Reverse Fed Transformer with Corner Delta Ground

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jwatts

Member
Location
Chicago, IL
Little confusion here about reverse fed transformers, grounding/bonding, and loads. Have a job site where the single phase 480V pole light feeds were damaged during excavation. There is an old transformer on site that was recently disconnected. The transformer is a step down, 480v to 208/120v. I know it is not listed for reverse fed, but I need to get the lights up and running ASAP as it is a shopping mall. I was planning on reverse feeding it with 208v, and getting 480v. Question I have is I believe I need to utilize a corner grounded delta on the 480v side, otherwise the system is not grounded. I have seen corner grounded deltas for 3 phase motors, but my situation is a bit different. I was hoping I could utilize this setup for the single phase 480v parking lot feeds, since each pole head has 2 hots, and no neutral is utilized in the system. Any clarifications or suggestions would be greatly appreciated.
 

tom baker

First Chief Moderator
Staff member
You can reverse feed the xmfr if that voltage is listed on the nameplate, 450.11.(B), this is a change in the 2014 NEC.
Great you recognize the need for grounding on the secondary. This type of system can be optionally grounded per Art 250. However you will need the system bonding jumper and the GEC connection.
 

jumper

Senior Member
You can reverse feed the xmfr if that voltage is listed on the nameplate, 450.11.(B), this is a change in the 2014 NEC.
Great you recognize the need for grounding on the secondary. This type of system can be optionally grounded per Art 250. However you will need the system bonding jumper and the GEC connection.

But the manufacture has to include that set-up in the installation instructions also. OP said it was not listed for back feed.

450.11(B) Source Marking. A transformer shall be permitted to
be supplied at the marked secondary voltage, provided that
the installation is in accordance with the manufacturer’s
instructions.
 

jwatts

Member
Location
Chicago, IL
From what I understand I can ground either of the 3 phases on the 480v side.....correct? Next question is since I am removing the jumper from X0 to case ground....do I then jumper from my grounded phase on the 480v to the case as well? The whole grounding a hot seems to guys against everything I thought. But seems to be not as uncommon as thought the more I research.
I was concerned that the corner delta was only intended for 3 phase loads......but was hopeful that since I have no need for a neutral and only two hots at each pole, I could get buy with what I proposed.
 

jwatts

Member
Location
Chicago, IL
But the manufacture has to include that set-up in the installation instructions also. OP said it was not listed for back feed.

450.11(B) Source Marking. A transformer shall be permitted to
be supplied at the marked secondary voltage, provided that
the installation is in accordance with the manufacturer’s
instructions.

I do understand the equipment is not listed for reverse fed....but I am in a bind and need the lights up and running ASAP, and there is no option of getting a listed transformer on site today.
 

Smart $

Esteemed Member
Location
Ohio
Is there 480 service on site?

Your OP mentioned light feeds damaged during excavation. Can you not just repair the feeds?

On backfed delta-wye step-up transformer, you can ground any [delta] H terminal but when you get to bused equipment, it is more conventional to rotate the phasing so that the ground phase is the B bus. Of course that would be the case if you are actually using Grounded B-phase equipment. Keep in mind that the grounded phase will need to have grounded insulation color if under #4, and appropriately marked if $4 and greater. On lighting circuits that don't have excessive voltage drop, that may require repulling the conductors.

Also keep in mind that you cannot fuse the grounded phase.
 

jwatts

Member
Location
Chicago, IL
Is there 480 service on site?

Your OP mentioned light feeds damaged during excavation. Can you not just repair the feeds?

On backfed delta-wye step-up transformer, you can ground any [delta] H terminal but when you get to bused equipment, it is more conventional to rotate the phasing so that the ground phase is the B bus. Of course that would be the case if you are actually using Grounded B-phase equipment. Keep in mind that the grounded phase will need to have grounded insulation color if under #4, and appropriately marked if $4 and greater. On lighting circuits that don't have excessive voltage drop, that may require repulling the conductors.

Also keep in mind that you cannot fuse the grounded phase.

Thats the tricky part about just repairing. 480 feeds damaged during demolition of a portion of the mall. Now local GC is expediting the portion where the local utility is removing the vault that was feeding the 480v lights as well. I just need a band aid to get through the remaining demolition.
Also, I never thought about not being able to fuse the grounded phase. So the grounded phase is now treated similarly to the neutral in a wye setup.....yet still would have 480v between the grounded phase and an ungrounded phase? Boy this corner delta is confusing.
 
Also, I never thought about not being able to fuse the grounded phase. So the grounded phase is now treated similarly to the neutral in a wye setup....

In a wye config, the "neutral" usually IS the grounded conductor, and you don't fuse the grounded conductor.

yet still would have 480v between the grounded phase and an ungrounded phase? Boy this corner delta is confusing.

Only if you want to be confused. You have 480v between any two phase leads of the delta, the fact that one phase lead is grounded* doesn't make any difference. (Same with 120/240, you will always have 120v between the "neutral" and a hot whether or not the "neutral" is grounded.)

*and only one lead of the system is grounded- in a wye, you wouldn't ground the center/neutral and a phase lead
 
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