Transfromer Secondary Conductors

[FaradayFF]

Greetings,
Is there a requirements in the NEC that the ampacity of the secondary conductors of a dry-type transformer should be greater than the transformer rated secondary current? For example, if the transformer output is 500Amp, the ampacity of cables connected to the secondary side of the transformer should be 500Amp or greater?
It would be logical to do this to get the max amps out of your transformer, just not sure if that's an actual code requirement, or a judgment call. In this specific application, the transformer would have primary overcurrent protection only.
Thank you,
EE

 

[david luchini]

Greetings,
Is there a requirements in the NEC that the ampacity of the secondary conductors of a dry-type transformer should be greater than the transformer rated secondary current? For example, if the transformer output is 500Amp, the ampacity of cables connected to the secondary side of the transformer should be 500Amp or greater?
It would be logical to do this to get the max amps out of your transformer, just not sure if that's an actual code requirement, or a judgment call. In this specific application, the transformer would have primary overcurrent protection only.
Thank you,
EE

There's no such Code requirement.

 

[petersonra]

As long as the secondary conductors are protected by the primary OCPD, I don't see an issue. But that would mean you probably have to lower the rating of the primary OCPD substantially.

 

[synchro]

Is there a requirements in the NEC that the ampacity of the secondary conductors of a dry-type transformer should be greater than the transformer rated secondary current? For example, if the transformer output is 500Amp, the ampacity of cables connected to the secondary side of the transformer should be 500Amp or greater?
... In this specific application, the transformer would have primary overcurrent protection only.

As long as the secondary conductors are protected by the primary OCPD, I don't see an issue. But that would mean you probably have to lower the rating of the primary OCPD substantially.

And lowering the OCPD device rating on the primary substantially could make it suceptible to tripping on inrush current.

 

[petersonra]

And lowering the OCPD device rating on the primary substantially could make it suceptible to tripping on inrush current.

Yep. Never liked the idea of protecting the secondary conductors this way. I like to make the primary OCPD at least 200%.

 

[don_resqcapt19]

As long as the secondary conductors are protected by the primary OCPD, I don't see an issue. But that would mean you probably have to lower the rating of the primary OCPD substantially.

It is pretty rare where you are permitted to use the primary OCPD to protect the secondary conductors. Only permitted for a 2 wire to 2 wire transformer or a 3 wire delta to 3 wire delta transformer....in all other cases you have to apply one of the other rules in 240.21(C).

 

[electrofelon]

Greetings,
Is there a requirements in the NEC that the ampacity of the secondary conductors of a dry-type transformer should be greater than the transformer rated secondary current? For example, if the transformer output is 500Amp, the ampacity of cables connected to the secondary side of the transformer should be 500Amp or greater?
It would be logical to do this to get the max amps out of your transformer, just not sure if that's an actual code requirement, or a judgment call. In this specific application, the transformer would have primary overcurrent protection only.
Thank you,
EE

Perhaps it is worth mentioning that for MV transformers, the primary feeder has to have an ampacity greater than or equal to transformer nameplate:

215.2(B)(1) Feeders Supplying Transformers. The ampacity of
feeder conductors shall not be less than the sum of the
nameplate ratings of the transformers supplied when only
transformers are supplied.