Transformer Load, for Calculations

[Npstewart]

If you have a 30 KVA Xfrmr being fed from a breaker in a 480v-3 panel, which is feeding a 208-3 panel, do you need to show the full load on the primary side breaker (ie 30 KVA) or can you use the demand load from the secondary side?

Ie) lets say there is a 30 KVA transformer being fed from a 3 pole breaker, should you show 10 KVA on each leg? (or) say the 208-3 primary side has a load of 20 KVA total, can you just show a load of 6.7 KVA on each leg on the primary side.

This is all assuming a theoretical balanced load on each phase, and is for panel load calculation purposes.

As always, thanks guys.

 

[skeshesh]

My general response would be that you can use the calculated or connected load and move the values upstream for calculations. You have to use your engineering judgment though to account for future growth and such to make sure the electrical system and distribution upstream provide adequate capacity.