Transformer Secondary Conductor Sizing

[minerman]

Its been over a year since I've sized conductors according to the NEC, so I'd love some help.

I have 2 legs of a 480/277Y feeding a 100A CB, which feeds a 25kVA step down 480V-120V transformer for a panelboard. I'm not sure how to correctly size the conductors on the secondary of the transformer. Could somebody walk me through that? On the primary side, I have 2-#3's for my current carrying conductor and 1-#8 for my ground conductor. Correct?

Thanks!

 

[david luchini]

Since your primary OCPD is greater than 125% of the primary rated current, your secondary must have overcurrent protection per 450.3(B). The maximum permitted size is 300A on the secondary.

The secondary conductors must have an ampacity that meets or exceeds the secondary OCPD rating. So if your secondary OCPD was 300A, you would need 350MCM. If the secondary OCPD was 250A, you would need 250MCM, etc.

(Your primary feeder appears correct.)

 

[jumper]

Here are a couple of links I keep bookmarked for transformers.

http://ecmweb.com/mag/electric_transformer_installation_made/

http://ecmweb.com/nec/code-basics/electric_transformer_secondary_conductors/

 

[minerman]

Thanks! Both replies helped greatly.