Sizing transformer neutral

[Jchagnon]

I am looking up and down article 450 and cannot find anything about how to size the neutral or ground for transformers.
We are replacing a three phase 70 kva with a new 70 kva and the neutral is smaller than the ungroundeds on the existing install.

I am trying to determine if this needs to change.

Any help or direction would be much appreciated. Thanks!

 

[HEYDOG]

Look at 250.30(A)(3).

 

[augie47]

Very good question. I agree with HeyDog on 2560.30 when the SBJ is not located at the transformer or dependent on the grounded conductor as a path. If the SBJ is at the transformer and not dependent on the grounded conductor as a path it seems 220.61 would apply as far as load but would 215.2(A)(2) apply ie: is a transformer secondary a "feeder" ??

 

[don_resqcapt19]

Very good question. I agree with HeyDog on 2560.30 when the SBJ is not located at the transformer or dependent on the grounded conductor as a path. If the SBJ is at the transformer and not dependent on the grounded conductor as a path it seems 220.61 would apply as far as load but would 215.2(A)(2) apply ie: is a transformer secondary a "feeder" ??

You can't apply the rule in 215.2(A)(2) as that references Table 250.122 and there is no OPCD that you can use to apply that table.
The code does not really address that case. The language in 250.30(A)(3) also does not address the cases where the load calculation would require a grounded conductor larger than one sized per Table 250.102(C)(1).

 

[augie47]

 

[ActionDave]

I think we can all agree it is ok to have a reduced neutral though.

 

[Tulsa Electrician]

What is your system voltage primary,secondary and phase.
just curious
The 70 kva cought my attention.
Is this a high leg delta secondary?