How we can decide the proper size of a fuse?

 

You should be more specific. What are you using the fuse for?

Generally the fuse/circuit breaker in wiring is to protect the cable. Sizing of fuses should be done by a competent person with knowledge of the relevant standards.

 

For that matter, there is a great deal more here that should be done by a competent person. Scares me to death sometimes the questions we get.

Regards
cww

 

I think I saw somewhere that there are 3 levels of ignorance:
There are those who don't know, and know that they don't know
-- no problem with these guys.

There are those who don't know, and don't know that they don't know
-- getting scary

There are those who don't know, and think that they do know
-- now THAT's scary.

At least the questions we get are from guys in the first category - it's the ones that don't come up on this or similar forums that are really worrying!

Bruce

 

My purpose is to protect the 25 kva transformer. Its ratings are 480V/208V, 3ph, delta star. I need to find proper fuse to protect it, either primary or secondary.

 

First of all, you have enough information to calculate the current. With the information you provided in the email, the capacity and the volatage rating you can calculate the rated current capacity. You should decide if you wants to protect the transformer from currents over the rated capacity, but consider that the wire you use will also limits your maximum current. this means maybe with a fuse you protect the transformer but the wire could be damaged. You shoul also make a load analisys to determine the current that will be circulating in your circuit.

The guidelines you need are avaialble in the National Electrical Code (NEC) there you can find, methods, restrictions, recommendations and examples.

 

I am surprised more people don't simply google an issue. I tried "fuse sizing transformers" and had excellent success.

I have seen Mike Holt's courses, and they are excellent. His site came up first in rankings. He also recommends obtaining information from http://www.bussmann.com/, a source that I have often used. Look at http://www.mikeholt.com/documents/calculations/pdf/TS14FuseSizing.pdf

An excellent magazine/web site is Electrical Construction and Maintenance. They have excellent articles and the magazine in the US is typically free to qualified individuals.

The above two sources I have used frequently. Hope this helps.