Adding loads on panelboard schedule?

[rrc14]

When adding loads by phase on a panel-board schedule how do you handle the 2-pole circuit breaker loads? Lets say for instance the single A-phase loads add to 12kVA, and the single B-phase loads =13.2kVA, and single C-phase loads = 10kVA. How do you calculate the two-pole loads? Would one split the kVA between the two phases? Lets say the 2-pole load was a stove rated at 1.5kW would you figure 750W per phase in the total per phase calculations?

Thanks,

 

[Smart $]

When adding loads by phase on a panel-board schedule how do you handle the 2-pole circuit breaker loads? Lets say for instance the single A-phase loads add to 12kVA, and the single B-phase loads =13.2kVA, and single C-phase loads = 10kVA. How do you calculate the two-pole loads? Would one split the kVA between the two phases? Lets say the 2-pole load was a stove rated at 1.5kW would you figure 750W per phase in the total per phase calculations?

Thanks,

Yes, but it is only an approximation when arithmetically summed by phase. The only way to get an accurate summation is to know the power factor of each load and figure that in while doing vector math or diagramming... yet the system summation will satisfy most all NEC calculations required.

 

[Karl H]

Yes, but it is only an approximation when arithmetically summed by phase. The only way to get an accurate summation is to know the power factor of each load and figure that in while doing vector math or diagramming... yet the system summation will satisfy most all NEC calculations required.

Could you explain this further?

 

[Smart $]

Could you explain this further?

Let's use a simple example to help explain.

208Y/120V 3? 4W System and two 208V 10A 1? purely resistive loads, one connected A-B, the other B-C.

On a panel schedule you would split each in two then sum by phase. The sums would include 1.04kVA in A_total, 2.08kVA in B_total, and 1.04kVA in C_total, and 4.16kVA in the system total.

The system total is as accurate as calculating with kVA can be. The reason should be obvious. However, it is not so obvious that the per phase totals can be inaccurate, especially for trying to determine amperes per Line.

To demonstrate, take the extreme case where these two loads are the only loads on the system. How many amps on each line?

On Line A and Line C, 1.4kVA ? 120 = 8.67A is incorrect, for there would be the full load current of 10A on each Line (1200kVA). For Line B, 2.08kVA ? 120 = 17.33 is correct (10A ? √3 = 17.32A). Yet if the two loads were unequal, or not purely resistive and had differing power factors, all three per phase totals would be inaccurate.

 

[Smart $]

On Line A and Line C, 1.4kVA ? 120 = 8.67A is incorrect, for there would be the full load current of 10A on each Line (1200kVA). For Line B, 2.08kVA ? 120 = 17.33 is correct (10A ? √3 = 17.32A). Yet if the two loads were unequal, or not purely resistive and had differing power factors, all three per phase totals would be inaccurate.

Every once and a while I review posts that I have made for accuracy. Most inaccuracies I let pass when nobody else discovered them :grin: However, there are a couple in the above quoted passage that I'd like to correct.

1.4kVA should have been 1.04kVA, and
1200kVA should have been 1.20kVA