Overcurrent Protection of Utility Transformers

[a10jp]

At one of the job site we have, the service transformer at the utility pole is 3x 75KVA which gives 225kVA max capacity. Our contractor has previously procured a 1200A main service panel with a 1200A main circuit breaker, before he realized what the utility service will be. At 225KVA, the max secondary current would be 625A or OCP would be 800A service.
Therefore 1200A is not effectively protecting the secondary, which according to NEC 2008, Art 450, cannot be more than 125% if secondary overcurrent protection is used based on NEC.

I am looking at the rationale whether we need to change out the 1200A service, and at the same time, looking at the angle that with 1200A secondary you are effectively eliminating the secondary protection, which results in only primary protection on the transformer fuse cutouts, which under certain condition maybe allowed by Code (Primary protection only).

Because it is my understanding that the utility can decide to put their equipment based on the burn rate of consumption, perhaps this is common, but in this case I want to be certain our installation is code compliant and safe. What are your opinions? Thanks for your help!

 

[dpc]

The utility system and the premises wiring systems are like two parallel universes in terms of the NEC.

You do not need to be concerned with protection of the utility service transformer. That is their problem. From experience they are betting that the actual loading will be much less than predicted by the NEC load calcs. If they are wrong, they will replace the transformer.

The demarcation in ownership between utility and owner is defined differently by different utilities. Just make sure any portion of the service lateral that is owned and maintained by the owner is sized per the NEC.

David Castor

http://www.cvoes.com

 

[waross]

Don't worry. You are not responsible for the protection of the utility transformer.
I once did an electrical design for a sawmill. We had the option of a primary service or a secondary service. If we went with a secondary service the utility would supply the transformer. If we went with a primary service we would have to supply our own transformer.
The utility would size their transformer according to their own demand factors that they had developed from experience and were appropriate for a sawmill of the type and size that we planned.
Our own transformer would be sized according to the demand factors in the Electrical Code which are based on a more severe generic load profile.
There was a two to one capacity ratio between our transformer and the utility supplied transformer.
Utility transformers are almost always smaller than client owned transformers.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[a10jp]

Thank you both. I just got confirmation from the utility that says something similar, stating that the main service is not to protect the utility transfomrer, but to protect the load side on the customer.

Logically, between the utility that tends to undersize (being conservative), and the customer that tends to oversize (being conservative, again), somewhere you have an uncoordinated zone.

 

[dpc]

Regarding coordination, it is unlikely that you have good coordination anywhere in your system. Molded case circuit breakers in series generally will not coordinate. But that is a separate issue from the protection requirements.



David Castor

http://www.cvoes.com

 

[itsmoked]

"the utility that tends to undersize (being [blue]cheap[/blue])"

.

There. Fixed that for you.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[davidbeach]

Not really being cheap, but just being efficient. The cost of all those service transformers shows up in everybody's bills. Far less expensive for all involved to go with the transformer that experience shows will work and change out 0.1% or so when they prove to be under sized than to incur (and pass on) the expense of a transformer one or two sizes larger everywhere.

 

[stevenal]

Don't forget the losses. Larger than needed transformers also have higher than needed no-load losses. Again, everyone gets to pay to keep these things humming 24/7.