ECN Forum Forums General Topics of the Trade General Discussion Area Sizing a Transformer

Better late then never I guess...that link has my head in a tail spin!!

That said, what's the math that concluded 625 amps @ 120volts??

1000 x 75 / 120

It's single phase so I didn't multiply x 1.73

The math for single phase is different than the math for multiphase.

If you had a single phase transformer you wouldn't use the square root of 3 at all.

Well, it's also shown in the table you linked on page 1.4

That's 625 Amps for a single phase 75kVA transformer.
For a 3 phase 75kVA transformer it's 416 Amps.

Isn't your question about a standard 75kVA 3-phase transformer? A 75kVA single phase transformer is an oddball special order.

I didn't. 1000 x 75 / 120 = 625

I think part of the confusion is the fact that the loads I am stepping down for are 120v but the primary is 460/3.

I think I need to look at it from the panel perspective instead?

Generally what I use for loads like yours is a 3-phase transformer. 460V 3phase 3wire primary and 208Y/120V 3phase 4wire secondary.
That will feed a 208Y/120V 3phase 4wire panelboard that will be filled with 1-pole circuit breakers that will then feed the branch circuits to the individual 120V loads.

Single panelboards generally come with either 125A or 225A bus in them, the usual setup is a 75kVA transformer feeding a 225A panelboard. If you need a larger transformer you can get a larger panelboard, but you'd probably be better off breaking the loads up so you could use 2 regular 225A panelboards.

With most PC and electronic loads wanting 1 (or only a few) loads on each individual breaker; you'll generally run out of breaker positions before you run out of power.

Our intention was to use 1 dedicated circuit to 5 users. We've done this in the past via extension cords and power strips without issue.... just not on this scale.

I see you are also from Chicago. I hope this isn't out of line but if you're interested in the job, I'd love for you to come take a look.

PM me for details?

If you're going to break it up by cubicle you should think about having a receptacle for the PC and separate general receptacles.
I've seen too many situations where the computer circuit ends up with the clock, printer, desk fan, and everything else under the sun on it.
If you're going to the trouble to deliver clean power for the PCs you should do whatever you can to keep people from piling on more loads.

In one office we provided a blue-colored duplex receptacle for the PC and monitor and required that they be plugged in DIRECTLY. If people can't use power strips it tends to keep them more honest about what they plug into those circuits.

I would be interested, but my boss keeps me pretty busy where I am; thanks for the invite though.