GEC where using Busway

[m sleem]

Ref # 250.66, What would be the GEC size of the transformer (separately derived system) where we use a busway from transformer to panel?

 

[don_resqcapt19]

See Note 2 to Table 250.66.

 

[m sleem]

See Note 2 to Table 250.66.

Here,

2. Where there are no service-entrance conductors, the grounding
electrode conductor size shall be determined by the equivalent size of
the largest service-entrance conductor required for the load to be
served.

Is that applicable where the transformer located inside the building? where the service conductor will be medium voltage cable.

 

[GoldDigger]

Here,

Is that applicable where the transformer located inside the building? where the service conductor will be medium voltage cable.

Paraphrasing the text of the Note:

... equivalent size of
the largest service-entrance conductor required...

I interpret that as the size that would have been required by the NEC for the service entrance conductor serving only those loads if there had been a service entrance conductor involved. That is, at the secondary voltage of the transformer (the actual load current of the load(s) being served from the transformer.)

 

[don_resqcapt19]

Here,

Is that applicable where the transformer located inside the building? where the service conductor will be medium voltage cable.

It applies anytime you are required to size a conductor per Table 250.66.

 

[m sleem]

It applies anytime you are required to size a conductor per Table 250.66.

Don, if it applies per table 250.66 it means, where the service conductor is 13.8kv & 185mm2(400AWG), the GEC will be 50mm2(1/0AWG) which is very small. Otherwise thinking as golddigger says, the equivalent cable size for the busway which it is not a service conductor anymore.

 

[don_resqcapt19]

Don, if it applies per table 250.66 it means, where the service conductor is 13.8kv & 185mm2(400AWG), the GEC will be 50mm2(1/0AWG) which is very small. Otherwise thinking as golddigger says, the equivalent cable size for the busway which it is not a service conductor anymore.

It applies to the equivalent size of the conductors that trigger the need for a GEC or other conductor that is sized per Table 250.66. If you are grounding the secondary of a transformer it would apply to the secondary conductors of the transformer.

 

[m sleem]

If you are grounding the secondary of a transformer it would apply to the secondary conductors of the transformer.

It's A bit confused, are we grounding each side of the transformer separately. I would think we have one grounding wire connected to the transformer's case.

 

[GoldDigger]

It's A bit confused, are we grounding each side of the transformer separately. I would think we have one grounding wire connected to the transformer's case.

The transformer case would normally be connected to the GES via an EGC running with the primary feeder. The problem is that with a separately derived system, with the primary and secondary side neutrals not directly connected, you will have to provide a bond between the secondary neutral (assuming that there is one) and the GES. That wire size is the one calculated based on the size of the secondary conductors.
But you must not bond the primary side neutral to ground at the transformer if it is bonded back at the service.