Grounding of utility transformer

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Pete@KC

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Hello,

What is the correct way of wiring single phase pad-mounted utility transformer (100 kVA, 12.47 kV primary 120/240 V secondary, Z% 2.84)

Specifically what size of the grounding wire should be used (#6 bare Cu, #4 ?) and how is this determined?

the transformer is loop feed by #2 Al concentric.
 
As a utility [service] transformer, there is no secondary grounding wire. The GROUNDED conductor is used for grounding and bonding the enclosure and any other non-current-carrying metals parts. Its minimum size is the greater of an ampacity equal to or exceeding the maximum calculated unbalanced neutral current or what a grounding conductor per Table 250.66 would be sized if there was one. There also must be a grounding electrode at the transformer, connected to the grounded conductor, GEC sized per Table 250.66. If there is a primary grounded/grounding conductor, it gets effectively bonded to the secondary grounding, either directly or through enclosure bonding.

PS: Welcome to the forum :thumbsup:
 
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Pete@KC said:
Hello,

What is the correct way of wiring single phase pad-mounted utility transformer (100 kVA, 12.47 kV primary 120/240 V secondary, Z% 2.84)

Specifically what size of the grounding wire should be used (#6 bare Cu, #4 ?) and how is this determined?

the transformer is loop feed by #2 Al concentric.
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Its a very, very vague question, since a lot of the requirements are governed by the poco requirements they have in place. Also by the pad mount spec itself.

The bare concentric cable conductors around your URD are used for grounding and bonding as well as a supplemental ground rod (s) or ring (s). If the pad has one of the HV windings grounded to the tank the bare concentric neutral must be full sized. No 1/3 or reduced size. The secondary LV XO should then also have a bond strap to the tank. The elbow lightning suppressors if present should also be grounded. The best way to do this is have a large ground "loop" wire with the concentrics bonded to it the, the ground rods and the tank grounds. Double crimp if you can since its MV we are dealing with. Also, don't forget to ground the elbows correctly. The stress cones need to be appropriately grounded as well as the elbows, this is another explanation in itself.

Most pocos publish a book with specs that show diagrams on how to connect equipment and what gauges to use. This would be the first thing to look at.
 
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mbrooke said:
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Most pocos publish a book with specs that show diagrams on how to connect equipment and what gauges to use. This would be the first thing to look at.
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FWIW, my answer was based on the NEC and the service point being the transformer secondary terminals or before. Connection of primary to secondary grounding is typical of all service transformers (grounded systems). POCO published connection requirements cannot conflict with, but can exceed NEC requirements where the installation is under NEC purview.
 
Smart $ said:
FWIW, my answer was based on the NEC and the service point being the transformer secondary terminals or before. Connection of primary to secondary grounding is typical of all service transformers (grounded systems). POCO published connection requirements cannot conflict with, but can exceed NEC requirements where the installation is under NEC purview.
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Correct when the NEC applies, however, large investor owned pocos do not follow the NEC but rather the NESC or similar published standards. The NEC may apply to the OP however.
 
mbrooke said:
Correct when the NEC applies, however, large investor owned pocos do not follow the NEC but rather the NESC or similar published standards. The NEC may apply to the OP however.
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FWIW, I've never seen a pad-mount service transformer connected any differently than I posted earlier. Where solely under POCO/NESC purview, the only difference has been wire sizes and types.
 
First of all thank you for your input.

The ring connection with the bare copper wire is what has been suggested to me connects in the large ring transformer lug then at two points concentric neutrals, grounding rod, X2 secondery side, and back to transformer tank grounding lug.


So bare copper wire should be full sized (the concentric neutrals are 10 x #14 Cu being Equal to #4) but on poco manual I've seen #6 SDBC wire (for grounding) however no diagram with connections was provided by poco and this is where confusion and my original question is coming from.

since FLA of this transformer is about 8A (100kvA/12.47kV) and #6 has ampacity of 95A I understand that short-time ampacity in the fault conditions would play role. Have you ever come across table showing short-time ampacity of bare copper wire?

In this case the secondary cables are 4/0 Al so based on NEC tab 250.66 grounding conductor should be #4 Cu but what if the the cables are oversized for the voltage drop? Additionally I wasn't able to find in NESC article that would specify size of the grounding wire.

Based on your experience and knowledge is #6 bare Cu sufficient for ring connection?

I appreciate your comments.
 
Pete@KC said:
Sorry for the confusion, I meant #6 for the "loop connection" like mbrooke presented in his post not grounding ring (witch would mean it has direct contact with earth)
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Show me a diagram of this loop, what parts are under POCO/NESC or NEC pruview, then I'll comment further.
 
Pete@KC said:
Sorry for the quality...
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If only the secondary is under NEC purview, and the grounding electrode is just a single ground rod, then #6 is fine for the part between X2 and the rod. Also your enclosure to X2 bonding jumper will have to be #4. With this being the case, I'd just use #4 for the whole loop. BTW, looping is neither required nor [commonly] practiced under NEC application.
 
Smart $ said:
If only the secondary is under NEC purview, and the grounding electrode is just a single ground rod, then #6 is fine for the part between X2 and the rod. Also your enclosure to X2 bonding jumper will have to be #4. With this being the case, I'd just use #4 for the whole loop. BTW, looping is neither required nor [commonly] practiced under NEC application.
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Thank you for your help.

Should the secondary cable be 3/0 Aluminum would the bonding jumper between enclosure and X2 be #6 Cu? base on table 250.66 or do other factors play role?

If the bonding jumper is ok to be #6 the whole loop could be #6 BCSD.
 
Pete@KC said:
Thank you for your help.

Should the secondary cable be 3/0 Aluminum would the bonding jumper between enclosure and X2 be #6 Cu? base on table 250.66 or do other factors play role?

If the bonding jumper is ok to be #6 the whole loop could be #6 BCSD.
Click to expand...


If this pad is made for an MGN system (HV winding bonded to the tank), a bonding strap on the LV neutral bushing is a standard requirement when they leave the factory.

I would make the grounding ring bigger. Is this a feed through?
 
mbrooke said:
If this pad is made for an MGN system (HV winding bonded to the tank), a bonding strap on the LV neutral bushing is a standard requirement when they leave the factory.

I would make the grounding ring bigger. Is this a feed through?
Click to expand...

Its a single phase transformer loop feed, I don't believe there was a bonding strap in place.
 
Pete@KC said:
Thank you for your help.

Should the secondary cable be 3/0 Aluminum would the bonding jumper between enclosure and X2 be #6 Cu? base on table 250.66 or do other factors play role?

If the bonding jumper is ok to be #6 the whole loop could be #6 BCSD.
Click to expand...
Where the largest secondary conductor is 3/0 AL, a #6 CU bonding jumper is the required minimum.
 
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