1/3 HP motor (with fuses) being fed from 75HP branch circuit... is this OK?

[sparkkey]

Hello,

I've seen this topic danced around on several boards, and I can't find a thread specific to this scenario, so I will just put it out there for commenting:
(being asked in the spirit of UL508A acceptability)

There is a 480 VAC 3-phase electrical panel that has door-interlocked Fusible disconnect with 200A fuses feeding a 75HP variable frequency drive with a MAX AMP rating of 168A.
This same 200A fusible disconnect is ALSO feeding a SECOND, small 1/3HP motor through a fuse block with 3 amp fuses + contactor + overload.

Question:
Lets assume for the sake of argument that the 200A fuses are sized correctly for the VFD, and the circuit current is in an acceptable amp range with or without the 1/3HP motor running.

Is it ok for the 200 amp fusible disconnect be used as the both the feeder to the 1/3HP motor as well as the protection for the drive, where both devices share the same 200A fused circuit?

...OR must the branch circuits be separated so that only one set of fuses is connected to each motor circuit (200A on one, 3A on the other) thereby requiring a third protection device (MCCB or Fusible disconnect) that becomes the feeder to both the 200A fuses for the VFD and the 3A fuses for the 1/3HP motor?

Make sense?

Thanks!

-JP

 

[jumper]

Welcome to the MH forum.

We have a lot of knowledgeable people here and someone will have an answer for you.

 

[Smart $]

First, JMO, there is nothing unsafe about the described installation.

As to compliance, however, Article 430 doesn't actually cover this installation with any specific wording, but somewhat implied in 430.53 general and (D) thereunder... covering single motor taps without individual GF/SC protection, but then says nothing about single motor taps with GF/SC protection. I can only surmise that it is permitted.

One other note is that providing a third protection device would not be required. It is already furnished as the feeder OCPD. The small motor could just be tapped off the line side. However, the issue of the interlock comes into play... but I assume it is only there to prevent unintended opening of the disconnect while in the closed position, and can be defeated rather easily for intended access while in the closed position. Either way, the feeder could still be energized. So whether the small motor OCPD is connected line or load side is really rather moot... but somewhat dependent on whether it is desired to deenergize both motor circuits when accessing in the open position.

I don't know the specifics of UL508A... but the question of whether this equipment is supplied this way by the manufacturer, or field modified. If the latter, UL508A is out the door unless provided for by manufacturer in the instructions... and otherwise would require NRTL certification.

JMO :angel:

 

[jim dungar]

Is it ok for the 200 amp fusible disconnect be used as the both the feeder to the 1/3HP motor as well as the protection for the drive, where both devices share the same 200A fused circuit?

In the old days, I wired many UL508A panels with this type of set up.
Effectively the 1/3HP motor is being applied using a tap rule.

 

[Jraef]

In the old days, I wired many UL508A panels with this type of set up.
Effectively the 1/3HP motor is being applied using a tap rule.

That's correct, nothing wrong with it for the following reasons:

1. The smaller motor has it's OWN set of fuses. This is no different than taping off to feed the primary of a control power transformer if you think about it.
2. The conductors between the 200A fuses and the 3A fuses do not leave the enclosure (I'm assuming so here). This doesn't really make a whole lot of difference, it just would add a few more rules to consider.

The risk is, that the inrush of the smaller motor added to the inrush of the larger motor, if they both started at the exact same time, might be too much for the 200A fuses. But with the smaller one being so much smaller and the larger one being fed by a VFD, no problem..

 

[petersonra]

perfectly acceptable the way it was done IMO.