Proper Grounding Connections of Customer Owned Wye-Wye Service Transformer

[DetroitEE]

The title says it all. I'm confusing myself trying to figure out all of the proper grounding connections for a wye-wye service transformer that will by owned by the customer, not the utility. This is an outdoor padmounted transformer that will feed a switchboard inside the building. Utility is 13.2Y/7.6kV grounded wye. Secondary is 480Y/277V. My questions are as follows:

1. Are both neutrals (primary and secondary) typically connected to the steel case of the transformer and the grounding rods at the transformer? It seems to me like all of these connections would be bonded together inside the transformer.

2. If the secondary neutral is connected to the ground inside the service transformer (main bonding jumper connection), then the neutral and ground should be kept separate from that point on, correct? In other words, the connection from the neutral to the ground bus inside the switchboard should be removed.

3. Does the fact that this is a customer owned transformer change how these connections are made vs if this were owned by the utility?

 

[DetroitEE]

Anyone?

 

[RB1]

DetroitEE,

The HO/XO terminal for some wye-wye transformers are physically the same terminal. If not they should be bonded together. If you choose to bond at the source rather than the switchboard, the neutral and ground should be separated at the switchboard. Where a parallel path with the neutral is not created (nonmetallic raceways) you can bond the neutral at both the transformer and the switchboard (250.30(A)(1) Ex. 2). In the 2011 Code a system bond is required at the transformer where the transformer is installed outdoors (250.30(C)).

For utility transformer installations: The utility here will bond HO and XO period. The neutral disconnecting link would not be permitted to be removed at the service equipment. That are the only differences that I am aware of.

 

[DetroitEE]

DetroitEE,

The HO/XO terminal for some wye-wye transformers are physically the same terminal. If not they should be bonded together. If you choose to bond at the source rather than the switchboard, the neutral and ground should be separated at the switchboard. Where a parallel path with the neutral is not created (nonmetallic raceways) you can bond the neutral at both the transformer and the switchboard (250.30(A)(1) Ex. 2). In the 2011 Code a system bond is required at the transformer where the transformer is installed outdoors (250.30(C)).

For utility transformer installations: The utility here will bond HO and XO period. The neutral disconnecting link would not be permitted to be removed at the service equipment. That are the only differences that I am aware of.

Thanks for the help. I found another post on here regarding 250.30(C) that has me confused. See post #4 http://forums.mikeholt.com/showthread.php?t=139423 Don states that a second connection between the grounded conductor and the earth at the structure served is not permitted. Can anyone verify this is correct?

I had planning on putting a ground bar in the main electrical room and connecting all of the usual bonding connections to it: (structural steel, water pipe, etc.) as well as connecting to a concrete encased grounding electrode in the rebar of a nearby column footing as required by 250.50. Would this be a legal installation with the new 250.30(C)?

 

[Gregg Harris]

Does 2011 250.30(C) pertain to your installation? Your transformer is the service transformer is it not? @50.30(C) is referencing a separately derived transformer.

 

[DetroitEE]

Does 2011 250.30(C) pertain to your installation? Your transformer is the service transformer is it not? @50.30(C) is referencing a separately derived transformer.

This is a customer owned outdoor pad mounted transformer, so I think 250.30 does apply here. The service point would be ahead of the tranformer in a customer owned primary switch, which is fed from the utility pole.

After reading through this thread http://forums.mikeholt.com/showthread.php?t=100003&page=2 i think i will just do exactly as the diagram in post #18 of this thread suggests: grounding electrodes at the transformer, bond the transformer, run a 4-wire feeder (A,B,C,N) in PVC into the switchboard, leave the system bonding jumper installed in the switchboard, and then make the building grounding electrode system connections to the switchboard. This seems like to most standard way of doing this type of installation, and it seems to be code compliant per 250.30(A)(1) exception No 2 since there will be no parallel path.

Does NEC 2011 addition of 250.30(C) affect this at all? It seems like they just added this to clarify that a grounding electrode system is required at an outdoor transformer, regardless of whether it's a service transformer or not.