VFD Cable and Disconnect

Sean.Day72

Member
Location
Florida
Did research on this but found conflicting answer. I have a 60-hp lift station with the VFD drives located in a adjacent electrical house. Utilities requested a local disconnect be located outside near the pumps for maintenance and make it easier for them to megger the cable. This also satisfies the NEC "line of sight rule" for motor disconnect. VFD cable manufacture says cable grounds shall be unbroken from drive to motor case and "carried through" any disconnects to reduce emf noise. I assume this means I need to run a separate EGC to the disconnect? How is the ground carried through with out touching the disconnect casing?

Also the disconnect will have aux. contacts to shut down the drive if someone turn the disconnect off while pump is running.
 

Besoeker

Senior Member
Location
UK
Did research on this but found conflicting answer. I have a 60-hp lift station with the VFD drives located in a adjacent electrical house. Utilities requested a local disconnect be located outside near the pumps for maintenance and make it easier for them to megger the cable. This also satisfies the NEC "line of sight rule" for motor disconnect. VFD cable manufacture says cable grounds shall be unbroken from drive to motor case and "carried through" any disconnects to reduce emf noise. I assume this means I need to run a separate EGC to the disconnect? How is the ground carried through with out touching the disconnect casing?

Also the disconnect will have aux. contacts to shut down the drive if someone turn the disconnect off while pump is running.
It's common here (UK) to have a padlockable isolator adjacent to the motor. I know of no instances where that has caused a problem.
I don't know if is a mandatory legal requirement these days.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Seems like the EGC in the cable is nothing more than an isolated ground, I'm guessing that if you provide another EGC for bonding the other metal parts you should be code complaint.
 

Wire-Smith

Senior Member
Location
United States
what cable manufacturer? different manufacturers recommend different things in this arena. some will say use glands that ground the shield at the disconnect, others will say keep shield isolated but if you also have an insulated ground you can ground the disconnect with that. the not grounding your referring to is likely the shield which some cables have bare grounds that are in contact with the shield and you may not want to ground it if electrical noise is a concern. drive cable may only be being used for capacitance though, if noise isn't a concern or the disconnect is close to the motor and no sensitive electronic equipment is in the area then it's very unlikely you will ever see a problem. i don't think its technically code compliant to not use a ground ran with the conductors (inductive reactance), but it sounds like you would be okay running a jumper, if you could bundle it with cable it might be better.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
You will have to strip the cable jacket off so that you can connect the ungrounded conductors to the disconnect. That will give you access to the EGC in the cable. You can connect the EGC to the disconnect enclosure without cutting it using some type of "lay-in" lug.
Running an additional EGC outside of the cable to bond the disconnect enclosure would be a violation of 300.3(B).
 
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