Overloading a Distribution Transformer 1

[CuriousElectron]

Hi Guys,

Is the design decision to provide an overload protection for pad-mounted transformer based on expectation that the transformer would not be carrying more than its nameplate rating? If so, than the feeder and the secondary conductors don't need to be sized at 125% of their nameplate ampacity.

I know some pole mounted and pad mounted transformers can come with internal overloads, and if these units are selected, than does that mean that the cable needs to be upsized as well?
For example, I've got 100A on the secondary of the transformer. If I have already oversized the xfmr for the load that it will supply, I'd just size the wire based on this ampacity and provide a 100A breaker. I'd not need a 125 breaker with the larger size wire.

Am I correct on this?

Thanks,
EE

 

[waross]

You asked about distribution transformers but your example seems to be for a customer's utilization transformer.
Different codes apply.
Utility owned distribution transformers are often allowed to run overloaded. Primary protection of distribution transformers often does not protect the transformer but protects the system by disconnecting a failed, shorted transformer.
The utilities do not fall under the same code as their customers.
Check your local codes.
In most jurisdictions the sizing of conductors and breakers is dictated by the code, not by what you think that it should be.

If so, than the feeder and the secondary conductors don't need to be sized at 125% of their nameplate ampacity.

Good luck explaining that to an AHJ who has just quoted the code section under which he has rejected your installation.

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

 

[dpc]

Agree with Bill. On the utility side of the meter, they have great leeway in sizing transformers, cables, fuses, etc. They are not under the jurisdiction of the NEC. Utilities routinely UNDERsize transformers compared to sizing requirements in the NEC. On the customer's side of the meter, the NEC is the law and there is no point in trying to apply logic or engineering judgement. Just size per the NEC and get on with your life. Unless the too small cable has already been installed, of course.

 

[waross]

As an example of the difference:
I did an electrical design for a small shingle mill.
We had a choice on the transformers;
Customer owned with primary metering or utility owned with secondary metering.
For a customer owned transformer the size for motor loads and other loads was close to 1 KVA per HP.
The utility applied a diversity factor based on experience and calculated the transformer size base on 0.5 KVA per HP.
When the transformer calculations were rounded up the the next common transformer size, a customer owned transformer would have been twice the KVA capacity as a utility owned transformer.
But: the code is the code.

Inscribed over the door of a cell block in Central America:
"DURO ES LA LEY PERO ES LA LEY"
"Hard is the law but it is the law."
The NEC may have something in common with that observation.

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

 

[7anoter4]

Thank you Bill.
The law may be harsh – but it’s the law. The Roman maxim from 300 a.D. (in the original, “Dura lex sed lex”)