Plant Electrician

Hello everybody. My company has asked me to take a 480vac delta to 208vac wye Xmfr and wire it backwards to get 480vac 3 phase from 208 3 phase. I have a couple of problems with this.

1. If i input 208vac on X1,2,3 and bond X0 to earth what will happen when the current is applied through the coils with no load? I think it would burn out the secondary side correct?
2. if i were to float the ground then there would be no way to properly ground the system?

Has anybody done something like this before this is a large Xmfr 225KVA so its not like i can bench test it like some science project. Also safety and workmanship and whether this is code compliant in our field are all issues i have debated. As of now ive told them "cant be done properly"

Thanks

"what will happen when the current is applied through the coils with no load?"

The inherent impedance of the coils is the load. Why do you think that this is a problem when back-feeding, although it is not a problem when forward-feeding the transformer? A coil is a coil.

Step-up/step-downs are done like that all the time, that is what transformers are made for.

BUT, If you don't know how to wire it, then find a someone who does to show you how to do it right.

plant electrician

Ok so i would wire the normally refered to secondary side or wye side of Xmfr X1,2,3 to 208vac Phases a,b,c, then bond X0 to earth for Systems grounding correct? Now the primary high side is a delta configuration with no ground connection. So will i be stuck with 480vac phase to phase seeing there is no center to tap for ground on this side " the now working side" im concerned about ground fault protection.
I have found Xmfr's that are 208delta to 480 wye made just for that purpose so what is the difference. Why would they make them if there wasnt some issue with what they are asking.

Regards

If you need 480 Y on the high side, then you need a 480 Y transformer.
480 Delta is 480 Delta, You can corner ground it (Scary but I have had the ... pleasure???... of working on a few of them), I hate the idea of installing one of them though...
So what is it that you need on the high side? Wye or Delta?

Ok so i would wire the normally refered to secondary side or wye side of Xmfr X1,2,3 to 208vac Phases a,b,c, then bond X0 to earth for Systems grounding correct?

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On the high side (now 480 Volts Delta), you would have no hard center neutral or grounding point. Personally I consider that dangerous for the typical plant with the typical maintenance crew. It is okay for a utility power line, but not inside a plant. If you can find a 480 volt Y to 208 volt Delta and reverse feed that, then you would still have your hard grounding point. But for the same money, you could buy the transformer that you really need (such as a Square D Class 7400, catalog number 225T64H, 225 kva, 208 volt Delta primary, 480/277 volt Y secondary). With a correctly labeled factory nameplate on the transformer there will be no danger of someone making a mistake.

If you do reverse-feed a transformer, make sure you add a durable label that shows what you did. Be sure to put your name on the label so they will know who to blame. Otherwise if someone gets hurt, guess where the plaintiff's lawyer is going to point his finger when you are called to the witness box while the crying widow stares daggers at you?

Understand that it is not that the transformer can't do the job. It is that is doing the reverse of what it was built for and labeled for. That is probably worth about $10 million to a good ambulance-chasing lawyer.

I believe i am stuck with the delta winding on the original primary side. As i only have taps for minimal voltage adjustments 0utputs are labled H1,2,3 only. I am not real thrilled about bonding any of the H1,2,or 3 outputs.

troylibby said:

I believe i am stuck with the delta winding on the original primary side. As i only have taps for minimal voltage adjustments 0utputs are labled H1,2,3 only. I am not real thrilled about bonding any of the H1,2,or 3 outputs.

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I agree that the delta as a secondary connection is not an ideal configuration. What you would gain by grounding one leg of the delta is that an unintentional grounding of another leg would lead to a circuit breaker trip, whereas a completely ungrounded delta could energize a structure without tripping the protective device should one phase become incidentally grounded.

Don't do it

Don't do it!!! For safety reasons. If you do have your "boss" that wants it done, sign a paper saying it is ok to do it in an unsafe way. Bet he won't but he would want you to. If it is only a for temp service to try something that is a differant story, you can lable it as such.

I'm not sure why they want this, but you should be fine. The primary 480 Delta would remain ungrounded (you don't see these often, but they are out there - one of our paint lines is fed from 480V ungrounded delta). Don't ground the secondary of the primary transformer (or the primary of the secondary transformer for that matter).

The system will remain ungrounded up to that point. The secondary of the secondary transformer will be 480ish volts (read JRB's post/link on this). You can corner ground one end if needed (or center tap ground it if you have the connection available). Or, leave it ungrounded, I suppose.

I have done the same thing on a smaller scale with 120 to 24 volt control transformers with the secondary of one transformer feeding the secondary of the other to get back up to 120 volts.

The purpose was that I needed 120 volts, *ungrounded* (this was done for testing purposes).

It posed no problems temperature-wise, and I was pulling zero amps from the 120 volt "secondary" (I just needed the pressure) and it didn't even get warm.

Are you doing this to provide grounding for your 480? If so, read up on Zig-Zag transformers.

As others have said, you can either corner ground the 480v side, or install ground detectors if you decide to leave the 480v side ungrounded. You MUST do one of these things. If you must reverse-feed your transformer, do not ground X0; the 208v side of your transformer will appear to be a delta connected load.

I personally hate corner grounded services, and hate ungrounded services even more.

Corner grounded services are a three-legged unicorn; no one's ever seen one in the wild. VFD's connected to corner grounded services can/will have issues with the supply.

Ungrounded services are hard to troubleshoot, many people are dangerously unfamiliar with them, and can float to weird phantom voltages that damage connected equipment. VFD's do NOT like ungrounded services, and will sustain damage. Ground detectors are a bad idea IMO, as they are often ignored or disabled.

Buy the correct transformer; 208v delta -to- 277/480Y. Install it correctly; bond the Y-point of the 480v side. You will sleep easier.


-rpoet

Agreed

rpoet said:

As others have said, you can either corner ground the 480v side, or install ground detectors if you decide to leave the 480v side ungrounded. You MUST do one of these things. If you must reverse-feed your transformer, do not ground X0; the 208v side of your transformer will appear to be a delta connected load.

I personally hate corner grounded services, and hate ungrounded services even more.

Corner grounded services are a three-legged unicorn; no one's ever seen one in the wild. VFD's connected to corner grounded services can/will have issues with the supply.

Ungrounded services are hard to troubleshoot, many people are dangerously unfamiliar with them, and can float to weird phantom voltages that damage connected equipment. VFD's do NOT like ungrounded services, and will sustain damage. Ground detectors are a bad idea IMO, as they are often ignored or disabled.

Buy the correct transformer; 208v delta -to- 277/480Y. Install it correctly; bond the Y-point of the 480v side. You will sleep easier.


-rpoet

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Absolutely AVOID floating 480 volt systems, and absolutely AVOID corner grounded 480 volt systems. Although both of the systems are allowed, the are numerous caveats, and special rules to follow. These systems stupefy John Q Electrician, and maintenance personnel alike.

Buy a "step-up" transformer with a 3-wire low voltage primary, and a 4-wire high voltage secondary.

If you do go with Plan A, and back feed a step-down transformer, do NOT make any connection to X0 on the incoming low voltage side.

Floating 480 volt system - DON'T DO IT.

You have already said you are not happy about it, as others have said get a proper transformer or better yet get a proper 480 volt supply. If it is not something you have been trained to do, if it is something you don't know how to do, and if it something you don't know how to test and certify that it has been done safely then DON'T DO IT.

I used to work (in the UK) for an American company, we were regularly circulated safety reports from head office warning us about electrical accidents that had happened (usually in the USA). As far as we were concerned most of them fell into the what the heck were they doing category, what you are suggesting is one of those. It was rather alarming the level of accidents resulting from lack of knowledge and people doing what any electrician in the UK would consider to be both completely mad and totally against our electrical rules.

If you do use this transformer DO NOT GROUND THE X0 on the wye side. The X0 should be connected to the neutral of the 208 volt supply. The 208 volt supply is already grounded at the supply transformer/panel and should not be grounded anywhere else.