Supply and Return fan on same fused switch

[Gategator37]

I have a step up transformer 208v to 480v (45kva) which feeds a fused switch that has 30 amp type J fuses. This switch then feeds a VFD panel which supplies a 480v 15 hp supply fan and a 480v 15 hp return fan. After reviewing this, aren't the 30 amp fuses too small?

A 15 hp 480v motor has a full load of 21 amps. If there are (2) motors on the same circuit don't I take the first motor X 125% ( I think i can go up to 175%) + the full load of the next motor 21 amps and come up with 47.25?

So I would have 50 amp fuses in a 60 amp disco?

Thanks in advance for the feedback

 

[CIECO]

With the VFD use 110%

 

[augie47]

In the case of drives, your guide on the line side is 430 Part X which calls for the calculations to be based on the input rating of the drive.
On the ones I have seen, the short circuit protection is normally specified by the drive manufacturer, but there in normally no minimum, however, the disconnecting means must be 115% of the rating of the drive.

 

[augie47]

With the VFD use 110%

I can't locate that in the NEC. Can you provide me the reference, please ?

 

[kwired]

I have a step up transformer 208v to 480v (45kva) which feeds a fused switch that has 30 amp type J fuses. This switch then feeds a VFD panel which supplies a 480v 15 hp supply fan and a 480v 15 hp return fan. After reviewing this, aren't the 30 amp fuses too small?

A 15 hp 480v motor has a full load of 21 amps. If there are (2) motors on the same circuit don't I take the first motor X 125% ( I think i can go up to 175%) + the full load of the next motor 21 amps and come up with 47.25?

So I would have 50 amp fuses in a 60 amp disco?

Thanks in advance for the feedback

If the fans operate without blowing the fuse they obviously are not operationg at or near full load. The fact that you are using VFD will reduce the load when not running at 100% speed. If the motors ever do both operate at or near 100% full load your 30 amp fuses will not hold.

125% of largest motor plus all others is for conductor sizing. Over current device sizing is different and is typically a maximum setting with no minimum. Minimum settings are more of a design issue than a code requirement.