Using Only One Leg

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Nathaniel.Kenney

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Maryland
Hey All,
I am working on a project where I have to supply a 100A, 120V circuit to a new inverter. In previous buildings where we've done this, we can always find a 120/208V panel to use. The current building is primarily 277/480V, though. Since we only need the one circuit, I'm trying to circumvent dropping a whole panel and use an enclosed breaker instead. Here is the question that's come up. Can I use a 480:120/240 transformer and only use one leg? Something doesn't quite seem right about that set-up and being able to use the transformer to its full capabilities (i.e.- is there some sort of de-rating factor if you do this)?

Any other suggestions on how to implement this?
Thanks for your help,
--Nate
 
Most dry type single phase transformers, over 3kVA, have secondaries that can be reconnected as 120V, 240V, or the common 120/240V.
 
jim dungar said:
Most dry type single phase transformers, over 3kVA, have secondaries that can be reconnected as 120V, 240V, or the common 120/240V.
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This.

The wiring diagram for the transformer will often show _4_ terminals on the secondary side. You connect series or parallel depending on if you want a 120V, 240V or 120/240V with center tap.

On top of this, with a single phase transformer wired with a 2 wire secondary (120V, no center tap) if you size the primary OCPD correctly it will also function as secondary OCPD. I forget the rules about secondary disconnect requirements in this situation.

I note you list your occupation as an engineer, which I presume means you are specifying an installation which your electrician will perform. Your electrician will know the detailed codes.

-Jon
 
Nathaniel.Kenney said:
Jim,
Helpful reply. And thanks, Winnie, for fleshing the idea out more. I think I've got my mind wrapped around the solution now.

Thanks everyone!
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You could look at this thread, last post for the official transformer schematic designations. Note you want the type where the secondary windings can be connected in parallel

240 x 480-120/240 V Transformer Ratio

Greetings, Gents - I'm a little confused on '240x480' designation on the high side of the transformer. This is different from ' 480-120/240' designation. Does this mean that I can connect either 240V or 480V supply on the primary and get 240V/120V on the secondary side? What does "X' mean in...
forums.mikeholt.com
 
You would need a 15kVA single phase transformer to get 100A at 120V.

Not sure what you mean by "one leg" though. You would use 2 legs on the primary, the secondary would also be two "legs", but you would ground one of them to make it a Neutral for the 120V.
 
Jraef said:
You would need a 15kVA single phase transformer to get 100A at 120V.

Not sure what you mean by "one leg" though. You would use 2 legs on the primary, the secondary would also be two "legs", but you would ground one of them to make it a Neutral for the 120V.
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I think a better choice would be to use a transformer with a 120V secondary instead of a 120/240 secondary and using "half" of the transformer
 
electrofelon said:
I think a better choice would be to use a transformer with a 120V secondary instead of a 120/240 secondary and using "half" of the transformer
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Definitely do not want to use only half of the secondary, so you don't want to use a transformer wired to produce a 120/240V output. However a common secondary arrangement is two 120V coils with four terminals. The coils get field connected to give 240V, 120/240V, or 120V as desired. This sort of transformer is likely described as having 120/240V secondary.

Jon
 
I never said to use half of the secondary. You use the entire secondary with the two windings in parallel for 120V... But you still ground one side to create the neutral.
 
electrofelon said:
Ok your use if the word "neutral" implied a center tap.
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We do this all the time. Especially for a 120/208 bank. That bank is essentially three 120 volt transformers. Nothing on the left bushing, (internally or externally) center is ground, and right is 120V.
Nothing really saying it can’t be reversed, just convention.
 
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