Proper Calculations of Full Load Amps for a UL508A Panel

It's simply clearly stated in section 49. Sum it all up with the a note that you take the primary of a transformer ampere rating if it has a transformer in it.

So what's your question?

It seems pretty simply to me as well

However, in my search i found a thread located here

http://forums.mikeholt.com/showthread.php?t=112174

And this confused the hell out of me. I have no idea what those guys are going on and on about

What they are talking about is how to determine the FLA of a panel when there is a mixture of 3 phase and single phase loads. However, they admittedly do NOT understand the PURPOSE of the rule in UL508A, which is so that when determining the conductor and feeder protection for the panel, you have the correct ampere value to work with. They get to the same place, their method is a tad more convoluted.

So lets say you have a set of 3 phase 460V motors that will all run at the same time with the following nameplate FLA ratings: 14A, 14A, 11A and 28A. The 3 phase load of those together is 67A. Then lets say that you have a 500VA control power transformer connected to phase A and B only. 500/480 = 1.04A. technically that 1.4A is divided across 2 of the phases, but not in a way that lowers the peak value on any one of them. So the maximum FLA for the circuit feeding that panel is 67 + 1.04 = 68.04A. Under their method you would convert all of them to VA, add all of the VA and divide by the voltage, but it comes out to 68.04 anyway.

Thanks for the reply

But here is the part that is concerning and they are noting it in that post

I have a similar setup

A bunch of 3P motors and a control transformer providing me my 110VAC circuitry. The transformer as you said is only on phase A and B for example

That means technically, The loads are not balanced and there could be a different amount of current draw on each of the phases that is feeding the cabinet.

So is it common practice to just pretend like the full load current of the transformer you calculated is on all 3 phases?

Also, what you did was calculate the full load current of the transformer of course with no power factor which will give us the worst case scenario.Is this common practice?

I find in practice we almost practically never know the power factor of the over all system. which is why so many people just use .8 as a generic power factor.

We are never going to get more purely resistive then .8

Almost everything in that post is wrong, or at least misunderstood.

Okay, so simply add up all the load current on each phase and select the transformer appropriately.

For the purposes of making sure your circuit conductors are sized properly, use VA.

How big is this control transformer? I’ve installed numerous MCC’s where the control transformers FLC loading would be about 0.1% of the MCC’s FLC.

The control transformer can be disregarded.

Is that how it works in BS7671--you can disregard the explicit instructions of certifying agencies?

We could include the transformer and adjust the FLC from 1000A to 1002.3A. I really can’t see the point.

In this case

The control transformer is about 12% of the panel

250 upstream supply to the panel and the control transformer is 15 KVA 480

so 30 amps on the primary.

15kVA is one WHOPPER of a "control transformer"!

Are you sure it's not 1500VA (1.5kVA)? If not, you must be powering something else besides "controls", in which case that's called a "distribution transformer".

ah, yes thanks for the clarification.

The transformer is pulling double duty. It is the supply for all my 110VAC control but also runs a few other things such as a few small motors, lights and cooling for the cabinets and so on. its about 100 Amp service on the secondary side.