Transformer

[hhsting]

I have 30kva transformer primary of 208V going to 480V feeding light poles. The transformer secondary within 25 feet has fused disconnect sized for 20A fuse. The load of light poles is 3 kva so 20A fuse is ok. However 20A fuse on 30kva 208v to 480v can it be sized way less than transformer fla nec 2014 Article 450?

 

[augie47]

are you asking if it can be sized less than FLA ?
If so, sure. The secondary protection can be sized to meet the load and protect the conductor.
If its being use to provide transformer protection, there is a maximum size.

 

[hhsting]

are you asking if it can be sized less than FLA ?
If so, sure. The secondary protection can be sized to meet the load and protect the conductor.
If its being use to provide transformer protection, there is a maximum size.

Secondary 20A fuse disconnect is used to protect transformer as well. So the maximum size to protect transformer would be 45A but can be lower?

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[augie47]

Yes. The secondary protection is often sized to protect the conductors being supplied.
It's possible to undersize primary protection due to inrush currents, but an undersized secondary still provides transformer protection.

 

[victor.cherkashi]

Assuming it's 3 phase transformer. Secondary (480V) could be 45A or below. Also see manufacturers manual if you're using step down transformer to step up voltage.

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