480V 3PH Transformer fed by 208 high leg service

[xWYEz]

Greetings everyone. Noob here in a jam hoping the resident experts can guide me in the right direction.
DILEMMA: I have a powder coating business and recently acquired an industrial oven that requires 3PH 480V and draws 189A (151 KVA). I have the following transformer...
TRANSFORMER: 225 KVA step down, high voltage side = 480V Delta, low voltage side = 208Y/120
IN-COMING SERVICE: 120/240 w 208 high leg delta bank (400A service).

I do not own the building so I am after the most cost effective solution of while still being safe. Would be nice if I could use the oven and transformer I already have.

What I've done so far:
1. I've had an electrician come in and he said I do not have enough service to power the transformer, the oven, and my existing single phase items in my shop since the oven will draw 400A by itself and that's my total service currently.
2. I had the POCO come out and they said they could increase the service, however I have phase to ground 208 3ph service and I need phase to ground 240 3ph to go to the transformer.

I have attached a pic of the transformer data plate. Any help is greatly appreciated.

 

[Little Bill]

I'm allowing this thread but only to the extent that posters keep the discussion towards advise on sizing and not actual install instructions.


I would probably have a couple different electricians come by that are experienced with transformers and larger voltages. Check around, especially at supply houses and they can usually suggest someone.

 

[augie47]

1st: Forget your transformer. Under the '17 Co0de (shown in your profile) you are no allowed to "back-wire" a transformer unless it's designed for such.
To me, the answer would be with POCO. If they will only provide you with more capacity on you 208v service, obtain the correct transformer to step that to 480v. Since POCO is looking at increasing capacity, you might be able to get them to install a 2nd service at 480v.

 

[xWYEz]

1st: Forget your transformer. Under the '17 Co0de (shown in your profile) you are no allowed to "back-wire" a transformer unless it's designed for such.
To me, the answer would be with POCO. If they will only provide you with more capacity on you 208v service, obtain the correct transformer to step that to 480v. Since POCO is looking at increasing capacity, you might be able to get them to install a 2nd service at 480v.

In regards to transformers designed for back-feeding. Isn’t that a slippery slope? I’ve researched and been told that any step down can be back-fed, however transformer manufacturers are reluctant to put it in writing.

I discussed a step up transformer that was 225 kva, 208v primary (line to line), 480v secondary (line to line), 277v (line to Neutral) with POCO and he stated even using that transformer my current 208 high-leg service would not work. This is what I did not quite understand. I guess he was saying my current service line to line is 240v which no transformer exists that uses that???

Getting a second service at 480v might be an option, however, I’m not sure if the slum-lord would approve since the electric bill is included in my rent (he foots the bill). At this point I can only ask, but I’m sure a second service will be pricey.

Thanks for the responses so far guys.

 

[electrofelon]

In regards to transformers designed for back-feeding. Isn’t that a slippery slope? I’ve researched and been told that any step down can be back-fed, however transformer manufacturers are reluctant to put it in writing.

There are several potential issues with backfeeding a transformer: 1) inrush is often higher making it sometimes harder to energize without tripping the primary OCPD. (depending on the application, really not that big a deal if you are willing to try a few times till it holds) 2) as mentioned the new code required a transformer be marked as suitable for backfeed. This is just a technicality really and shouldnt be a code IMO, there is no safety issue 3) There will not be a neutral point on the secondary side thus the secondary system has to be either ungrounded with fault detectors or corner grounded which some equipment such as VFD's may not like. An oven is probably fine.

I discussed a step up transformer that was 225 kva, 208v primary (line to line), 480v secondary (line to line), 277v (line to Neutral) with POCO and he stated even using that transformer my current 208 high-leg service would not work. This is what I did not quite understand. I guess he was saying my current service line to line is 240v which no transformer exists that uses that???

Your current service is 240 phase to phase. The high leg is 208V phase to ground. You dont feed a transformer with the neutral/grounded conductor just the phases, so the high leg doesnt come into play - you need a 240V delta to 480/277 wye transformer.

Getting a second service at 480v might be an option, however, I’m not sure if the slum-lord would approve since the electric bill is included in my rent (he foots the bill). At this point I can only ask, but I’m sure a second service will be pricey.

I would think the new 480 service is the way to go. Since the 240 service would need to be upgraded anyway, I dont see why it would cost any more for the new 480, but utilities sometime have strange pricing structures, i.e. some will charge a lot for a second service and not help you out with the capital cost at all. I installed a 200A 480 service recently for some 480 equipment, and the POCO charged the customer 17k, and I charged the client 7k.

 

[winnie]

You have a 240V service. Your service happens to have a parasitic 208V that you can't use for anything. You can't use a 208V transformer with it.

You need a 240V to 480V transformer. They certainly exist but may be less common and less available on the used market.

You might want to have your electrician look at the oven schematic. Perhaps the heating elements could be reconfigured for 240V operation.

Jon

 

[ptonsparky]

In regards to transformers designed for back-feeding. Isn’t that a slippery slope? I’ve researched and been told that any step down can be back-fed, however transformer manufacturers are reluctant to put it in writing.

I discussed a step up transformer that was 225 kva, 208v primary (line to line), 480v secondary (line to line), 277v (line to Neutral) with POCO and he stated even using that transformer my current 208 high-leg service would not work. This is what I did not quite understand. I guess he was saying my current service line to line is 240v which no transformer exists that uses that???

Getting a second service at 480v might be an option, however, I’m not sure if the slum-lord would approve since the electric bill is included in my rent (he foots the bill). At this point I can only ask, but I’m sure a second service will be pricey.

Thanks for the responses so far guys.

Your electric bill is included in the rent and you are going to significantly increase the load? Expect a significant increase in rent. Talk to your LL before you commit to anything. Slum-lord or not he should know and you may end up having to leave whatever you have installed when he kicks you out. (I don't know the terms of your lease or the laws of your State.)

There are reasons for not back feeding a transformer but someone else will have to explain them.

 

[LarryFine]

Possibility: Change service to 480, and use this transformer to lower 480 to serve existing equipment.

 

[ptonsparky]

Possibility: Change service to 480, and use this transformer to lower 480 to serve existing equipment.

Better option.

 

[kwired]

Greetings everyone. Noob here in a jam hoping the resident experts can guide me in the right direction.
DILEMMA: I have a powder coating business and recently acquired an industrial oven that requires 3PH 480V and draws 189A (151 KVA). I have the following transformer...
TRANSFORMER: 225 KVA step down, high voltage side = 480V Delta, low voltage side = 208Y/120
IN-COMING SERVICE: 120/240 w 208 high leg delta bank (400A service).

I do not own the building so I am after the most cost effective solution of while still being safe. Would be nice if I could use the oven and transformer I already have.

What I've done so far:
1. I've had an electrician come in and he said I do not have enough service to power the transformer, the oven, and my existing single phase items in my shop since the oven will draw 400A by itself and that's my total service currently.
2. I had the POCO come out and they said they could increase the service, however I have phase to ground 208 3ph service and I need phase to ground 240 3ph to go to the transformer.

I have attached a pic of the transformer data plate. Any help is greatly appreciated.

Upgrade your service to 480 volts. Install necessary gear to supply your new load plus the transformer. Use transformer to supply what is existing. Note it will be 208/120 instead of 120/240 when done. May need to change/adjust other items if they are not compatible with 208 volt supply vs 240 volt supply, most loads will work though.

Your electrician was right, your new equipment uses nearly all the capacity you currently have, you have no choice but to increase capacity no matter what route you take.

 

[xWYEz]

You guys are awesome!! I feel like I just learned more in this thread than in all discussion I’ve had with my electrician & poco combined.

Im going to price out the secondary 480V service route since that would completely eliminate the transformer.

I’ll also price out trying to source a 240V - 480V/277V transformer and increasing my service.

Once I decide on which way I decide to go, I will definitely discuss with the landlord. Never know, he may be against all of it.

I’ll keep you all posted regardless. The knowledge level on this forum is something to behold.

 

[xWYEz]

:rotflmao: Good point!! Very good point indeed!!

Your electric bill is included in the rent and you are going to significantly increase the load? Expect a significant increase in rent. Talk to your LL before you commit to anything. Slum-lord or not he should know and you may end up having to leave whatever you have installed when he kicks you out. (I don't know the terms of your lease or the laws of your State.)

There are reasons for not back feeding a transformer but someone else will have to explain them.

 

[xWYEz]

Single or three phase 240 Delta transformer?

Single or three phase 240 Delta transformer?

So are you saying I need a “single phase” 240V delta primary to 480V/277V wye secondary transformer since the 208V high leg is useless? If that’s so then I would have to get a phase converter to convert that 480V single-phase to 480V three-phase am I correct?

There are several potential issues with backfeeding a transformer: 1) inrush is often higher making it sometimes harder to energize without tripping the primary OCPD. (depending on the application, really not that big a deal if you are willing to try a few times till it holds) 2) as mentioned the new code required a transformer be marked as suitable for backfeed. This is just a technicality really and shouldnt be a code IMO, there is no safety issue 3) There will not be a neutral point on the secondary side thus the secondary system has to be either ungrounded with fault detectors or corner grounded which some equipment such as VFD's may not like. An oven is probably fine.



Your current service is 240 phase to phase. The high leg is 208V phase to ground. You dont feed a transformer with the neutral/grounded conductor just the phases, so the high leg doesnt come into play - you need a 240V delta to 480/277 wye transformer.



I would think the new 480 service is the way to go. Since the 240 service would need to be upgraded anyway, I dont see why it would cost any more for the new 480, but utilities sometime have strange pricing structures, i.e. some will charge a lot for a second service and not help you out with the capital cost at all. I installed a 200A 480 service recently for some 480 equipment, and the POCO charged the customer 17k, and I charged the client 7k.

 

[winnie]

So are you saying I need a “single phase” 240V delta primary to 480V/277V wye secondary transformer since the 208V high leg is useless? If that’s so then I would have to get a phase converter to convert that 480V single-phase to 480V three-phase am I correct?

Not quite. You have a three phase 240V delta, and you can convert it three phase 480V with a 3 phase transformer. The 'parasitic' 208V is between one of the 240V terminals and ground. You can use this terminal along with the other phase terminals to supply 3 phase power, but you generally should not connect loads between this terminal and the grounded neutral.

AFAIK there is no explicit prohibition against using this 208V but generally it simply is not used.

-Jon

 

[xWYEz]

Not quite. You have a three phase 240V delta, and you can convert it three phase 480V with a 3 phase transformer. The 'parasitic' 208V is between one of the 240V terminals and ground. You can use this terminal along with the other phase terminals to supply 3 phase power, but you generally should not connect loads between this terminal and the grounded neutral.

AFAIK there is no explicit prohibition against using this 208V but generally it simply is not used.

-Jon

So basically a transformer like this?

 

[electrofelon]

So are you saying I need a “single phase” 240V delta primary to 480V/277V wye secondary transformer since the 208V high leg is useless? If that’s so then I would have to get a phase converter to convert that 480V single-phase to 480V three-phase am I correct?

I think Jon clarified. Sorry, some of my wording may have been a bit sloppy. Yes the picture you posted looks like it should do it. A "240 Delta" transformer can be fed from either a 120/240 high leg system or a 240/138 wye system (although the latter is rare, just making a point).

 

[xWYEz]

Thank you Jon & Ethan for the transformer explanation. It appears to also be a proper step-up transformer and if so, would eliminate the need for corner grounding or a ground fault system correct?

 

[electrofelon]

Thank you Jon & Ethan for the transformer explanation. It appears to also be a proper step-up transformer and if so, would eliminate the need for corner grounding or a ground fault system correct?

Correct. Because it has wye connected windings, that point would (almost always) be grounded. It would then be a grounded neutral conductor. Even if you didn't need the neutral for anything, it's still nice to ground the system there for standardization, and the balanced lower phase to ground voltage.

 

[kwired]

If utility won't supply you with two services, some will some won't, convert your service to 480 volts and use the transformer you already have to supply what is existing, as I mentioned before you will drop from 240 to 208 for line to line volts, so make sure that is acceptable with what you are already running.

Either way you need new service gear or add to what you already have.

Another option is keep same voltage, have them increase transformer size, unless they think they can supply you with what is there, and add a second service disconnect to supply your transformer if you end up going that route. Basically need to add a second 400 amp service disconnect that is grouped with the existing 400 amp service disconnect.

 

[winnie]

kwired makes a good point, the transformer that you have would do a pretty good job of taking 480V and converting it to supply your existing loads. (Not perfect, however, because the output would be 208/120V, not 240V, so it would serve all of your 120V loads perfectly, but your 240V loads would see low voltage. Some can tolerate this, some cannot...and some could be converted to operate on 480V).

I am also going to re-state: having an electrician look at the oven to see if it can be reconfigured for 240V use. This would open the option of increasing the size of the 240V service without having to find a proper transformer.

-Jon