GEC required for 208-480 delta transformer?

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greenspark1

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The contractors hooked a standard 480-208/120V transformer backwards to power a crane. Does it require a new GEC connected to the case? Seems a bit redundant to re-ground the case, but probably safe. I can't find a clear answer in the NEC.
 
Yes, a GEC is required. I assume you are asking because the 480V secondary is ungrounded. See 250.30(B)(1).
 
Smart $ said:
Yes, a GEC is required. I assume you are asking because the 480V secondary is ungrounded. See 250.30(B)(1).
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Wow I was scouring 250 for this application and missed these paragraphs. Thanks!

I'm still trying to wrap my head around it. So both the LV and HV equipment grounds get landed on the xfmr enclosure plus you have a GEC landed there?
 
greenspark1 said:
Wow I was scouring 250 for this application and missed these paragraphs. Thanks!

I'm still trying to wrap my head around it. So both the LV and HV equipment grounds get landed on the xfmr enclosure plus you have a GEC landed there?
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Exactly. Grounded and ungrounded systems grounding is essentially identical except there is no main or system bonding jumper with an ungrounded system.
 
kwired said:
You have a separately derived system, there is no ground reference on the secondary side until you make one.
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Well not exactly, the xfmr enclosure is already grounded via the equipment ground on the low side of the transformer. The new GEC from 250.32 just establishes a direct and lower impedance path to ground.
 
greenspark1 said:
Well not exactly, the xfmr enclosure is already grounded via the equipment ground on the low side of the transformer. The new GEC from 250.32 just establishes a direct and lower impedance path to ground.
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Your separately derived system has no ground reference until you make one, unless you intend to use it as an ungrounded system.

H1, H2, H3 (in your instance) are the secondary conductors and they have no connection to the transformer case.

Even if you were using it as a step down X0 usually doesn't have a connection to the case either until you make that bond. NEC gives you the option to make that bond at the transformer or at the first disconnecting means, and is part of why they seldom make that bond for you as is shipped.
 
kwired said:
Your separately derived system has no ground reference until you make one, unless you intend to use it as an ungrounded system.

H1, H2, H3 (in your instance) are the secondary conductors and they have no connection to the transformer case.

Even if you were using it as a step down X0 usually doesn't have a connection to the case either until you make that bond. NEC gives you the option to make that bond at the transformer or at the first disconnecting means, and is part of why they seldom make that bond for you as is shipped.
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I agree, but am talking about the EGC on the high side, not the neutral. The EGC is already grounded through the xfmr enclosure.

The bigger question for me is whether you have to ground one of the legs of the 480V delta. As-is, we have the 3 hot legs and the equipment ground feeding a 480V crane. I don't see this listed under the acceptable ungrounded systems of 250.21.
 
greenspark1 said:
I agree, but am talking about the EGC on the high side, not the neutral. The EGC is already grounded through the xfmr enclosure.

The bigger question for me is whether you have to ground one of the legs of the 480V delta. As-is, we have the 3 hot legs and the equipment ground feeding a 480V crane. I don't see this listed under the acceptable ungrounded systems of 250.21.
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That is exactly what I was trying to tell you. That 480 volt secondary has no reference to ground until you ground one of the phases.

If you don't ground one of them you have an ungrounded system. EGC does nothing for facilitating overcurrent protection if you don't ground one of the high side conductors. It is still necessary to bond things to prevent having voltage potential between those other items though. That is why GES and EGC's are still necessary on an ungrounded system. The first fault on an ungrounded system just makes it become a grounded system, it is when there is a second ground fault you get undesired and/or high level of current flow. Because of that ungrounded systems also require ground fault detection and indication to warn the user there is a problem. Equipment will continue to operate with just the first fault, but a second fault may result in operation of overcurrent devices and cause a shutdown.
 
kwired said:
That is exactly what I was trying to tell you. That 480 volt secondary has no reference to ground until you ground one of the phases.

If you don't ground one of them you have an ungrounded system. EGC does nothing for facilitating overcurrent protection if you don't ground one of the high side conductors. It is still necessary to bond things to prevent having voltage potential between those other items though. That is why GES and EGC's are still necessary on an ungrounded system. The first fault on an ungrounded system just makes it become a grounded system, it is when there is a second ground fault you get undesired and/or high level of current flow. Because of that ungrounded systems also require ground fault detection and indication to warn the user there is a problem. Equipment will continue to operate with just the first fault, but a second fault may result in operation of overcurrent devices and cause a shutdown.
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Yup, agree that OCPD won't do any good (until 2nd fault) if we're running an ungrounded system. Since we don't have a ground detector seems like we have to ground a leg. So then we have both a grounded hot leg and equipment ground running to the crane? Seems strange, but I don't typically do ungrounded systems. Is this correct?

2nd issue- Doesn't grounding a leg put the low voltage EGC at the same 480V potential via the xfmr case. Is this OK? Sorry if I'm getting this jumbled up.
 
greenspark1 said:
Yup, agree that OCPD won't do any good (until 2nd fault) if we're running an ungrounded system. Since we don't have a ground detector seems like we have to ground a leg. So then we have both a grounded hot leg and equipment ground running to the crane? Seems strange, but I don't typically do ungrounded systems. Is this correct?

2nd issue- Doesn't grounding a leg put the low voltage EGC at the same 480V potential via the xfmr case. Is this OK? Sorry if I'm getting this jumbled up.
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Yes you leave the point of the system bonding jumper with both a grounded conductor and an equipment grounding conductor. Just like you would do with a system with a neutral conductor. The current carrying grounded conductor is just that a grounded conductor. The equipment grounding conductor is bonded to the grounded conductor at the system bonding jumper but from that point on is not intended to carry current except for abnormal conditions.

Bond that grounded conductor somewhere downstream you have the same violation and similar safety issues as bonding a grounded neutral conductor in a similar situation.

Are you going to ground a phase? If not then you must have ground fault detection and indication equipment. Either way you run a 4th conductor as an equipment grounding conductor to bring all non current carrying metallic components to the same potential.


Grounding a leg puts that leg at ground potential. All three legs of your secondary are floating free in reference to ground until you ground one of them. POCO does the same thing when they step down to your facility. A wye system or single phase with a center tap usually gets the neutral connected as the grounded conductor, but it could be any conductor. A delta system, we generally call the B phase the one to be grounded, but you can ground any of the three. And you must have a A/B/C designation in the first place before you can demand grounding the B phase. Now in a switchboard or panelboard they may say B phase primarily because it is the center bus, regardless of what you might call it at the source.
 
kwired said:
Yes you leave the point of the system bonding jumper with both a grounded conductor and an equipment grounding conductor. Just like you would do with a system with a neutral conductor. The current carrying grounded conductor is just that a grounded conductor. The equipment grounding conductor is bonded to the grounded conductor at the system bonding jumper but from that point on is not intended to carry current except for abnormal conditions.

Bond that grounded conductor somewhere downstream you have the same violation and similar safety issues as bonding a grounded neutral conductor in a similar situation.

Are you going to ground a phase? If not then you must have ground fault detection and indication equipment. Either way you run a 4th conductor as an equipment grounding conductor to bring all non current carrying metallic components to the same potential.


Grounding a leg puts that leg at ground potential. All three legs of your secondary are floating free in reference to ground until you ground one of them. POCO does the same thing when they step down to your facility. A wye system or single phase with a center tap usually gets the neutral connected as the grounded conductor, but it could be any conductor. A delta system, we generally call the B phase the one to be grounded, but you can ground any of the three. And you must have a A/B/C designation in the first place before you can demand grounding the B phase. Now in a switchboard or panelboard they may say B phase primarily because it is the center bus, regardless of what you might call it at the source.
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A word of advise to the OP.
Be careful if your hesitant about the correct way to back feed a transformer.
They make transformers more tailored for specific application rather than having to backfeed a transformer out of convienience.

Get this wrong and this transformer will let you know in very short order.

JAP>
 
kwired said:
Yes you leave the point of the system bonding jumper with both a grounded conductor and an equipment grounding conductor. Just like you would do with a system with a neutral conductor. The current carrying grounded conductor is just that a grounded conductor. The equipment grounding conductor is bonded to the grounded conductor at the system bonding jumper but from that point on is not intended to carry current except for abnormal conditions.
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That makes a lot of sense. Thank you for the explanation. We will ground a high phase, bond @ xfmr, and run an EGC.

I just learned that contractors have obtained a real 208Y-480D step up transformer. I don't think this changes any of our discussion though.
 
greenspark1 said:
That makes a lot of sense. Thank you for the explanation. We will ground a high phase, bond @ xfmr, and run an EGC.

I just learned that contractors have obtained a real 208Y-480D step up transformer. I don't think this changes any of our discussion though.
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Delta/Delta more than likely.

JAP>
 
greenspark1 said:
I guess so. This xfmr is currently on site. If you suggest a different way to wire it up I'm all ears.
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If I had 208 volt available and wanted a 480v secondary in 15kva for instance, I wouldn't back feed a 480v Delta/ 120/208y secondary Transformer just because I happened to have it.

I'd be going with an equal to an Eaton V29M47T1516, 208v primary 480/277v secondary and bond the 480/277v secondary as I would normally and only use delta out of the 480v secondary if that's all I needed but that's just me.

JAP>
 
By the way if that number given previously was actually V48M28T1516 instead of V28M48T1516 it is an Eaton Transformer 480v delta primary to 120/208v Y secondary.

JAP>
 
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