3 phase panel for single phase loads?

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Jpflex

Jpflex

Electrician big leagues
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Victorville
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Electrician commercial and residential
We have a 30KVA, 480 volt Delta primary to 208Y 120 secondary transformer feeding a 3 phase breaker panel

We have one fixed appliance unit requiring a 3 phase 220 volts source (which transformer was stepped up to 220 volts from transformer taps). All other loads are single phase being fed from individual 1 of 3 phases and a neutral derived from transformer XO terminal to 3 phase panel neutral buss (later to be tied in with ground buss as first point of disconnect)

Can anyone refer me to NEC code which would stipulate this as a violation, such as 3 phase panels having to supply 3 phase loads and single phase panels supplying single phase loads?

My coworker says this is not a violation


Also would system bonding jumper be installed in first point of disconnect 3 phase panel or in transformer? Thanks
 
I am an engineer and not an electrician, but every shop I've ever worked in has had 3 phase 208Y/120 panels supplying a mix of single and 3 phase loads.

Also, if I understand your post correctly, you adjusted the taps on the 480 primary side of the xmfr to get 220 on the secondary? If that's correct, your single phase L-N outputs from the Wye side will be running a little hot. Like 127 VAC instead of 120. Not sure how worried I'd be about that, but just something to be aware of.
 
MD Automation said:
I am an engineer and not an electrician, but every shop I've ever worked in has had 3 phase 208Y/120 panels supplying a mix of single and 3 phase loads.

Also, if I understand your post correctly, you adjusted the taps on the 480 primary side of the xmfr to get 220 on the secondary? If that's correct, your single phase L-N outputs from the Wye side will be running a little hot. Like 127 VAC instead of 120. Not sure how worried I'd be about that, but just something to be aware of.
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Yes the single phase loads were originally too high around 130 volts so I stepped down the taps a few positions to get closer to 120. I don’t remember exact voltage but it’s likely between 123 to 127 volts. This seems to be within 5% plus or minus threshold
 
I agree that powering 3-phase and single-phase loads from the same panel is perfectly fine. I echo the concern that your L-N loads will be running at a higher than normal voltage.

The N-G bond can be at either the transformer secondary (which I have never seen) or the panel ir serves (which is almost the universal method).
 
charlie b said:
I would suggest a buck-boost transformer to serve the 3-phase load at 220V, rather than using transformer taps to seek a middle ground.
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If we use a buck and boost transformer I believe you cannot ground xo terminal of transformer since primary and secondary coils have physical contact. How would you apply proper ground to this set up? Would you ground only transformer cabinet and what about ground for first point of disconnect?
 
Sorry. Wiring schemes for transformers is not my gig. Perhaps someone else can answer this question.
 
Jpflex said:
If we use a buck and boost transformer I believe you cannot ground xo terminal of transformer since primary and secondary coils have physical contact. How would you apply proper ground to this set up? Would you ground only transformer cabinet and what about ground for first point of disconnect?
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With a buck or boost transformer, you don't have a Separately Derived System, as one or more of the load side conductors is connected directly to one or more of the line side conductors. You don't need to do any grounding at the transformer, as the load side system is still grounded by the line side system's grounding.

Cheers, Wayne
 
charlie b said:
I agree that powering 3-phase and single-phase loads from the same panel is perfectly fine. I echo the concern that your L-N loads will be running at a higher than normal voltage.

The N-G bond can be at either the transformer secondary (which I have never seen) or the panel ir serves (which is almost the universal method).
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I agree. The book shows the system bonding jumper being installed at the transformer as an option but I always see it at the main panel
 
Note that your grounding electrode conductor should be connected at the same point the system bonding jumper is connected.
 
220Y/127 systems are standard in South America. As long as you don't have any incandescent lighting I would not be concerned about a 220Y/127 system in the US.
 
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