Mike Dolt Enterprises
Mike Dolt Enterprises
User: PhaseShift, Location: Los Angeles, Ca
Asked on: Wednesday, March 16, 2011 at 1:40 pm EDT
Answered on: Tuesday, May 14, 2024 at 2:53 am EDT
Question: I'm looking at a 120/208V panel schedule that shows the following loading:Phase A: 12380VAPhase B: 9586VAPhase C: 11356VATotal: 33322VASo 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?
Yes, if the panel is evenly loaded, you can calculate the total load by adding up the VA load on all three phases and dividing by the L-L voltage (208V in this case). This will give you the total current load in amps. In this case, the total load is 33322/208/1.73 = 92.6A.However, if the panel is not evenly loaded, you need to calculate the current load on each phase individually. To do this, you divide the VA load on each phase by the L-N voltage (120V in this case). This will give you the current load in amps on each phase.In this case, the current load on phase A is 12380/120 = 103.16A, the current load on phase B is 9586/120 = 79.88A, and the current load on phase C is 11356/120 = 94.63A.As you can see, the current load on phase A is over 100A, which means that the panel is overloaded on this phase.To verify that no single phase is overloaded, you need to compare the current load on each phase to the ampacity of the circuit breaker that is protecting that phase. In this case, if the circuit breaker on phase A is rated for 100A, then the panel is overloaded on this phase.If the panel is not evenly loaded, you need to use the highest current load on any of the phases to determine the minimum size of the main breaker. In this case, the highest current load is 103.16A, so the minimum size of the main breaker would be 100A. Flag for review