Calculating current from panel schedule load

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PhaseShift

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I'm looking at a 120/208V panel schedule that shows the following loading:

Phase A: 12380VA
Phase B: 9586VA
Phase C: 11356VA
Total: 33322VA

So if I look at the total VA of 33322VA does this mean that the panel has a load of 33322/208/1.73 = 92.6A? I would then use this number as the load, and the minimum that the main breaker could be?

If I take any one of the phase let's say phase A and use its load of 12380VA and divide it by the L-N voltage will that give us the current on this phase or 12380/120=103.16A? I'm guessing not since this is over 100A? Cant this be used as an estimation though without considering phase angles?

What if panel wasn't evenly loaded wouldn't you have to look at single phases to verify that no single phase was overloaded? How would you go about this.

So I guess with an evenly loaded panel you always take the total VA added up on all 3 phases and use this total divided by the L-L voltage to come up with the panel load? Is this correct?
 
Your consideration of the high leg when dealing with an unbalanced load is correct. When I prepare panel schedules I do add up the phase loads and divice by 208V and sqrt(3), but I also do a quick sanity check on my legs. It is definitely possible to exceed the rating of the main breaker as determined by the average current if one leg is overloaded due to excessive imbalance.
 
If you have a generator, it will get grumpy. It's like a washing machine. It will vibrate like hell with that much imbalance.
 
skeshesh said:
Your consideration of the high leg when dealing with an unbalanced load is correct. When I prepare panel schedules I do add up the phase loads and divice by 208V and sqrt(3), but I also do a quick sanity check on my legs. It is definitely possible to exceed the rating of the main breaker as determined by the average current if one leg is overloaded due to excessive imbalance.
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O.k. so looking at the current on phase A we see that we have 103A which is larger than the 100A main breaker. Is this not correct or do you have to look at divirsity factors?
 
Using diversity and demand factors is a different question. Of course you should use all such factors as they apply, but the point of the original question is if after all calculations are applied correctly you don't want the high leg to exceed the breaker rating or the rating of the panel bus. If you're asking my opinion on what will happen in reality, I've never seen the measured load on a panel come near calculated load for what it's worth - but that doesn't mean you can overlook calculated values.
 
skeshesh said:
Using diversity and demand factors is a different question. Of course you should use all such factors as they apply, but the point of the original question is if after all calculations are applied correctly you don't want the high leg to exceed the breaker rating or the rating of the panel bus. If you're asking my opinion on what will happen in reality, I've never seen the measured load on a panel come near calculated load for what it's worth - but that doesn't mean you can overlook calculated values.
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So do you think one phase with 103A calculated load with only a 100a main breaker is a problem?
 
PhaseShift said:
So do you think one phase with 103A calculated load with only a 100a main breaker is a problem?
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Adding the total VA of the three phases does not give you the calculated total load. Nor does adding the loads in the Phase A column of a panel schedule give you the calculated total load on Phase A. Those numbers are "total connected load." You do get to apply demand factors to some types of loads. The panel schedule spreadsheets I have used all included the demand factors at the bottom of the page. That is, they added up the phase totals, applied the demand factors, and only then did you have a calculated total load. So without knowing what the loads were, I can't answer the question of what to do about a load of 103 amps on Phase A.
 
charlie b said:
Adding the total VA of the three phases does not give you the calculated total load. Nor does adding the loads in the Phase A column of a panel schedule give you the calculated total load on Phase A. Those numbers are "total connected load."
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So these numbers would give you the total Connected Load as well as the Connected Load for each phase? Assuming that there were not demand factors to apply then these numbers would represent the total VA and Amps as well as the individual phase VA and Amps?

Just for clarification sake.
 
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