Summation of apparent power

[m sleem]

If mathematically we cannot summation the apparent power for different loads, why always see the panel schedules make a summation for the VA even the same you could find in Revit.

 

[winnie]

Because a rough approximation is often good enough.

The exact total load on a conductor or bus bar requires adding the individual load currents while taking into account things such a phase angle and distortion.

But simply adding up the RMS current for each load will give an answer that is pretty close with normal loads, close enough that you can reasonably know you are properly sizing the equipment.

Since the supply voltage is the same for all loads, adding VA is effectively the same as just adding current.

It's important to know that this is just a usually acceptable approximation, not exact truth.

Jon

 

[m sleem]

Thanks for your explanation, but if we make the summation for wattage/load the calculation would be more accurate.

 

[winnie]

IMHO it depends upon what you are sizing.

Things such as circuit conductors and panelboards are limited by the system voltage and the _current_ they have to carry. In this case it makes more sense to size based on A or VA.

For sizing something such as a generator prime mover, what matters is the actual power delivered, and you would want to make the summation based on wattage.

For sizing the generator alternator itself, we are back to a VA limitation. (Thus generators are limited by both the maximum VA they can deliver and the maximum W they can deliver.)

-Jon

 

[drktmplr12]

power flow and summation analysis is more important for analysis of failure modes like fault currents and arc flash.

for sizing panelboards it really is going too far.