1-Phase Panel Fed By 3-Phase Transformer

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Robert Maddox

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Jonesboro, Ga.
Discovered someone installed a 1-Phase 120V/ 240V/ 1Ph/ 3W/ G
load center on secondary of a 480V Delta to 208Y/ 120V/ 3Ph/ 4Wire dry-type general use transformer. This is all new work & is the only item on the transformer secondary. Do not yet know the loads or details. Is this a code violation?

It does not make a lot of sense - why buy 3-Phase transformer to serve 1-Phase load? Will load imbalance create voltage imbalance?

Thanks
 
You can feed a single phase load from a 3ph transformer getting 208v from lets sat the A-B phases with 120v from A-N and B-N and ignoring the C phase. However, as I can recall only 1/3 of the kva of the transformer is available. As long as you assure that the proper OCPDs devices as provided the is no code .
It looks as if the right hand didn't know what the left hand was doing and probably a 3ph4w panel was intended would be my best guess.
 
templdl said:
You can feed a single phase load from a 3ph transformer
Click to expand...

I cannot comment on a power transformer in this configuration, but I will tell you that doing this with a control transformer such as is found in a combination starter will result in the smoke escaping from inside the transformer, thus bringing all electromagnetic activity to a halt. (sad, sad voice of experience.) [208v into the high side of a 240/120 control transformer]
 
We have a 12-pole 1 ph 3w load center at my plant wired like that. The contractor installed it to serve 120v single-phase loads, and fed it from a 2-pole CB in an upstream 3ph 4w panelboard. I think you'd have 2/3 of the transformer capacity there, based on having direct connection to two of the three secondary windings. It does seem a waste if the third phase winding isn't being used. You're sure there aren't any other loads intended for the transformer secondary? Oopsie!

Dan
 
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thinfool said:
I cannot comment on a power transformer in this configuration, but I will tell you that doing this with a control transformer such as is found in a combination starter will result in the smoke escaping from inside the transformer, thus bringing all electromagnetic activity to a halt. (sad, sad voice of experience.) [208v into the high side of a 240/120 control transformer]
Click to expand...

This statement confuses me. I don't understand how feeding a transformer on the high side with a lower voltage could cause the smoke to be released.
 
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