Panel Schedules

[Zepplin96]

Hello,

I recently was tasked with creating panel schedules for large custom equipment that have panel boxes installed into them and are then shipped and field wired to a feeder by a onsite electrician.

On these schedules if I have a piece of equipment that is 208v 2080VA they will divide the VA between A and B phase. So they would enter 1040 on A phase and 1040 on B phase. Then they will at the bottom of the form add the 2 phases back together to get a total VA for the schedule of 2080VA.

Now when i look at that i see 1040VA on each phase and instantly think it's a 5A load and that a 10A breaker would be needed. However, I'm being told i'm incorrect.

In all my years as a electrical engineer I have never heard of dividing the VA of a piece of equipment between the 2 phases. The 2 phases share the load equally. So there would in fact be 10A on each phase. Which is 2080VA on phase A and B alike. I would never add the VA from phase A to Phase B to get a total VA for the panel. It just doesn't make sense to me.

This has become a heated debate here and as I am not an Electrician I'm a little unsure of what is really correct. I just posed the question.

So my question is what is the correct way to do this and why/how is it the correct way?

Thanks,
Kyle

 

[steve66]

Hello,

On these schedules if I have a piece of equipment that is 208v 2080VA they will divide the VA between A and B phase. So they would enter 1040 on A phase and 1040 on B phase. Then they will at the bottom of the form add the 2 phases back together to get a total VA for the schedule of 2080VA.

That's common, and that's the way its normally done.

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Now when i look at that i see 1040VA on each phase and instantly think it's a 5A load and that a 10A breaker would be needed. However, I'm being told i'm incorrect.

No, its a 10 amp load. 2080/208 = 10A. The 10 amps flows from one phase to the next , say from A to B. Its just not as simple as your calculation, because the voltage on each phase is the line voltage, not the line to line voltage.

In all my years as a electrical engineer I have never heard of dividing the VA of a piece of equipment between the 2 phases. The 2 phases share the load equally. So there would in fact be 10A on each phase. Which is 2080VA on phase A and B alike. I would never add the VA from phase A to Phase B to get a total VA for the panel. It just doesn't make sense to me.

It takes a while to get used to, but that's the way its calculated. Its not that that current is less than the total on either phase A or B, but more just a means to get the total at the bottom of the panel to be something reasonable. And if I remember right, this method doesn't give the exact total current on each phase, but a pretty close approximation.

If it makes you feel better, remember that the phase voltages are less than the line to line voltage (120V vs. 208V), so it doesn't make sense to put the full VA's for a load on one phase.

 

[kingpb]

2080VA, 208V, 2pole;

2080VA/208 = 10A

on each phase 2080VA/2 = 1040VA
phase voltage 208/sqrt 3 = 120V

1040VA/120V = 8.7A

This is a good example whereby it shows why you do not want to work in amps on a panel, you want to keep it in VA or KVA

 

[LarryFine]

Another advantage to you for using VA is that you don't have to use that math or convert anything; just addition.