Trip unit/ transformer current

[jimbo123]

I have a question on a existing installation that i believe maybe undersized to the main cb there now. The utility is suppling power from a 750kvatransformer, secondary is 480v The main cb has a trip unit rated for 1600amps ,the maximum for that frame size cb.
Should the trip unit be sized to the transformer? I believe is more like 900 amps . I am thinking if there is a overload condition 1600 amps is way above the transformer rating. Am i missing something ? thanks for any help.

 

[iwire]

The only thing you are missing is the NEC has nothing to do with the utility transformer.

The power company has a much better idea of the buildings real needs, the NEC load calculations are very conservative which makes sense. On the other hand if the power company placed an NEC sized transformer at each location that would cause all of our rates to go up due to the additional costs of the larger units and all the additional wasted energy used keeping the spare capacity on line.

The power company also counts on the fact that short overloads do not kill a transformer, they are fine with running it over it's tagged ampacity but they do not guess about it, they have formulas that indicate how the overload condition will effect the life of the transformer.

 

[iwire]

Should the trip unit be sized to the transformer?

Most likely reducing the service size from 1600 to 900 would be an NEC violation as the NEC load calculations are likely above 900 amps.

 

[jimbo123]

OK . Thanks again for the help with this . Learning more each day.

 

[jimbo123]

I been thinking about the response on the transformer protection. It seems to me there is a big difference in 900 amps and 1600amps . on a transformer. Any idea how the poco size their protection on primary side ?

 

[don_resqcapt19]

I been thinking about the response on the transformer protection. It seems to me there is a big difference in 900 amps and 1600amps . on a transformer. Any idea how the poco size their protection on primary side ?

In general the utility OCPD on the line side of their transformers are sized to protect the distribution system, not the transformer itself.