone experienced in elec design --
    -- a quick glance is not enuf!

    and to correct some errors --

    using 60C wire rating will save energy!
    harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!

  11. #11
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    And that was not the question.

    If you had answer the real question first.
    And then elaborated, that would have been fine.
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  12. #12
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    Quote Originally Posted by cem-bsee View Post
    what I attempted to indicate is that this installation needs someone experienced in elec design --
    -- a quick glance is not enuf!

    and to correct some errors --

    using 60C wire rating will save energy!
    On a similar note. While sizing the cable for my geothermal HP, one of the calculations I performed was a power loss calc. I realized that by going up to the next size wire than what I needed the money saved within a year was more than the additional cost of the larger wire.

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