Transformer feeder

[cppoly]

Are transformer feeders supposed to be sized for the full kva of the transformer or the load the transformer is serving? For instance, if a 75 kva transformer feeds a 50 kva load, does the feeder need to be sized for 75 kva or 50 kva?

This would impact feeder calcs too on a larger scale. For instance, if there's ten 75 kva transformers does the service need to be calculated to handle this load or only the load that is connected to these transformers?

 

[iwire]

In my opinion all the calculations are all based on the load and not the capacity of the electrical distribution equipment chosen.

 

[jumper]

This may help. A MH page on trannies.

https://www.mikeholt.com/mojonewsarchive/NEC-HTML/HTML/TransformerInstallation~20020516.htm

 

[Dexie123]

This may help. A MH page on trannies.

https://www.mikeholt.com/mojonewsarchive/NEC-HTML/HTML/TransformerInstallation~20020516.htm

I just read this article in the link and have a follow question regarding it.


One sentence reads "Author’s Comment: The grounding electrode conductor must connect directly to the grounded neutral conductor terminal. It cannot be terminated to the case of the transformer".

Does not 250.30(A)(5)ex 1 not apply here?

 

[infinity]

I just read this article in the link and have a follow question regarding it.


One sentence reads "Author’s Comment: The grounding electrode conductor must connect directly to the grounded neutral conductor terminal. It cannot be terminated to the case of the transformer".

Does not 250.30(A)(5)ex 1 not apply here?

The exception would allow you to use a common terminal but you still cannot use the case to connect the GEC to the neutral conductor.

 

[Dexie123]

The exception would allow you to use a common terminal but you still cannot use the case to connect the GEC to the neutral conductor.

Ah ok. In other words the case can't act as the SBJ (system binding jumper) ?

 

[infinity]

Ah ok. In other words the case can't act as the SBJ (system binding jumper) ?

In order to apply the exception the SBJ needs to be a wire or busbar. That would eliminate using a screw or strap as the bonding jumper and applying the exception.

250.30(A)(5)Ex1:If the system bonding jumper specified in
250.30(A)(1) is a wire or busbar

 

[don_resqcapt19]

The issue with using a smaller feeder for the primary of the transformer is that the feeder conductors must be protected at or below their ampacity. This may result in an OCPD that will trip on inrush when the transformer is energized.