OT. Transformer theory/Overcurrent protection

Guys,

It's been awhile, and my transformer theory is failing me. Can someone explain why NEC states that for transformers less than 600V, less than 2A, primary side protection is ok up to %300, less than 9A is %167, and over 9A is 125%? I had a 500VA transformer primary protected at 125%, and a colleage pointed this out, stating concerns of nuisance tripping. I've never had this issue arise? Have you? Why the change in percent allowable?

Russ

your best bet is to post on an electrical website like www.mikeholt.com.

It's been a while, but here is my .02 worth.
It all has to do with the size of the transformer.
Transformers conduct heat due to their construction and the
collapsing magnetic fields.

The laminate plates hold the coils of wire. they also have what is known as a stacking factor. the wire ranges in size and length based on kva. The wires are coated with varnish to insulate them. The electrical current produces a magnetic field. Adding all this together, you get heat and vibration. the smaller transformers have less and can handle more over current before the insulation breaks down.

If i'm wrong, someone please correct me. It has been several years since i studied transformers.

regards,
james