Service Entrance verses Generator Feeder

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roger

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Looking for opinions of the design below, does anything look wrong?

MV.JPG


Roger
 

wirenut1980

Senior Member
Location
Plainfield, IN
Roger, just from the drawing alone:

It looks like the alternate feed is a resistance grounded wye, but the normal service is a solidly grounded wye. Is this the case and if so, is that kosher? Never seen that setup before, so I don't know.

Also, the number of poles on the transfer switch is not shown, is that specified elsewhere?

The normal supply transformer KVA is not shown.

Why are there four 5 kV cables on the normal supply service conductors, but three 5 KV on the generator service conductors? Seems only three would be needed for both.

Also the cables are not specified here whether copper or aluminum, single conductor or 3 conductor, so not sure on the ampacity.

I am guessing many of these questions are answered elsewhere. Other than that, looks ok to me!
 

roger

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Roger, just from the drawing alone:

It looks like the alternate feed is a resistance grounded wye, but the normal service is a solidly grounded wye. Is this the case and if so, is that kosher? Never seen that setup before, so I don't know.

Wirenut, you are correct and it is kosher, only the generator needs the NGR.

Also, the number of poles on the transfer switch is not shown, is that specified elsewhere?

Ooops, I should have included that, it is a three pole switch and there are no neutral loads, all loads are phase to phase.

The normal supply transformer KVA is not shown.

It is 2500 KVA

Why are there four 5 kV cables on the normal supply service conductors, but three 5 KV on the generator service conductors?
That is the question I have

Seems only three would be needed for both.
I agree. The 350 THWN is fine for the grounded conductor for both the utility and generator sides.



Also the cables are not specified here whether copper or aluminum, single conductor or 3 conductor, so not sure on the ampacity.

I am guessing many of these questions are answered elsewhere. Other than that, looks ok to me!

The cables are single conductor copper EPR 5/8kv 133/100% CT rated and are not a problem.


Roger
 

skeshesh

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Los Angeles, Ca
Interesting question Roger but I'm afraid the real answer is probably the most boring one: someone messed up the number of conductors while making the drawings and it slipped through...
 

roger

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Interesting question Roger but I'm afraid the real answer is probably the most boring one: someone messed up the number of conductors while making the drawings and it slipped through...

Skeshesh, you hit the nail on the head but the kicker is, they are trying to say it is correct.

I threw it out here wondering if someone may have come up with the same reasoning they are using to justify the design. ;)

Roger
 

bob

Senior Member
Location
Alabama
Skeshesh, you hit the nail on the head but the kicker is, they are trying to say it is correct.

I threw it out here wondering if someone may have come up with the same reasoning they are using to justify the design. ;)

Roger

One wild guess is that I have seen utilities require and EGC and neutral in the service. Maybe one of the 5 kv cables should have been noted as a 600 v cable.
When you find out please let us know. Also do you need to size the fused disconnect below the transfer sw? what does it feed?
 
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skeshesh

Senior Member
Location
Los Angeles, Ca
I threw it out here wondering if someone may have come up with the same reasoning they are using to justify the design. ;)

Roger

So you mean to say they provided a reason? I'd like to hear it if you don't mind (I guess I could wait a few hours so the others can participate in the fun guessing game!)
 

Mike01

Senior Member
Location
MidWest
I'll bite

I'll bite

Why 4#350 from the utility incoming xfmr. if the neutral ground bond is located in the main switchboard? (as noted already)

Are there any life safety loads? if so is the transfer device UL rated for the application of an automatic transfer switch? Does UL1008 or UL2200 apply?

If the system is resistance grounded is 600V insulation on the ground conductor adequate? (when the ground fault occurs dosen't the ground voltage become elevated??)

Is the incoming service "#C7" a customer or utility owned transformer.

Is 6"C required seems like 5" would be sufficent?

What about a conduit for generator controls (engine start signals, etc.)

Any overcurrent protection at the generator?

thats all I can think of for now...
 

roger

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Retired Electrician
One wild guess is that I have seen utilities require and EGC and neutral in the service. Maybe one of the 5 kv cables should have been noted as a 600 v cable.
When you find out please let us know. Also do you need to size the fused disconnect below the transfer sw? what does it feed?

Bob, in this case the utility does not require an EGC and in fact they have agreed with my stance that the grounded conductor only needs to be a bare conductor.

The fused disconnect is already sized, I had removed some of the non pertinent notes when I was cropping the drawing.

I will post their response to my question later.

Roger
 

roger

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So you mean to say they provided a reason? I'd like to hear it if you don't mind (I guess I could wait a few hours so the others can participate in the fun guessing game!)

Well, kind of, reason may not of been the best choice of wording :grin:

It may be tomorrow before I can post it, it was sent to me in an email at my work address and I will have to retrieve it from a remote server, I am posting from home at the moment.

Roger
 

roger

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Why 4#350 from the utility incoming xfmr. if the neutral ground bond is located in the main switchboard? (as noted already)

Are there any life safety loads? if so is the transfer device UL rated for the application of an automatic transfer switch? Does UL1008 or UL2200 apply?

Mike, this particular service and generator is strictly running large equipment and is only an Optional Standby System, there are six more large generators running other optional and emergency loads but they are all 480/277.

If the system is resistance grounded is 600V insulation on the ground conductor adequate? (when the ground fault occurs dosen't the ground voltage become elevated??)

You are correct and from the center of the star to the NGR the conductor is MV, on the secondary side of the NGR it is THWN.

Is the incoming service "#C7" a customer or utility owned transformer.

Utility owned.

Is 6"C required seems like 5" would be sufficent?

What about a conduit for generator controls (engine start signals, etc.)

Any overcurrent protection at the generator?

thats all I can think of for now...

There are many more conduits involved but they are not really in question, The OCPD at the generator is through CT's and relays.

Roger
 

benaround

Senior Member
Location
Arizona
On the c7 service, they bring in the N to the 1st disconnect to ground the system, or it

would be an ungrounded system. On the genset the HRG on the feeder is probally to limit

the GF current to the genset and to keep the process(s) going, but only 3 wires are needed

because only L-L loads are allowed on the HRG system. OK, so their reason is 250.186 A-D.

Roger, I have never worked on MV systems but I thought I would take a stab at trying to

answer this, as it is kind of a contest at this point. I'll be looking for the real answer

tomorrow.
 

roger

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Fl
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Here is their technical reason for the four MV conductors and one THWN from the service, note that the utility has already said they would have used a bare for the grounded conductor and no EGC is needed.

Roger
I talked with a few other electrical engineers here at .......... and below are responses to the questions you brought up while I was on-site on Wednesday.

1 ? The neutral (or grounded) conductor run from Incoming Service #C8 to MSB-C8, and Incoming Service #C7 to MSB-C7, must be the medium voltage (5 KV) cable type, as shown on Sheet E605. The ground conductor (or in this case supplemental ground) shall also be run from Incoming Service #C7 to MSB-C7, and Incoming Service #C7 to MSB-C7 (as shown on Sheet E605); however, this ground conductor shall not be installed within the 6? Conduit with the four (4) medium voltage cables for each service. This supplemental ground conductor shall connect the grounding system at the utility XFMR with the grounding system at the main switchboard (for each medium voltage service).

Per you current installation, you have not run a medium voltage cable for the (neutral) grounded conductor and the THWN/THHN ground conductor is in the 6? conduit (with the three medium voltage phase cables). Please correct both of these items before energizingthe CEP.

Maybe I'm missing something, but I don't see any technical or electrical reason (in case it was missed, I was being sarcastic in my opening sentence;)) for using an MV grounded conductor included in their reply.


I also don't understand what they are wanting to do with the "supplemental ground", (their term for this conductor) being that each end is bonded to the grounded conductor we have simply put the two conductors in parallel by connecting the padmount GE and our GES together.

Roger
 

skeshesh

Senior Member
Location
Los Angeles, Ca
This statement they sent you is a written equivalent of what the drawing shows. I think it's incorrent. If I send you an email writing 2+2 = 5, it wouldnt matter if I follow it with an official statement saying "Roger, please note that two plus two shall equal five" - it's still wrong.
 

Smart $

Esteemed Member
Location
Ohio
Bob, in this case the utility does not require an EGC and in fact they have agreed with my stance that the grounded conductor only needs to be a bare conductor
Since the generator/standby system is an impedance grounded neutral system, the neutral is required to be fully insulated with the same insulation as the phase conductors: ref. 250.186(B). I understand a neutral conductor is not run from the generator. However, because the transfer switch is three-pole with solidly-connected neutral, the neutral (grounded) conductor from the service must be equivalently insulated because the code does not relinquish the requirement when the source neutral is not run to the system disconnecting means.

As for the grounding conductor, I speculate this must be the engineers' concept of a Multigrounded Neutral System: ref. 250.184(C), and being it establishes a parallel neutral current path, they want it insulated from the conduit embedding concrete.
 

roger

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Fl
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Retired Electrician
This statement they sent you is a written equivalent of what the drawing shows. I think it's incorrent. If I send you an email writing 2+2 = 5, it wouldnt matter if I follow it with an official statement saying "Roger, please note that two plus two shall equal five" - it's still wrong.

And I agree with you whole heartedly.

Roger
 

roger

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Smart$, I will have to reply later, I am in the middle of a woodworking project with my grandson. :)

Roger
 

RUWired

Senior Member
Location
Pa.
Roger,I would call the supplemental ground conductor an equal potential grounding electrode conductor. This is also used for manhole grounding and for all the drains of the termination to be grounded to. I see this in multi building campus primary distribution systems.

The grounded conductor is in parallel but since these are service conductors, it is allowed up to the main service OCPD's.

Rick
 
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