3 motor drives

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spark master

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I was asked to review an installation on a 3 motor mixing tank installation. 1 circuit, #8's to a disconnect. Trough under the disconnect. In the trough is what concerns me. 3 VFD's mounted on a wall. 3 circuits come out of the trough, #10's for 2 small motors, & #8 for the big 10HP motor. Obviously the 2 small motors don't need #8's, and #8's won't even fit in the VFD terminals. So they used #10's from the trough to the drives, and also #10's from the VFD to the motor. It's about a 5' run of undersized wire.

Considering the VFD kicks out if the motor goes over current, do I need to fuse where the #8's go into #10's ?

Normally I'd see drives mounted in a big control panel, with circuitry, and starters, & contactors. But this one was done simple, with just 3 drives mounted on a wall, and a speed control.
 
It would help to know the HP & voltage, but in general if the wires are adequate for the motor load, No, do not add extra fusing. The drives will provide OC protection for the wires if properly set up.

This is no different than increasing the wires to compensate for VD.
 
All 208volts.
10HP 29.5 amps #8 wire
1.5HP 5.3 amps #10 wire
1HP 4 amps. #10 wire.

#8 wire into trough protected by 50amp 3 pole breaker. The trough is just a splice box, with 3 feeds coming out.
The 2 smaller motors are fed by #10's coming out of the trough.

How is this legal, without fusing the #10's in the trough?
and yes, the VFD's protect the motors. The VFD's were programmed by the vendor, to match the motors.
 
spark master said:
All 208volts.
10HP 29.5 amps #8 wire
1.5HP 5.3 amps #10 wire
1HP 4 amps. #10 wire.

#8 wire into trough protected by 50amp 3 pole breaker. The trough is just a splice box, with 3 feeds coming out.
The 2 smaller motors are fed by #10's coming out of the trough.

How is this legal, without fusing the #10's in the trough?
and yes, the VFD's protect the motors. The VFD's were programmed by the vendor, to match the motors.
Click to expand...
You're saying they all are fed from the 50 amp 3 pole?

That changes things a bit. You will need to look at the manual to determine what the max short circuit protection is for each drive. If they are happy with that 50 then I guess you can be happy to.

VFDs seem to have some extremely large allowances for fusing ahead of them. Note fusing. The CB may not be correct for any of them. Gonna hafta RTFM.
 
So confirm that this is correct. You have a 50A CB feeding the drive supply circuit with #8s, which go into a splitter block or something that separates that into another #8 feed into the bigger drive and then two #10 circuits feeding the two smaller drives, and that total distance from the split to the drive terminals is 5' or so. Right?

Technically, that may be legal from the conductor standpoint because of the tapped conductors being only one size smaller and under 6' (or maybe 10', can't remember without looking); commonly called the "Feeder tap rule". There are a number of qualifying conditions to that so you would need to review everything, but from what you said, it may be legit.

Yet ptonsparky is right, chances are about nil that the 1HP and 1.5 HP drives can be legally used with a 50A breaker ahead of them. The VFD manual should tell you the maximum OCPD that can be used ahead of that drive, and even if CBs are OK in their UL listing (many are only UL listed with fuses).
 
yes, It's not a splitter block, it's just big blue wire nuts. But whatever..... Comes out of the disconnect, into a trough, and then goes to 3 different motors.
No fusing, just the 50a 3 pole breaker in the panel.

So it appears I am NEC legal, using the line tap rule.

But you're saying, potentially not ok with the drive manufacturer....
So I need to learn about drives, and review the drive specs...... somehow.
 
We used to have some stuff built like this. I fixed all of that by adding separate protection to all drives.
That said, if something goes wrong with your drives especially the smaller ones they will smoke up pretty good.
 
So... I found out a 1hp drive has a MCOP 10 amps. 1.5hp drive MCOP 12amps. So even if both of these drives were on a 20amp circuit, with a twist lock 20 amp plug. I'm NEC compliant, but the drive will fry if something goes wrong internally.

Not sure how much of a problem this is.........

Seems every drive, even if it comes in a full enclosure, needs to be in a big control panel with fuses, and stop / start circuitry.
 
I don't know how many drives will save themselves if something goes wrong internally. I believe the fuses are there to eventually stop the arcing, sparking and smoking. Small VFDs are disposables. We have a 200 Hp in for repairs and the repair on that is just over half of a new.
 
ptonsparky said:
I don't know how many drives will save themselves if something goes wrong internally. I believe the fuses are there to eventually stop the arcing, sparking and smoking. Small VFDs are disposables. We have a 200 Hp in for repairs and the repair on that is just over half of a new.
Click to expand...
I have to agree that small VFD's are disposables. Most will protect themselves if there is a fault on the load side. If you have a rectifier failure or something of that nature, the following overcurrent specifications won't really save the drive. Even if it minimizes damage, you are not going to repair it you are going to replace it in most cases. This especially true for 10 hp and less drives, probably still applies to many up to about 50 HP.
 
spark master said:
Seems every drive, even if it comes in a full enclosure, needs to be in a big control panel with fuses, and stop / start circuitry.
Click to expand...

There are drives made to be placed outside a cabinet and mounted on walls, conveyors, carts etc. Usually these are weather rated. But any OCP and misc controls
do need to be enclosed. The only real difference I have noted in my experience is that exterior drives usually have screw on covers and electrical entry is made through
knockouts or connectors. No way to get any fingers inside without use of tools.
 
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