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Thread: 3 phase load calculation

  1. #11
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    individual circuit ratings

    Quote Originally Posted by kwired View Post
    What is the individual VA ratings of the 7 individual circuits. If you calculated this wrong you could easily have high totals. I see you as reporting a total of about 52.7 KVA. I could also see you actually having a total load of about 26 KVA depending on how you came up with the numbers you posted.

    Find out what the total load is @ 208 volt single phase then work on dividing it up across the three phases. If all seven loads are equal you can easily come up with a balanced per phase load for six of the circuits and then add the unbalanced portion of the seventh to two of the phases.

    First thing is to know the VA ratings of the individual circuits.
    I hope I got this part of it right. The seven loads are not equal. I think they are as balanced across all three phases as much as possible. I calculated the individual circuit rating by taking the load current and multiplying by 208. For example, one heat pump has a MCA of 30A. 208x30=6240VA. I applied that amount (6240) to each of the two phases that circit is connected to. Correct?

  2. #12
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    Quote Originally Posted by KHall View Post
    I hope I got this part of it right. The seven loads are not equal. I think they are as balanced across all three phases as much as possible. I calculated the individual circuit rating by taking the load current and multiplying by 208. For example, one heat pump has a MCA of 30A. 208x30=6240VA. I applied that amount (6240) to each of the two phases that circit is connected to. Correct?
    That is where I was going with my question. If you balance the load each phase carries 1.73 times the single phase current.

    You also only need to calculate with actual load not MCA marked on the unit. The MCA is for sizing the conductors for the branch circuit and is likely 125% of the compressor RLA plus all other loads in the unit or very close to that. Your feeder will need to be at least 125% of largest plus all others

    Lets assume your 30 amps or 6240 VA is the actual load and you have 6 of the same units evenly balanced across all three phases. You have a total 37440 VA.
    Divide by 208 gives you total single phase amps of 180 amps. Since you are balancing this across three phases you need to divide that by the square root of 3 (1.73) to get the amps per phase. Which will be about 104 amps. Now your seventh unit will be an additional 30 amps connected to each of the two phases you put it on, so you will have something like

    Phase A = 134 amps
    Phase B = 134 amps
    Phase C = 104 amps

    Remember you used the MCA to get your individual load of 30 amps. The actual full load of the unit is probably closer to 21-24 amps. If your seven units are not all the same size you will want to connect them to the phases in a way that comes closest to balancing the loads between the phases for best results as well as allowing a smaller feeder to supply them.

  3. #13
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    If you are calculate load then you would not use the mca. The mca of a heat pump/ac unit already has the 125% factored in.

  4. #14
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    Create an Excel spreadsheet just like a panel schedule with A,B,C phases on the left and right 22 +/- down each side. Total each column of A,B,C’s to view the balance of each, similar to below the put the math in from Art 220 for the totals at the bottom of the spreadsheet.


  5. #15
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    Quote Originally Posted by tryinghard View Post
    Create an Excel spreadsheet just like a panel schedule with A,B,C phases on the left and right 22 +/- down each side. Total each column of A,B,C’s to view the balance of each, similar to below the put the math in from Art 220 for the totals at the bottom of the spreadsheet.


    I like that. Only question is does the totals for each phase factor in the 1.73 multiplier for three phase. This would be an easy spreadsheet to make for single phase. Three phase is not impossible but the formula for total amps is a little more tricky for three phases that are not the same load per phase.

  6. #16
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    very helpful!!

    Quote Originally Posted by kwired View Post
    That is where I was going with my question. If you balance the load each phase carries 1.73 times the single phase current.

    You also only need to calculate with actual load not MCA marked on the unit. The MCA is for sizing the conductors for the branch circuit and is likely 125% of the compressor RLA plus all other loads in the unit or very close to that. Your feeder will need to be at least 125% of largest plus all others

    Lets assume your 30 amps or 6240 VA is the actual load and you have 6 of the same units evenly balanced across all three phases. You have a total 37440 VA.
    Divide by 208 gives you total single phase amps of 180 amps. Since you are balancing this across three phases you need to divide that by the square root of 3 (1.73) to get the amps per phase. Which will be about 104 amps. Now your seventh unit will be an additional 30 amps connected to each of the two phases you put it on, so you will have something like

    Phase A = 134 amps
    Phase B = 134 amps
    Phase C = 104 amps

    Remember you used the MCA to get your individual load of 30 amps. The actual full load of the unit is probably closer to 21-24 amps. If your seven units are not all the same size you will want to connect them to the phases in a way that comes closest to balancing the loads between the phases for best results as well as allowing a smaller feeder to supply them.


    This has been very instructive, but I'm not sure I've got it. If you don't mind, I am going to run thru the whole calculation ...

    There are (3) 208V/1ph Heat Pumps rated 30 MCA. If I reduce them by .8, and then multiply by 208V, that will give me a 4992VA load to each of two phases.

    Next, I have (3) 208V/1ph Air Handlers at 9 MCA. Same calculation results in 1498VA per phase.

    Finally, one 208V/1 ph Energy Recovery Ventilator at 9.7 MCA adds 1622VA to each phase connected, which are A & B.

    The HPs and AHUs can all be connected to result in a balanced load across all 3 phases. The ERV will produce a larger imbalance in the two phases (A & B) to which it is connected.

    Therefore ...
    Phase A = 4992+4992+1498+1498+1622 = 14,602VA
    Phase B = (same as A) 14,602VA
    Phase C = 4992+4992+1498+1498 = 12,980VA

    Calculating the balanced portion of the load ...
    12,980VA / 208 / 1.73 = 36A on each phase A, B, & C

    the unbalanced portion ...
    1622VA / 208 = 7.8A on phase A & B.

    Compiling balanced & unbalanced amperage results in ...
    A = 43.8, B = 43.8, C = 36

    Final note ... add in 25% of largest motor of sizing feeder.

    The final numbers seem low to me. Did I miss something??

  7. #17
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    Quote Originally Posted by kwired View Post
    I like that. Only question is does the totals for each phase factor in the 1.73 multiplier for three phase. This would be an easy spreadsheet to make for single phase. Three phase is not impossible but the formula for total amps is a little more tricky for three phases that are not the same load per phase.
    This panel schedule calculates either 1 or 3 phase, there is an "IF" function that calculates based on the phase type entry at the top of the spreadsheet (again 1 or 3).

    The panel schedule is designed to reveal an Art 220 calculated load (1 or 3 phase). The phase balances reveal using VA rather than amperes. The circuit entries are done in VA so if there is a 1ph unit with nameplate amperes 32A at 240 it would be 3840VA per phase in the entry (or 3840VA for 208 as well).

    The spreadsheet calculates at the bottom based on circuit specs entered at the top.

  8. #18
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    Quote Originally Posted by KHall View Post
    Calculating the balanced portion of the load ...
    12,980VA / 208 / 1.73 = 36A on each phase A, B, & C

    the unbalanced portion ...
    1622VA / 208 = 7.8A on phase A & B.

    Compiling balanced & unbalanced amperage results in ...
    A = 43.8, B = 43.8, C = 36

    Final note ... add in 25% of largest motor of sizing feeder.

    The final numbers seem low to me. Did I miss something??
    I think you missed something. Your balanced load is three (3) 4992VA loads and three (3) 1498VA loads. That gives a balanced load of 19470VA.

    19470VA balanced load at 208V, 3ph give a current of 54.04 Amps. So A, B and C would see 54.04 Amps. And adding the last unbalanced load (of 1622VA) between A and B would add 7.8 Amps to A and B.

    So the total unbalanced load would be 61.84 Amps on A and B, and 54.04 Amps on C.


    (As a side note, you probably can't multiply the MCA of the heat pump by 0.8 to get the load current. This would only work for a single motor load.)
    Last edited by david luchini; 03-28-11 at 03:20 PM.

  9. #19
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    Quote Originally Posted by KHall View Post
    …There are (3) 208V/1ph Heat Pumps rated 30 MCA. If I reduce them by .8, and then multiply by 208V, that will give me a 4992VA load to each of two phases…
    Use the whole MCA in your load otherwise you need to calc it per 440 part IV. Also the 4992 would be half on each phase 2496 (4992 / 208 = 24A) but again use the 30A.
    Using your numbers here’s the result:

  10. #20
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    Calc at full MCA and largest motor as energy recovery vent


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