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Thread: VA continuous load question

  1. #1
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    VA continuous load question

    the maximum volt ampere loading permitted on a 20 ampere branch circuit with a continuous load would be

    20x120x1.25 percent=3000va. Correct?
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  2. #2
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    Quote Originally Posted by panhandle444 View Post
    the maximum volt ampere loading permitted on a 20 ampere branch circuit with a continuous load would be

    20x120x1.25 percent=3000va. Correct?
    No.

    20x120/1.25=1,920 va

  3. #3
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    Quote Originally Posted by iwire View Post
    No.

    20x120/1.25=1,920 va
    Alternatively, 80% of 2400VA (20x120) also equals 1,920VA. Division by 125% (5/4ths) is the same as multiplication by 80% (4/5ths).
    Electricians do it until it Hertz!

  4. #4
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    Quote Originally Posted by JFletcher View Post
    Alternatively, 80% of 2400VA (20x120) also equals 1,920VA. Division by 125% (5/4ths) is the same as multiplication by 80% (4/5ths).
    Of course it is, but the OP and the NEC reference 125% so that is what I chose to show for clarity.

  5. #5
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    Quote Originally Posted by iwire View Post
    Of course it is, but the OP and the NEC reference 125% so that is what I chose to show for clarity.
    Agree. It's just that some people learn differently. I can do it either way, tho I'm sure there are folks more comfortable multiplying by .8 than dividing by 1.25.

    2400
    ------
    1.25

    or 2400 x .8

    I can do the latter in my head easier. and when I see 2400/5/4, I convert it mentally to 2400 x .8. Actually, I've memorized the answer, and know 16A is the max continuous load on a 20A circuit. That's a third way to do it. (20 x 4/5 = 80/5 = 16).
    Electricians do it until it Hertz!

  6. #6
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    To the OP. Just remember that when you are having a continuous load, you put more heat into the conductors and over current protection so you need to have a result that is lower than 20 amps x 120 volt which = 2400 VA.

    You cannot allow more VA when you know the load is continuos. You have to reduce the allowable VA to compensate for the heat generated from the 3 or more hours of load.

    Sent from my SM-G920V using Tapatalk
    Randy

  7. #7
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    Could someone provide the code reference stating you cannot have a continuous load more than 80% of a branch circuit's rating? This would imply you have the ability to reduce the draw on the appliance in use. How do you reduce the current draw on an appliance pre-assembled with plug n cord ready out of the box? I know we use 80% of circuit size as a good rule of thumb, but I haven't seen it as rule in the code book.

    My understanding is: 20A x 120v = 2400VA is the load. Since it is a continuous load you would need a conductor rated at 20a x 1.25 or 25amps. "...minimum branch conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load."

    My question is your terminals are rated at 75 degrees and you ran 12AWG THW (or any THHN for that matter), would that be code compliant? The OCPD wouldn't change, but the code is satisfied (even 240.4[D] ) to carry the calculated load now.

  8. #8
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    Most breakers, especially of the branch circuit sizes, are not rated for 100% with a continuous load. So a 20 amp breaker can only carry 16 amps (80% of a 20 amp breaker).

    210.20 Overcurrent Protection. Branch-circuit conductors
    and equipment shall be protected by overcurrent protective
    devices that have a rating or setting that complies with
    210.20(A) through (D).
    (A) Continuous and Noncontinuous Loads. Where a branch
    circuit supplies continuous loads or any combination of continuous
    and noncontinuous loads, the rating of the overcurrent
    device shall not be less than the noncontinuous load plus
    125 percent of the continuous load.
    Exception: Where the assembly, including the overcurrent devices
    protecting the branch circuit(s), is listed for operation at 100 percent of
    its rating, the ampere rating of the overcurrent device shall be permitted
    to be not less than the sum of the continuous load plus the noncontinuous
    load.
    210.19 Conductors — Minimum Ampacity and Size.
    (A) Branch Circuits Not More Than 600 Volts.


    (1) General. Branch-circuit conductors shall have an ampacity
    not less than the maximum load to be served. Conductors shall
    be sized to carry not less than the larger of 210.19(A)(1)(a)
    or (b).


    (a) Where a branch circuit supplies continuous loads or
    any combination of continuous and noncontinuous loads, the
    minimum branch-circuit conductor size shall have an allowable
    ampacity not less than the noncontinuous load plus
    125 percent of the continuous load.
    (b) The minimum branch-circuit conductor size shall
    have an allowable ampacity not less than the maximum load to
    be served after the application of any adjustment or correction
    factors.
    Exception: If the assembly, including the overcurrent devices protecting
    the branch circuit(s), is listed for operation at 100 percent of its rating,
    the allowable ampacity of the branch-circuit conductors shall be permitted
    to be not less than the sum of the continuous load plus the noncontinuous
    load.

  9. #9
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    Quote Originally Posted by 2Broke2Sleep View Post
    Could someone provide the code reference stating you cannot have a continuous load more than 80% of a branch circuit's rating? This would imply you have the ability to reduce the draw on the appliance in use. How do you reduce the current draw on an appliance pre-assembled with plug n cord ready out of the box? I know we use 80% of circuit size as a good rule of thumb, but I haven't seen it as rule in the code book.

    My understanding is: 20A x 120v = 2400VA is the load. Since it is a continuous load you would need a conductor rated at 20a x 1.25 or 25amps. "...minimum branch conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load."
    You are stating an entirely different scenario than the op. He asked what is max continuous VA on 20A circuit. Your question is about a continuous 20A-rated load.

    Quote Originally Posted by 2Broke2Sleep View Post
    My question is your terminals are rated at 75 degrees and you ran 12AWG THW (or any THHN for that matter), would that be code compliant? The OCPD wouldn't change, but the code is satisfied (even 240.4[D] ) to carry the calculated load now.
    210.21(A) states "Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load." Minimum OCPD is 20A × 125% = 24A, rounded up to next standard size 25A.

    240.4(D)(5) you cannot use #12 copper with greater than a 20A breaker.
    I'll never get there. No matter where I go, I'm always here.

  10. #10
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    Quote Originally Posted by Smart $ View Post
    You are stating an entirely different scenario than the op. He asked what is max continuous VA on 20A circuit. Your question is about a continuous 20A-rated load.


    210.21(A) states "Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load." Minimum OCPD is 20A × 125% = 24A, rounded up to next standard size 25A.

    240.4(D)(5) you cannot use #12 copper with greater than a 20A breaker.
    Got it. I overlooked 210.21 Would 10 AWG (romex/thhn/thw) with a 25 amp breaker satisfy this installation? Or as I stated previously, a 20A breaker rated at 100% with 12AWG THW?

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