VFD Load Side Conductors - Multiple Motor

We have applications at times that utilize one VFD for up to 4 motors (overhead cranes). If I branch out the output of the VFD to each motor through thermal overload relays to meet 430.124(C), is there an additional code section that addresses the conductor sizes in this situation? For instance, if I have (4) 25A motors and a 100A VFD output, directly out of the VFD I'd size conductors per that 100A, but I think due to the tap rules, each of the 4 motor branch conductors would also need to be rated for 100A (likely longer than 25' runs per 240.21 and would not meet 240.21(4)). I think if I did size per the 25A on each circuit, I'd need appropriate branch OCPD. Thoughts?
 

Besoeker3

Senior Member
Location
UK
We have applications at times that utilize one VFD for up to 4 motors (overhead cranes). If I branch out the output of the VFD to each motor through thermal overload relays to meet 430.124(C), is there an additional code section that addresses the conductor sizes in this situation? For instance, if I have (4) 25A motors and a 100A VFD output, directly out of the VFD I'd size conductors per that 100A, but I think due to the tap rules, each of the 4 motor branch conductors would also need to be rated for 100A (likely longer than 25' runs per 240.21 and would not meet 240.21(4)). I think if I did size per the 25A on each circuit, I'd need appropriate branch OCPD. Thoughts?
Or the nearest size up maybe.
Is there any diversity in the running of the motors?
 

Besoeker3

Senior Member
Location
UK
Yes, always should have equal loads on all motors.
I'd size each of the four cables for each motor and have individual fuse switch and protection for each motor that would shut down the VFD. Belts and braces.

But I live in UK so NEC doesn't generally apply.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Revisiting this, generally speaking, we don't want the load side circuit to open up under load or it could damage the VFD (unless we include a reactor which isn't always the case). On the example listed above, if we have a 25A thermal trip, the magnetic of the same device is 325A which effectively wouldn't open under normal conditions correct? Essentially i want to protect the wires, but I don't want the circuit to open under a slight overcurrent and fault the VFD. Thoughts?
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
... but I don't want the circuit to open under a slight overcurrent and fault the VFD.
"Slight" overcurrent is vague. You have a 100A VFD, it can put out 100A. It has no idea if that is split up as 25-25-25-25 or 10-10-10-70. So if the motor is rated for 25A and it pulls more than 25A for as long as an I2t trip curve will allow, you will WANT the OL to trip the drive. The drive will have no idea what's going on. The chanced of damaging a drive from an emergency opening of a circuit is very low now days (it used to be more risky).
 
Yeah I guess I worded that funny. Dry-contact of the overload device should fault the drive from a digital input standpoint, but I'd rather not open the power circuit at that time. That would also likely require manual reset.
 
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