Transformer Question

Richard -

I can't answer your NEC question but I would like you to verify something. Small dry type transformers like this have three standard temperature rises - 150=B0C, 115=B0C, and 80=B0C. I cannot tell which is the correct temperature rise on the unit you have because there is a number missing.

Here's what the transformer can do based on temperature rise. A 150=B0C rise does not allow for any overloading. As a matter of fact, we recommend to our customers that if your load will exceed 80% of the rated KVA, you should install the next largest available size. A

115=B0C rise, using modern design principles and materials, will allow for a 15% overload above the rated KVA. Again, some consideration does need to be given to the life of the transformer. The larger the load, the less life the transformer will have.

At 45 KVA and 208V, the transformer is rated for 125A three-phase. At

115=B0C rise (51.75 KVA) and 208V, the transformer is rated for 143.6A three-phase.

At 200A, your panel will have a higher rating that your transformer. Again, I don't deal with NEC in my business, but to have your panel have a higher capacity than your transformer seems opposite of what it should be. The panel should protect the transformer. If your main doesn't trip until 200+ amps, that could allow the transformer to fail first. The answer may be to use appropriate fusing prior to the transformer.

I have a friend who is very familiar with NEC. I will ask him tomorrow and post another reply.

Steve Lockridge Manager Alfa Transformer

The full load rating of the transformer is 125A. The NEC allows secondary protection at 125% of rating, or 156A. SInce it is over 150, you are allowed to go to the next higher standard rating, which is 175A.

You need to determine if this would adequately serve the panel loads. You also need to verify proper protection of the wiring, transformer life, and any other factors that you may not have mentioned in your post.

Ben Miller

Thanks for the information! It was very helpfull and confirmed what I thought!