Overloading Transformer

[cabletray]

Would it be an NEC violation to slightly overload (say 3-5%) a rarely-used standby (1500 kVA) transformer?

 

[peebee]

Nope. NEC does not mention transformer loading.

All NEC covers is transformer overcurrent protection -- there, you're allowed to go to 25% over transformer rating. So the max you could possibly overload your transformer would be 25% (assuming you're running close to nominal voltage).

Your 3-5% overload doesn't bother me one bit. The worst you'd do is knock a couple years off the transformer's expected life.

Note that utilities, who are not held to NEC, will commonly load transformers to 25, 40, even 100% over their nameplate rating. Of course, they don't mind replacing them a bit more frequently than we normally would. Matter of fact, this works out to be cost effective for them as it helps keep them from oversizing transformers (that's why they do it!). Most customer-owned transformers are only loaded to about 30-50% of their rating -- that's negligable extra cost for a customer, but that kind of overcapacity would be super expensive for a utility.

 

[cabletray]

Our utility 500 kVA xfmr operated at least at 660 kVA. We were told this was acceptable because the nightime and weekend load was very small. When we went to primary metering, the load dictated a 750 kVA xfmr (customer-owned) per NEC.

I seem to recall an NEC rule that prohibits operating equipment outside its ratings, but can't find it now.

 

[torslum]

I agree with the above.

Also worth mentioning, the only thing that is really interesting when it comes to overloading a transformer, is the winding temperature. If you have measurements of this temperature, then you can overload your transformer whatever you want within the insulation class temperature limit.

Therefore you can also uprate a transformer by installing fans to keep the temperature down.

Good luck.

 

[alehman]

Liquid insulated transformers will withstand overloading much longer than dry-type transformers.

The NEC requires overcurrent protection no larger than 125% of the transformer rating. It also prohibits loading a standard (80% rated) overcurrent device at more than 80% of it's rating for continuous (>3 hours) loads. This effectively prevents overloading the transformer for more than 3 hours.

 

[peebee]

But not too effectively, given the availability of 100% rated breakers.

 

[advidana]

If it is a new installation use a larger transformer for possible future loads that are usually always added.