Sizing Primary Feeders of a Transformer

[Nittanyengineer]

I am trying size the primary feeders on a dry type transformer. To help with coordination and per Table 450.3(B) we are sizing the primary OCP at 250% (secondary is at 125%). My question is, are the primary feeders sized to 250% or 125% of the primary FLA?

Sizing the feeders to 250% FLA will match the over-current rating and is not wrong. I wonder if you could size the feeders to 125% and treat the transformer more like a motor. I cannot find anything on primary feeders in the NEC.

Thanks.

 

[don_resqcapt19]

The primary feeders to a transformer must be protected at or below their ampacity. In other words if you size the primary OCPD at 250% of the transformer primary current, you must also size the primary conductors at 250% of the transformer primary current.
The secondary conductors will also have to be protected per one of the rules in 240.21(C).

 

[Larry Bohn]

Let's see if I understand correctly. If I'm installing a 50KVA single phase transformer for a lighting panel, 480V primary, 240/120 secondary, with a 200A main breaker in the lighting panel, I'm going to have 4/0 secondary conductors from the transformer to the breaker in the lighting panel. Then if I'm sizing the primary OCD at 250%, (250A), then I've got to have 4/0 conductors on the primary also?

 

[augie47]

That would be correct.
I would think that it why I seldom see a 250% primary transformer feeder except on small transformers.

 

[jumper]

Here is a link I often use for transformer sizing rules.

http://ecmweb.com/contractor/transformer-installation-made-easysort