Xfmr primary ocpd rating

[jwoody]

I have a 45 KVA transformer (480 V primary/ 400 V secondary) to provide power to a single piece of equipment with a 33KW rating. Can I size the primary overcurrent protection based on the equipment 33 KW rating, with 125% of course (50A)? Or does it need to be sized for 45 KVA? In addition, I will have secondary overcurrent protection of 33KW at 125% (60A).

 

[don_resqcapt19]

You can size it any size at or below 125% of the primary full load current, assuming primary only protection, but there is a risk of the breaker opening when you turn the transformer on. Not sure there is anyway to know for sure without trying it. I think I would size the primary OCPD and conductors for 60 amps.

 

[jim dungar]

You can size it as small as you want, but any device sized less than the transformer 125% is likely to trip/ blow when you try to turn on you transformer.

 

[tortuga]

I will have secondary overcurrent protection of 33KW at 125% (60A).

If you have secondary protection of 125% that is based off the transformer 45kVA not your calculated load 33kVA, so I come up with 80 amps on the 416V secondary and then you can have 250% on the primary so 135A for the 480V primary. ( T450.3(B) )

Note I use 416V as the standard voltage for calculations (220.5) for (416Y/240 60hz) and 400V for motors on these 400V systems, much like we use 208 & 200V respectively in my experience most of the loads out of a 400V system are actually line to neutral (240V).