Bonding Pad Mount Transformer

[Bob Root]

We are changing a service from 208/120 volt to 480/277 volt (600 A) by replacing the pad-mounted transformer and main service panel. The service is not metered. Our new service will run directly into the main panel approximately 40 feet away in an electrical equipment room. The existing service has Xo and Xn bonded at the transformer secondary and separate ground and neutral conductors to the main panel inside. There isn't a bonding connection between the main panel neutral and ground. The electrical contractor is insisting on bonding at the main panel only. I prefer bonding at the transformer secondary only and running separate neutral and ground conductors. This would keep all of the unbalanced current flowing in the neutral conductors. Please give your opinion.

 

[RUWired]

If the conductors from the transformer to the first main breaker in the building are service conductors, a main bonding jumper is required at both locations.

If the conductors are feeder conductors, bonding would be at either location but not both because you have an EGC run in the raceway.

Rick