Top-Fed Transformer Restrictions

[Cjmeziere12]

Hello, all.

Got a question regarding requirements for conductor entry into dry-type transformers.
Here's the situation: We have a 400Hz/600kVA step-up transformer (200Y-575Y) being installed. The installation team wanted to top-feed the transformer to cut down on install time (long story but it checks out) and I reasoned that it would be okay due to the transformer being largely oversized for the existing load and that, given an increase in load, we would go back and side-feed the transformer when we replaced the upstream conductors.

So, when my Senior Engineer found out, he informed me that he believed it to be against code to top-feed. I have done some research and haven't found it in the NEC yet, and it appears that it may be an installation requirement by the manufacturers in most cases (though there is no "wire below this line" tag on this transformer).
Is there somewhere in the code (NEC or otherwise) that lays out this restriction? Is it just a manufacturer requirement? I'm seeing a lot of discussion mentioning the prohibition of top-feed on dry transformers up to a certain kVA, would this somehow not apply to a 600kVA?

Any help appreciated,
Thank you!

 

[winnie]

As far as I recall there isn't a specific prohibition in the NEC about 'top feeding', simply the general requirement to follow manufacturer's instructions. My understanding is similar to yours, that most manufacturers of 'dry type' transformers have a 'now way above this line' somewhere in the transformer case.

The issue is, of course, heat. The top of the transformer gets _hot_. But the manufacturer certainly puts wire above the 'line'. The manufacturer makes the transformer with high temperature rated wire, and is instructing that the field wiring must be below the line, both to not impede airflow and to protect wire with 90C insulation from the potentially higher transformer temperatures.

You might be able to get the manufacturer to sign off on a 'top fed' installation with a derating of the transformer or a requirement to use higher temperature rated field conductors.

Just remember that if something is installed 'slightly wrong' because it gets the job done faster, it will probably stay 'slightly wrong' for many years to come.

-Jon