Transformer grounding

[infinity]

With multiple transformers you should just run a #3/0 common grounding electrode conductor and tap off to each transformer.

 

[edf]

What is the correct thing to do in the following scenario. Installing a 15kva Delta 480 3 phase pri. To a wye 400 3 phase sec. The Transformer will be feed from a 277/480 Load center. The building has wood post frame, so I can not cad welding the ground to the building structure. Do I install a ground rod for the grounding electrode of the secondary side? Or would that not be correct?

The transformer creates a separately derived system. If you are feeding a single machine or panel you will need over current protection for the secondary of the transformer and ground it as you would an out building. If you Ground the secondary load to the primary circuit grounded wire a fault on the secondary will not trip that local overcurrent protection. If you ground the transformer secondary a fault would have a low impedance path to the over-currrent protection and do its job, Tripping the breaker or fuse.

 

[kwired]

If the primary circuit originates from the service equipment enclosure why wouldn't you be able to have a dual function EGC/GEC conductor to the transformer and make the secondary system bond in the transformer enclosure?

If you Ground the secondary load to the primary circuit grounded wire a fault on the secondary will not trip that local overcurrent protection. If you ground the transformer secondary a fault would have a low impedance path to the over-currrent protection and do its job, Tripping the breaker or fuse.

You must ground the secondary, and you must bond to the GES. It will have low impedance path to trip a secondary fault. If you (intentionally) don't ground the secondary then it is an ungrounded system, and you would need ground fault indication equipment.