Whipsawed between the utility and the electrical inspector and the GFI requirement

[Paul the BIG Finn]

We requested a 3,000A, 480V delta service from the utility for a large industrial user. The utility service told the contractor they could not connect the green ground wires that had been pulled with the power conductors and also the ground conductors would have to be removed before they would connect the power conductors. They assured the contractor and only the power conductors were required for a delta service.

When the electrical inspector came to the site for final inspection he told the contractor that because the utility had installed a delta/wye transformer we would need a neutral connection, even though there was no neutral distribution in the building. In his opinion the ground fault relays on the service would not not have a true reference point and would not function without the neutral. Since it required for reference only, i.e. there is no unbalanced load on the neutral the contractor ask it we could we run a single reference conductor. The inspector cited NEC 250.24(C)(2) which refers to NEC 250.24(C)(1) which refers to NEC 250.102(C)(1) which references Table 250.66 with the addition "the supply-side bonding jumper shall have an area not less than 12 1/2% percent of the area of the largest set of ungrounded supply conductors."

In short, they are insisting we provide a full size neutral for the service in spite of the fact there will not be any current on that neutral.

I have searched the code and can't find an exception. Does anyone have any thoughts on this other than pulling in a 600 kCMIL neutral in each of the parallel conduits?

 

[infinity]

So the utility transformer is a WYE secondary not a Delta?

 

[Jraef]

So the utility transformer is a WYE secondary not a Delta?

Yeah, that makes all the difference.

Just because the user is not PLANNING on having any loads that require a neutral is NOT a good justification for a Delta-only service. Delta is now a really BAD idea for industrial plants because so much of what gets installed has power electronics, such as VFDs, UPS, other big 3 phase power supplies etc. Delta is the kiss of death for those devices. Utilities have been wary of allowing Delta services now because they tend to take the brunt of blame when VFDs blow up, so they are often insisting on a detailed analysis of WHY you insist on Delta. Saying "because there are no loads that need a neutral" no longer cuts it.

So, if they supplied a solidly grounded Wye secondary transformer, it needs to be bonded at the service entrance. You can chose NOT to run the neutral out from there, many places do that because that does depend on what you have as loads.

 

[texie]

Like infinity, I'm understanding you to say this is a grounded Y service. If so the inspector is correct, you must have the grounded conductor brought to the service and be at least as large as 250.102(C)1. There are very important reasons for this.

 

[Bigjabe]

Yup you don't need to carry a neutral conductor to your distribution or anything, but you do need to ground the Xo to your plant ground.

If this is an industrial customer with no 277V loads, you should really throw an NGR on that transformer - and even if you have 277V loads, the play then would be to get 480-277/480V transformers so just those are on a solidly grounded connection.

 

[electrofelon]

Just to clarify though, "delta" does not necessarily mean an actual delta connected secondary. We often use delta to mean "ungrounded". Utility could certainly supply an ungrounded service with a wye secondary transformer, and I would go out on a limb snd say that's what they would use for inventory purposes. First thing to do is clarify if a everyone is on the sam page about the grounding topology.

 

[infinity]

Just to clarify though, "delta" does not necessarily mean an actual delta connected secondary. We often use delta to mean "ungrounded". Utility could certainly supply an ungrounded service with a wye secondary transformer, and I would go out on a limb snd say that's what they would use for inventory purposes. First thing to do is clarify if a everyone is on the sam page about the grounding topology.

Doesn't 250.20 distinguish between the two?

 

[Ingenieur]

imho an ungrounded wye is not a delta
nor should it be called one

 

[electrofelon]

Doesn't 250.20 distinguish between the two?

I don't believe so. If I am not using the neutral as a circuit conductor, I don't see anything prohibiting me fron getting a three wire ungrounded service from the utility.

imho an ungrounded wye is not a delta
nor should it be called one

Would the user notice anything different between a 3 wire wye vs a three wire delta?

My chain of though is that if it originates from a wye supply and is supposed to be ungrounded, I could see a lineman accidently, out of habit grounding the wye point.

 

[Ingenieur]

I don't believe so. If I am not using the neutral as a circuit conductor, I don't see anything prohibiting me fron getting a three wire ungrounded service from the utility.


Would the user notice anything different between a 3 wire wye vs a three wire delta?

My chain of though is that if it originates from a wye supply and is supposed to be ungrounded, I could see a lineman accidently, out of habit grounding the wye point.

that is not the point
they are entirely different things
words matter
it confuses the issue

 

[electrofelon]

that is not the point
they are entirely different things
words matter
it confuses the issue

The utility is free to choose the winding configuration, it's on their side of the service point. I have heard they may supply an ungrounded service with a wye connected secondary. Do you disagree?

 

[Ingenieur]

The utility is free to choose the winding configuration, it's on their side of the service point. I have heard they may supply an ungrounded service with a wye connected secondary. Do you disagree?

yes
they may supply an ungrounded wye
but it's not called a delta

 

[Jraef]

I agree, utilities are careful about the semantics on this issue. They may give you and ungrounded service, but they would not call it Delta if it is Wye.

And from the standpoint of risk to line voltage power supplies like VFDs and Servos, that distinction is irrelevant. Ungrounded (or even resistance grounded) is the problem for them.

 

[electrofelon]

yes
they may supply an ungrounded wye
but it's not called a delta

Of course they are different. Common trade jargon is not always technically correct. 99% of costomer don't care about the winding configuration, they care if it's ungrounded. It's common in the field, depending on the context to call something that is not grounded "delta"

 

[Paul the BIG Finn]

So the utility transformer is a WYE secondary not a Delta?

Yes, the utility transformer's secondary is wye.

 

[Paul the BIG Finn]

Yup you don't need to carry a neutral conductor to your distribution or anything, but you do need to ground the Xo to your plant ground.

If this is an industrial customer with no 277V loads, you should really throw an NGR on that transformer - and even if you have 277V loads, the play then would be to get 480-277/480V transformers so just those are on a solidly grounded connection.

You are correct. The client requested a Delta service, The load letter requested a delta service, the utility called and questioned if we really wanted a delta service which the owner insisted they did, but the utility provided a transformer with a wye secondary citing inventory control. We did not find this out till the wire was already roughed into the vault.

 

[electrofelon]

You are correct. The client requested a Delta service, The load letter requested a delta service, the utility called and questioned if we really wanted a delta service which the owner insisted they did, but the utility provided a transformer with a wye secondary citing inventory control. We did not find this out till the wire was already roughed into the vault.

BINGO

This is also a perfect example of what I have been saying. You say "delta" , you never said ungrounded. Do you know if the left the wye ungrounded? If so, any reason you can't use an ungrounded wye?

 

[Ingenieur]

Of course they are different. Common trade jargon is not always technically correct. 99% of costomer don't care about the winding configuration, they care if it's ungrounded. It's common in the field, depending on the context to call something that is not grounded "delta"

I've been in the field for 30 years
dealing with utilities from the start (my internship was with Duquesne Light)
I've never heard a wye called a delta
agree to disagree
there are some differences such as how unbalances and harmonics are handled

 

[infinity]

Never mind.

 

[Paul the BIG Finn]

You are correct. The client requested a Delta service, The load letter requested a delta service, the utility called and questioned if we really wanted a delta service which the owner insisted they did, but the utility provided a transformer with a wye secondary citing inventory control. We did not find this out till the wire was already roughed into the vault.

You make a good point. They may have grounded the wye connection at the service transformer, either directly or through an NGR or it may be floating.