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Thread: Current Calculation

  1. #1
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    Current Calculation

    I have a 100A 2 pole 480V Main circuit breaker. The cables then splice to feed two separate distribution panelboards rated 240/480 volts. The panelboards then feed LED light fixtures using 240V single phase using 30A 1P breakers. The total lighting load is 23.5kW pretty much balanced. What is the calculation to determine whether the main circuit breaker is sized appropriately? Is 100A sufficient?

  2. #2
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    Quote Originally Posted by Malteser View Post
    I have a 100A 2 pole 480V Main circuit breaker. The cables then splice to feed two separate distribution panelboards rated 240/480 volts. The panelboards then feed LED light fixtures using 240V single phase using 30A 1P breakers. The total lighting load is 23.5kW pretty much balanced. What is the calculation to determine whether the main circuit breaker is sized appropriately? Is 100A sufficient?
    Assuming 100% power factor 23500 watts / 480 volts = 48.95 amps. That is if it is balanced.

    Your luminaires may not state power factor, but likely will state rated amps.

  3. #3
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    That's what I thought but, I keep second guessing myself. I also found this response on Mike Holt's Forum which helped clarify my issue.

    Question: Do 2 & 3 pole breakers protect each phase at it's rated ampacity? For example, on a single phase panel with a 200amp main breaker, can you load each phase to 200amps for a total of 400, or 600amps on a 3 phase panel? Or does the breaker trip when it senses a combined load of 200amps? Not sure why this is difficult for me to wrap my head around, maybe you can help.

    Response: The part you need to wrap your head around (and this is not an easy thing to get at first), is that if you load each phase to 200 amps, the total is 200, not 400. On a 3 phase panel with 200 amps on each phase, the total is still 200, not 600.

    Let�s stick with the single phase panel, and let�s load it to 200 amps on each phase. The current that leaves the panel on Phase A will return to the panel on Phase B. It is the same current. That is why you don�t add the 200 to the 200 to get 400.

    Once you get this concept, the answer to your question becomes obvious.

    Oh, and by the way, we don't use the word "ampacity" in connection with breakers. That term only applies to wires, and indicates how much current they can handle without overheating their insulation systems.

  4. #4
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    Quote Originally Posted by Malteser View Post
    That's what I thought but, I keep second guessing myself. I also found this response on Mike Holt's Forum which helped clarify my issue.
    I do believe that was a comment of mine. I am glad it helped.

    But now, let me ask you to clarify your situation. You started by saying you have a 480 volt, two pole breaker. Is that part of a 240/480V single phase system or a 277/480V three phase system? If it is the later, then your LED fixtures are getting 277 volts, not 240 volts. Are they rated for that voltage.

    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

  5. #5
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    200 amps @ 120 volts is 24000 VA

    200 amps @ 120 on line A and 200 amps @ 120 on line B is 48000 VA, since there is no imbalance on the neutral you have the same 48000 VA effectively connected to 240 volts. This for single phase 120/240 volt supply.

    200 amps @ 120 on all three phases of a three phase wye system - you have 72000 VA, but each line measures 346 amps

    200 amps @ 120 on just A and B of a three phase wye system -you have 4800VA, A and B both measure 200 amps, but the neutral is also very near 200 amps.

    Chew on that for a while

  6. #6
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    Quote Originally Posted by kwired View Post
    200 amps @ 120 on all three phases of a three phase wye system - you have 72000 VA, but each line measures 346 amps
    I think not, at least not the way you described this. Please chew on it again.

    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

  7. #7
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    Quote Originally Posted by charlie b View Post
    I think not, at least not the way you described this. Please chew on it again.

    OOPs, I forgot one step and blew that one.

    Let me try again with what I was trying to set him up for:

    Say you had three 200 amp 208 volt single phase loads balanced across all three phases,

    you would have 3 x 41,600 = 124,800 VA, each individual load draws 200 amps but each common supply line measures 346 amps.

  8. #8
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    Yes, Charlie B., that was based on one of your previous responses. Yes, the voltage from the utility is 240V/480V single phase. Thank you all for your help and the additional information to "chew on"..

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