Sizing Conductors Serving Transformers

[richwaskowitz]

We are seeing more and more drive isolation transformers (480 delta in and 460 wye out) in industrial conveyor installations since VFDs are becoming more common. Many of our transformers are in the range of 100 to 250 kVA and are larger than we are used to using. My question is about how to size the conductors between the bus plug and the control panel and then to the primary of the transformer. Without being specific as to the class of fuses being used, but knowing that the fuses on the primary of the transformer will be sized at between 125 and 250% of the transformer primary current and then having the bus fuses sized at 150 to 200% of the panel main fuses for coordination purposes, we can end up with a bus plug and fuse sized at as much as 5 times the transformer secondary and therefore 5 times the actual load. So, for example, a system with a 100 amp load would require a 500 amp fuse in the bus plug. Do I really need to size the feeders between the bus plug and the panel / transformer at 500 amps, even though the transformer secondary is protected at 125% of its rated current?????? Do we really need 500 amps worth of copper for a load of 100 amps? Code citations and inspector opinions would really be appreciated.

 

[david]

Assuming your conductors are limited in length to be compliant with the tap rules for transformers. Your conductor sizes have to be correct for the overcurrent protection that you are providing for them.

You mentioned 500 amp buss fuses. I have to assume that is for the 250 KVA transformer on the primary side. I would expect to see 400 amp overcurrent protection on the secondary side. And secondary conductors sized based on the overcurrent protection provided for them.

If you had 400 amp overcurrent protection on the primary side and the secondary conductors comply with the tap rules I would expect to see conductors sized based on the over current protection provided in the secondary panel.