Transformer Overcurrent Protection

[SteveHuck]

I know from the code book that when protecting a transformer with a fuse or breaker that the maximum sizes can be calculated.

When it comes to transformers over 600v primary, what is the optimum fuse or breaker size. For a 13.2kv primary tranny with only primary fusing the max fuse may be 250% and the max breaker may be 300%.
Well thats very nice but i don't think anybody would go that high except in extreme circumstance.

What is normal!
What is optimum!

 

[dpc]

It depends on the type of fuse and the secondary protection. A standard fuse link will not begin to melt until about 150% of its rating.

The other issue you must be concerned with is the transformer inrush.

S&C has some good guidelines on fuse sizing for transformer protection.

http://www.sandc.com

 

[DanDel]

This is typically done by a coordination study, taking into account the transformer damage curve and inrush point, along with downstream cable protection. The secondary device is used for overload protection, and the primary device is used for short-circuit protection. I never rely on just the NEC limits for this, since it really does not take transformer protection into account. Remember, the NEC is only the required minimum protection. Transformer manufacturers will have a different requirement, that's why they list the maximum kVA rating. Check out IEEE Std 242-1986, 10.8.3.2.