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changing a 3Ø 3W feeder to 3Ø 4W feeder #128313
01/14/03 04:25 AM
01/14/03 04:25 AM
Gwz  Offline OP
Joined: Apr 2002
Posts: 197
Scott 35,

I've looked at an installation, but do not have the prints, where it appears the premise is supplied by a 34.5KV to 4160V 3Ø 3W secondary load transformer and the 4160v is underground feeders to various remote loads on the premise.

At one location, a 150KVA 4160Y / 480Y transformer is installed on one of the feeders.

The Ho terminal is Bonded to the transformer case and to a ground rod within the pad mount slab opening. The 3 - 1/C - 4/0 Aluminum cable shields are also connected to the this Ho bonding (GEC?) to the ground rod.
The bonding jumper continues into the 480Y side of the pad mount transformer and is again connected to the case and to the Xo bushing with a factory bond to the Case.
Except for the HV cable shield(s), the ground rod bonding does not leave that transformer location.

There is not a Xo conductor with the 480V feeders into the building.

My question is:

If this is a 3Ø 3W delta system at the Service Point, can it be made a 4W system by grounding an Ho terminal on a transformer some 2000' feet away from the service point?

[This message has been edited by Gwz (edited 01-14-2003).]

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Re: changing a 3Ø 3W feeder to 3Ø 4W feeder #128314
01/14/03 03:17 PM
01/14/03 03:17 PM
Scott35  Offline

Broom Pusher and
Joined: Oct 2000
Posts: 2,708
Anaheim, CA. USA
Let me see if I have the setup correct:

This Y-Y stepdown Transformer is an Isolated Transformer, with the common starpoint on each side driven to earth ground?

Simply, the "Starpoint" (common connection point for the ends of each coil) on the Primary side (tagged as Hø) is bonded to a local GES - along with the drains from each input feeder conductor; and the Starpoint on the Secondary side (tagged as Xø) is also bonded to this local GES.

As far as I can make out, what you described to do for making this a 4 wire system should work, but again I am stumped for NEC compliance!
Also, will it be a problem not having the drains (Shields) grounded at the Transformer? I am thinking along the lines of Coupled EMF / Amperes.

I'll check my Engineering books regarding the operation stuff, then see if I can uncover something per NEC compliance.

Just FYI, if the system was left connected as it is, and a Grounded Conductor was derived from Xø, this would be a kind of similar to an MGN setup (Multi Grounded Neutral).

Need a Member that has much experience per these types of installations to chime in... (Please!).

I'll see what I can find for you.

Scott s.e.t.

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
Re: changing a 3Ø 3W feeder to 3Ø 4W feeder #128315
01/14/03 11:30 PM
01/14/03 11:30 PM
ElectricAL  Offline
Joined: Oct 2001
Posts: 597
Minneapolis, MN USA

The first transformer, the 34.5 kV / 4160, what is the configuration of the secondary. Is it wye or delta? As I understand your question, this is the service point, yes?


Al Hildenbrand
Re: changing a 3Ø 3W feeder to 3Ø 4W feeder #128316
01/15/03 05:13 PM
01/15/03 05:13 PM
Bjarney  Offline
Joined: Apr 2002
Posts: 2,527
West-Southern Inner-Northeast ...
On the “{34.5KV to} 4160V 3Ø 3W secondary,” is it a solidly-grounded-wye secondary, or something else? There may be a significant problem if it is not.

Can you post pictures of the two transformer labels: “34.5KV to 4160V 3Ø 3W transformer” AND “150KVA 4160Y / 480Y transformer”? Or, can you carefully record all the label information from them? The rules can change significantly above 600V.

If the 150kVA-transfomer secondary is anything other than a solidly-grounded-wye configuration, you cannot change it to 4 wire.

Also, exactly where is the transition between utility-owned equipment and customer-owned equipment?

[This message has been edited by Bjarney (edited 01-15-2003).]

Re: changing a 3Ø 3W feeder to 3Ø 4W feeder #128317
01/15/03 11:29 PM
01/15/03 11:29 PM
Gwz  Offline OP
Joined: Apr 2002
Posts: 197
Have been back at the site. The 4160V is a 3Ø 4W system.

Some of cable(s), I can not absolutely sure of their ratings, as they were apparently installed about 1983 and some being outside or in dirty enclosures are not readily read-able markings.

The premise power source is a 34.5KV .
Label 34.5KV / 4160Y / 2400V - 7500 KVA 4.2%Z.

The Service Point is 4160Y.

One of the HV feeders from the sub-station structure , through fused cut-outs, appear to have 1 - 1/C 4/0 Al HV cable per phase with shielded with concentric grounding ( neutral ) about 25' to a switching cubicle with 150A fuses. Then underground some 500 feet with the same type of 1/C shielded/concentric grounded conductor to a 2500KVA 4160V delta/480Y/277V 5.93%Z transformer.

The HV terminals on the 2500KVA transformer has been double-lugged with 1 - 1/C 4/0 Al 15KV shielded/concentric grounded conductor, per phase, ( rated 1/3rd of total ampacity ) about 700' underground to to a 500 KVA 4160Y/2400x12470/7200 to 480Y/277 which does have HV feed through terminals. This section cable was installed by the utility company and is the first point on this feeder that the grounded conductor has be utilized by connecting to the Ho terminal, a ground rod and to the Xo terminal. Both the Xo and the HO terminals are also bonded to the transformer case.

Then the same type 1/C 4/0Al 15KV shielded/concentric grounded conductor cables installed by the utility company continue 700' to a utility owned 150KVA 4160Y/2400x12470/7200 to 480Y/277V transformer. Again the Ho and the Xo terminals are bonded together and to a ground rod and of course, the shields/grounding conductors of each cable. The shields have been bonded to the concentric grounded conductor(s) at each transformer location.

Since the system is a 3Ø 4W systemn 4160V system, my original question is mute.

Now this 4160V circuit has been extended another 1000' with 1 - 1/C 4/0 Cu 5KV cable(s) with shield to a 150KVA 4160V delta / 208Y/120V transformer. The manufacture says the shield is not of ample size for a Grounded Conductor.

There is not grounded path ( other than the shields ) for the 4160V back to the the grounded conductor of the premise 3Ø 4W system to meet 310.6, 250.4(A)(5), 250.96,
250.122, 250.134(B) requirements.

Any comments on the last paragraph would be appreicated.

Re: changing a 3Ø 3W feeder to 3Ø 4W feeder #128318
01/16/03 04:56 PM
01/16/03 04:56 PM
Bjarney  Offline
Joined: Apr 2002
Posts: 2,527
West-Southern Inner-Northeast ...
[Sorry for the delay in followup.] A critical question—regardless of who installed it: “Are the cables and equipment under utility jurisdiction?”

Based on the 99NEC Art 250 Part K ≥1kV, there are what may be seemingly odd but historically acceptable methods such as grounding and bonding methods of transformer enclosures and windings. A big one is application of 250-2(d) Performance of Fault Current Path and typically fall under other but accepted IEEE/ANSI national-consensus standards that have evolved much like the NEC, but may differ somewhat from the National Electrical Code. Also, utilities can legally avoid practices like “industrial/institutional” gang-operated disconnecting means for transformer primaries, based on specific operational training of their personnel. A significant philosophical difference is that, for medium-voltage systems that are usually not considered premises wiring, the earth can be considered an acceptable fault-current path in utility-type distribution circuits.

If the shielded cables are of the jacketed, ⅓ -neutral URD type, then they are likely capable of acceptable performance during faults in limiting potential differences and facilitating overcurrent-device operation at the medium-voltage level. If they’re merely tape or drainwire shielded, an engineering evaluation should have been performed. In industrial circles, an additional ground conductor is characteristically included, routed with medium-voltage cables.

Having a combination of grounded-wye and delta- transformer primaries can complicate performance in fault conditions, but unless a grounded-wye-primary transformer has a delta secondary, there is no significant operational disadvantage.

It’s not clear whether the transformer or cable manufacturer objects to the cable construction, and specific and detailed clarification should be formally requested of them. Other than in a very general, offhand manner, it is doubtful whether a transformer manufacturer is in a position to make that judgment without knowing details of the whole installation. Fault studies and protective-device coordination determine suitability of a system, with transformers having wholly different considerations in the design and safety of the facilities at hand.

Another aspect may bear scrutiny. If indeed the transformers and primary overcurrent/disconnecting means are not under utility jurisdiction, then application of NEC Table 450-3(a), Overcurrent Protection for Transformers Over 600 Volts is likely enforceable.

Re: changing a 3Ø 3W feeder to 3Ø 4W feeder #128319
01/16/03 10:44 PM
01/16/03 10:44 PM
Gwz  Offline OP
Joined: Apr 2002
Posts: 197
It is my understanding that the Utiliy Company, by the ( Indiana Utility Requlatory Commission { IURC }) is permitted to have one (1) transformation on the premise.

Again, it is my understanding that the building served by the utility owned 150KVA transformer, was two or three years ago, served by an overhead pole installation.
The utility and the private premise owner chose to make that location the Utility owned transformation location. By the utility company extending the existing premise feeder that supplied the 2500KVA transformer to a new 500 KVA tranformer and on to the 150KVA transformer, the pole line was remove.
Then I understand, the Utility sold the new 15KV cable installation to the premise owner.
It is may understanding that the Utility now only own the 150 KVA 4160Y to 480Y/277V transformer at the end of the 15KV cable installation.

A contractor has installed some 1000' of 1 - 1/C 4/0 Cu, per phase, 5KV cable with only the shield ( not a grounding conductor ) from the 4160Y /480Y transformer to a new 4160 delta / 208Y tranformer. This new transformer primary FLA is approximately 20.8 Amps. It does have internal fuse links. The 2000' 4/0 Aluminum and 1000' 4/0 copper feeder is protected by 150 Amp fuses.

The cable maker's engineer says the Shield is not suitable as a grounding conductor.

There-in 310.6, 250.4(A)(5), 250.134(B) is the problem. The system starts out as 3Ø 4W 4160V.

[This message has been edited by Gwz (edited 01-16-2003).]

Re: changing a 3Ø 3W feeder to 3Ø 4W feeder #128320
01/16/03 11:53 PM
01/16/03 11:53 PM
Bjarney  Offline
Joined: Apr 2002
Posts: 2,527
West-Southern Inner-Northeast ...
Sounds like a reasonable approach to me.


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