# Wiring a Transformer backwards



## jayare813 (Mar 12, 2009)

I have120/208 at my Jobsite. I need 277/480 to supply a temp material hoist. My shop has a step down 480 primary to 208 secondary transformer. They want me to use this transformer and wire it reversed. My primary being my load for my hoist and my secondary being my line from my mdp. Voltages will still match the transformers rating. Just the line and load will be reversed. Any one installed a transformer this way? Any thoughts?


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## 480sparky (Sep 20, 2007)

You won't have a neutral on the (original) primary side, which will end up being your secondary.


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## wwilson174 (Apr 25, 2008)

jayare813 said:


> I have120/208 at my Jobsite. I need 277/480 to supply a temp material hoist. My shop has a step down 480 primary to 208 secondary transformer. They want me to use this transformer and wire it reversed. My primary being my load for my hoist and my secondary being my line from my mdp. Voltages will still match the transformers rating. Just the line and load will be reversed. Any one installed a transformer this way? Any thoughts?


All single phase?


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## jayare813 (Mar 12, 2009)

I understand that concept just wondering if i should be concerned with it going bad. Being wired differently


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## jayare813 (Mar 12, 2009)

Three phase


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## guest (Feb 21, 2009)

No it won't go bad. 

Be aware that the inrush on the new "primary" side will be pretty high, you'll need to size your 208 breaker accordingly.


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## brian john (Mar 11, 2007)

1. You will only be able to feed 3 phase 480 VAC and single phase 480 VAC as Ken said no neutral.

Additionally you will need to corner ground the transformer or install ground alarms.

Higher inrush as noted, and a few other minor drawbacks

Or you could order a 208 delta to 480/277 wye.


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## btharmy (Jan 17, 2009)

Don't bond "XO" if you choose to go this route. I removed these from 2 "back fed" xfmrs after a single phasing incident. The smoke from the melting insulation set off the F.A. in the building. Yes, the one on the left melted completely in two!


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## Cl906um (Jul 21, 2012)

btharmy said:


> Don't bond "XO" if you choose to go this route. I removed these from 2 "back fed" xfmrs after a single phasing incident. The smoke from the melting insulation set off the F.A. in the building. Yes, the one on the left melted completely in two!


I am wondering why you wouldn't want to bond xo on the low side? I call it this for less confusion, but it still is wound as a wye. I always thought you still had to bond xo. Kinda confused now.


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## nolabama (Oct 3, 2007)

brian john said:


> 1. You will only be able to feed 3 phase 480 VAC and single phase 480 VAC as Ken said no neutral.
> 
> Additionally you will need to corner ground the transformer or install ground alarms.
> 
> ...


Can you explain grounding the corner. What specific lugs get bonded to ground when this is done.


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## Cl906um (Jul 21, 2012)

H2 grounded b


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## RIVETER (Sep 26, 2009)

jayare813 said:


> I have120/208 at my Jobsite. I need 277/480 to supply a temp material hoist. My shop has a step down 480 primary to 208 secondary transformer. They want me to use this transformer and wire it reversed. My primary being my load for my hoist and my secondary being my line from my mdp. Voltages will still match the transformers rating. Just the line and load will be reversed. Any one installed a transformer this way? Any thoughts?


Never have done it but a transformer does not know if it is a step up...or a step down. Just wire it and if the current on your secondary is okay, all should be well.


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## BBQ (Nov 16, 2010)

btharmy said:


> Don't bond "XO" if you choose to go this route. I removed these from 2 "back fed" xfmrs after a single phasing incident. The smoke from the melting insulation set off the F.A. in the building. Yes, the one on the left melted completely in two!


Yes, this is what can happen for sure.


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## brian john (Mar 11, 2007)

cl219um said:


> I am wondering why you wouldn't want to bond xo on the low side? I call it this for less confusion, but it still is wound as a wye. I always thought you still had to bond xo. Kinda confused now.


For one it is a code violation grounding a system down stream from the main neutral ground bond.

The current imbalance that can flow on this connection (I BELIEVE THIS IS WHY)exceeds the sizing requirements for the conductor.


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## BBQ (Nov 16, 2010)

brian john said:


> The current imbalance that can flow on this connection (I BELIEVE THIS IS WHY)exceeds the sizing requirements for the conductor.


I don't understand it fully but when you bond XO as long as the supply is either on or off things will be fine. But if the supply loses a phase the current starts to flow on the XO connection and yes at much higher current than that conductor is sized for. But even if it was sized larger you would not want (as you already know) current to be flowing on the bonding jumper to the GEC / EGCs.


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## RandyM (Apr 5, 2012)

When hooking up a transformer backwards you do not bond the X0 or connect to the neutral. Some transformers have a label to remind you.
X1, X2 & X3 are your 208 inputs. H1, H2 & H3 are your 480 outputs.

Ground one output leg. You will have 0 volts, 480 volts & 480 volts to Ground. Will drive the lift repair man crazy unless he has seen a corner ground system.

You can order a 208 to 277/480 transformer to get your standard Y output.


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## MaxFuse (Oct 23, 2011)

just dont let the smoke out.


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## Cl906um (Jul 21, 2012)

brian john said:


> For one it is a code violation grounding a system down stream from the main neutral ground bond.
> 
> The current imbalance that can flow on this connection (I BELIEVE THIS IS WHY)exceeds the sizing requirements for the conductor.


But I am just set on thinking if you hook up a wye system and something happens to it, the only path is phase to phase. MAybe I should not have said to bond xo, but to bring neutral to it. Xfmers are not smart.they can deliver unlimited amps until they pretty much blow up or start on fire. I do agree on you with getting the right one for its intended use.


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## Cl906um (Jul 21, 2012)

Why do you bring neutral to a wye service even if you don't use it? Fault current.


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## brian john (Mar 11, 2007)

cl219um said:


> But I am just set on thinking if you hook up a wye system and something happens to it, the only path is phase to phase. MAybe I should not have said to bond xo, but to bring neutral to it. *Xfmers are not smart.they can deliver unlimited amps until they pretty much blow up or start on fire.* I do agree on you with getting the right one for its intended use.


Transformers CANNOT deliver unlimited amps, they are limited on the primary by what the supply source can deliver and the impedance of the distribution system, they are limited on the secondary by the supply source and the impedance of the transformer.

I need to check this, but I am sure you do not need the neutral on the primary/input (208 VAC).

You really are not really back feeding/reverse feeding the transformer, you are supplying a transformer primary and obtaining a need distribution voltage on the secondary. 


Nothing can deliver unlimited amps, we are limited by the source and what it can provide instantaneously during a fault.


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## papa (Sep 15, 2011)

What would happen if you did not corner grd. the transformer ? What is a grd. alarm ?


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## brian john (Mar 11, 2007)

papa said:


> What would happen if you did not corner grd. the transformer ? What is a grd. alarm ?


If you did not ground the transformer, you would have an ungrounded system and everything would work fine.

When you have the first ground fault, you would then have a corner grounded system, but the ground would be remote from the transformer.

Ground alarms can be as simple as lights tied to each phase, with an inline resistor to prohibit the lamp from lighting under normal conditions (due to capacitance coupling*). When you have a fault the lamp lights on the grounded phase.

*I THINK or as I understand it.

Or this
http://www.bender.org/ground_fault_for_ungrounded_large_systems.aspx


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## wwilson174 (Apr 25, 2008)

brian john said:


> If you did not ground the transformer, you would have an ungrounded system and everything would work fine.
> 
> When you have the first ground fault, you would then have a corner grounded system, but the ground would be remote from the transformer.
> 
> ...


 
A simple ground detector consists of three lamps connected in a wye configuration with the center point grounded,under normal conditions the lamps burn dimly, when a phase wire becomes grounded the lamp on that phase goes out and the other two lamps burn brightly.


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## BBQ (Nov 16, 2010)

wwilson174 said:


> A simple ground detector consists of three lamps connected in a wye configuration with the center point grounded,under normal conditions the lamps burn dimly, when a phase wire becomes grounded the lamp on that phase goes out and the other two lamps burn brightly.


That is certainly a great old school method, I brought it up on a forum before and the EEs were horrified, called it unsafe, unprofessional and insisted you must buy a listed ground detector.


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## wwilson174 (Apr 25, 2008)

wwilson174 said:


> A simple ground detector consists of three lamps connected in a wye configuration with the center point grounded,under normal conditions the lamps burn dimly, when a phase wire becomes grounded the lamp on that phase goes out and the other two lamps burn brightly.


The Ground Detection Lights are designed for operation on a 3 Phase, Ungrounded System or Hi Resistance Grounded System. It will not work on any type of Solid Grounded System.

The 3 lights are wired in a “WYE” Configuration with the center of the “WYE” grounded.

The Lights are rated at the Full Voltage of the system, thus a 480V System requires 480V Pilot Lights.

In normal operation all 3 lights will be at approx 57% Brightness. 

When one phase of the power system shorts to ground, the Light the Ground is on will go out, the other 2 lights will be at Full Brightness.


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## brian john (Mar 11, 2007)

wwilson174 said:


> The Ground Detection Lights are designed for operation on a 3 Phase, Ungrounded System or Hi Resistance Grounded System. It will not work on any type of Solid Grounded System.
> 
> The 3 lights are wired in a “WYE” Configuration with the center of the “WYE” grounded.
> 
> ...


While I agree, Bob's poin t was and is



> That is certainly a great old school method, I brought it up on a forum before and the EEs were horrified, called it unsafe, unprofessional and insisted *you must buy a listed ground detector*


I would bet your system was utilized for years and then some one came up with a ground detector that listed and, thats the way it is.

On the system I noted I an almost positive I saw a ground detector system installed like this, but I have made at least one mistake in my life, this could be number 2?


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## BBQ (Nov 16, 2010)

brian john said:


> While I agree, Bob's poin t was and is
> 
> 
> 
> ...


The thing is, there is nothing in the NEC that requires the ground fault indicators to be listed. 


The lamp method works fine if someone is keeping an eye on it. The few times I have seen them they were in prominent spots that mantainace personal would see many times a day.


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## guest (Feb 21, 2009)

BBQ said:


> That is certainly a great old school method, I brought it up on a forum before and the EEs were horrified, called it unsafe, unprofessional and* insisted you must buy a listed ground detector*.


I'd tell them to look up the theory and go pound sand. Ain't nothing wrong with the light bulb method. 



brian john said:


> While I agree, Bob's poin t was and is
> 
> 
> 
> ...


Bob beat me to it:



BBQ said:


> The thing is, there is nothing in the NEC that requires the ground fault indicators to be listed.
> 
> 
> *The lamp method works fine if someone is keeping an eye on it. *The few times I have seen them they were in prominent spots that mantainace personal would see many times a day.



And consider the possible failure modes: If a bulb burns out it would be obvious as it would be the only one NOT glowing at reduced intensity. 

And if...well that's it. If any bulbs burn out they would be dark *without* any other bulbs being bright, so it pretty much tells you the fault is you need a new bulb. 

Now do those new fancy "ground fault indicators" have such a foolproof method of telling you if they suffer an internal breakdown? Doubt it. :no:

I would not hesitate at all to use the lamp method on a new installation. Simple, cheap, foolproof.


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## geomx (Aug 27, 2014)

Dont forget to corner ground, bond H2, B phase, to case ground on the transformer. Without this imporatant part, the conduit and all the equipment could become energized without a ground path. Just found this problem at local machine shop. Very dangerous. As someone mentioned earlier it will drive the start up guys crazy as you will have H1, 480 volt to ground, H2, 0 volt to ground, and H3, 480 volt to ground. Across each phase will be 480 volt.


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## Meadow (Jan 14, 2011)

mxslick said:


> No it won't go bad.
> 
> Be aware that the inrush on the new "primary" side will be pretty high, you'll need to size your 208 breaker accordingly.


Good point. Inrush may be higher, which often gets left out of the equation.


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## Meadow (Jan 14, 2011)

And I fell for a 2 year old thread...


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## guitarchris (Jan 16, 2009)

does anyone have a picture of a back fed trans corner bonded? I'm doing my first one like this and would like to see one all wired up!


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## Magnettica (Jan 23, 2007)

jayare813 said:


> I have120/208 at my Jobsite. I need 277/480 to supply a temp material hoist. My shop has a step down 480 primary to 208 secondary transformer. They want me to use this transformer and wire it reversed. My primary being my load for my hoist and my secondary being my line from my mdp. Voltages will still match the transformers rating. Just the line and load will be reversed. Any one installed a transformer this way? Any thoughts?


If it's not rated to do that you're in violation of the NEC. :whistling2:


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## McClary’s Electrical (Feb 21, 2009)

Magnettica said:


> If it's not rated to do that you're in violation of the NEC. :whistling2:


Any transformer can be wired backwards


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## RIVETER (Sep 26, 2009)

mcclary's electrical said:


> Any transformer can be wired backwards


Yes, but can you wire a three way switch backwards?


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## rocky01 (6 mo ago)

Does having a high leg effects the secondary on reverse feeding a transformer?


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## just the cowboy (Sep 4, 2013)

rocky01 said:


> Does having a high leg effects the secondary on reverse feeding a transformer?


FYI if you want to step up buy the correct transformer, it needs to state it can be used for step up now. 
OP was for an test situation, same answers apply with regards to grounds.
You should start a new thread if you want more information on this. It is an old one and may not get answered.


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## rocky01 (6 mo ago)

Thanks for the feedback. I've called 4 different suppliers. I know it's rare but they don't even know where to order one. Shm..


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## just the cowboy (Sep 4, 2013)

rocky01 said:


> Thanks for the feedback. I've called 4 different suppliers. I know it's rare but they don't even know where to order one. Shm..


Don't know what size you need but here are examples.
Platt electric.


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## micromind (Aug 11, 2007)

rocky01 said:


> Does having a high leg effects the secondary on reverse feeding a transformer?


The high leg does not matter at all provided there is absolutely nothing connected to the neutral.

The high leg is higher voltage only when referenced to neutral/ground. If the neutral is unconnected then you have a 240 3Ø 3 wire system.


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