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Old 11-15-2010, 06:34 PM   #1
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transformer wiring


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?

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Old 11-15-2010, 07:28 PM   #2
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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.

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Old 11-15-2010, 07:46 PM   #3
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I would think you really need an electrician
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Old 11-15-2010, 07:49 PM   #4
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Quote:
Originally Posted by nap View Post
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.
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.
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Old 11-15-2010, 07:54 PM   #5
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Quote:
Originally Posted by Steelhead View Post
I guess what I'm looking for in the secondary is 120V to neutral and 208V phase to phase
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
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Old 11-15-2010, 07:57 PM   #6
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Quote:
Originally Posted by Steelhead View Post
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.
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.
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Old 11-15-2010, 08:16 PM   #7
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Originally Posted by nap View Post
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.
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?
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Old 11-15-2010, 08:21 PM   #8
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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.
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Old 11-15-2010, 08:33 PM   #9
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Quote:
Originally Posted by nap View Post
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.
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?

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