Drive Isolation Transformers: 450.3 vs 430.52 OCPD

[Tip DS]

Ok, Section 430.3 says a transformer has to be protected at 125% of the rated current. 430.52 says a motor can be protected at 250% of rated current. If I'm simply trying to provide isolation, it seems I'm being forced to over-size my transformer just to meet the 430.3 protection requirements.

Looks like I have to put a 51kVA transformer ahead of a 15HP VFD, or else use a motor branch circuit protection of only 125% of FLC. What am I missing?

 

[winnie]

Don't confuse permission to use a 250% rated OCPD with a requirement to oversize the OCPD.

Higher rated OCPD is often permitted to prevent nuisance tripping on inrush or starting, with overload protection provided elsewhere in the system.

But if your system prevents inrush or excessive starting current, then you don't need the higher rated OCPD.

A 15 HP motor might draw 75 kVA on an across the line startup. Feed that same motor with a VFD and the starting kVA will be much lower.

Size the transformer for the input current to the VFD, and pick appropriate protection for that.

Jonathan