Sizing OCPD and conductors of transformer that is oversized for secondary load

[tjeff008]

I have a 3PH 208V service. The equipment is rated at 3PH 480V 60A. I have a 75KVA 208V primary/480V secondary transformer that i already have in stock. My question is, can I size the 208V primary OCPD and corresponding conductors based off the calculated load of the equipment, or do I need to size the primary OCPD and conductors off of the 75KVA transformer?

So either my calculations would be:

75000 / 208 / 1.732 = 208.19 x 1.25 = 260.24 *sized based off transformer nameplate*

OR

60 x 480 x 1.732 = 49881.60 / 208 / 1.732 = 138.46 x 1.25 = 173.08 *sized based off secondary load nameplate*

If anyone can point me in the right direction or let me know if I have calculated anything incorrectly I'd appreciate it. Thanks for the help.

 

[Jraef]

it tells you right in article 450.

Table 450.3(B) Maximum Rating or Setting of Overcurrent Protection for Transformers
10OO Volts and Less (as a Percentage of Transformer-Rated Current)...

The NEC is concerned with maximums, not minimums (in this case). You can do things at way UNDER their ratings if you so choose.

BUT, from a practicality standpoint, there are pitfalls. The primary OCPD, if under sized, may not hold in during initial inrush of the transformer when you first power it up. As a general rule then, I like to always size primary protection at the maximum allowable. Secondary... it makes no difference, you can do anything you like UNDER the maximum allowable.