transformer wiring

Steelhead

Hi guys,
I have a question on how to wire a transformer and use it to feed a panel for general branch circuit use. The transformer is 3 phase 9 kVa 480V primary with the secondary being 208/120V. I fiqured a 30A breaker for my panel using the FLC chart for three phase transformers in Ugly's. The nameplate on the transformer has been damaged so I'm not entirely sure how I would connect the wires on the secondary side. From what I can see this is what I was thinking :

X0 - neutral
X1 - line
X2 - line

Is this okay? And is my calculation for the panel right as far as my main breaker is concerned?

nap

what happened to X3?

you are missing a winding. With a 3 phase wye secondary, you will have 4 terminals



X0 being neutral and the others being phase conductors.


my little Square D sliding data chart says 9kVA 480 volts would be 10.8 amps. I can't confirm that being correct right now but it has generally been correct.

Anti-wingnut

I would think you really need an electrician

Steelhead

I guess what I'm looking for in the secondary is 120V to neutral and 208V phase to phase. Can I get that from this transformer? In my Ugly's book it says 25A FLC at 208V phase to phase. 25A x 125% = 31.25A (30A breaker) for my overcurrent protection.

Anti-wingnut

You need an electrician. Your answer depends on your specific transformer, which we, and you, know nothing about.

You are literally playing with your life

nap

I guess I just don't understand what you are after. You say you have a 3 phase transformer but then list connections for a single phase transformer.

Steelhead

I'll try to explain this better. We have a three phase transformer at work that is currently supply 120V (one phase conductor and a neutral) to a safety switch for some general circuits. I want to replace the safety switch which was there for a long gone piece of equipment and put in a 120/208V panel in its place. Can I use this transformer to get the setup I want which I explained earlier or do I have to install a panel with three phase conductors and a neutral?

nap

Oh, you want to run the transformer in a really unbalanced fashion by only using two phases of the 3.

It will work but you can have 50% more power available if you take all 3 phase conductors to a 3 phase panel. Just because it will work doesn't mean you should do it.

Steelhead

I totally agree with you. I am all about doing things the right way. My explanation earlier is kind of what I'm stuck with at work and I was suspicious of the current setup. What I'm accustomed to is a single phase trans 480V to 120/240V and I don't know why this setup with the three phase trans was used. If the powers that be give me the okay I would definitely want to do it the right way. Regardless of which way I do this was my calculation correct as far as the overcurrent setting I came up with?