On a recent project installation, the engineer specified the use of VFD cable [shielded, 3-symmetrical grounds, etc.] to be installed in RMC from the drive to the motor [less than 125’ in my opinion belts and suspenders], at the motor there is also a local non-fused safety switch. Wanted to get opinions from others as how they addressed correctly grounding the conductors. The drive manufacturers indicated the shield / ground conductors need to be continuous from the drive to the motor termination without any intermediate grounding, this also appears to be the stance from the cable manufacturers [still waiting for response from one of the vendors]. So, the question is how to handle this at the local safety switch? If the grounds are continuous can you use the conduit system [provided it is properly installed with the correct fittings] as the safety switch ground or would [as this engineer recommended] install a separate 600V ground conductor in the conduit with the VFD cable for safety switch grounding, making the pull much more difficult with the VFD cable and a separate ground conductor…thoughts??
VFD Rated Cable
- Thread starter Mike01
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Ragin Cajun
Senior Member
- Location
- Upstate S.C.
Why is a disconnect switch needed? Not by code. And in VFD installations I have designed, the VFD supplier didn't like one between the VFD and the motor.
RC
RC
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Is your concern about bonding the disconnect enclosure?
(Disconnect within sight is the typical reason for it RC).
(Disconnect within sight is the typical reason for it RC).
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
NEC requires disconnect within sight of the motor and within sight of the controller, with some exceptions, IE submersible motors..
430.102
The engineer included a local safety switch at the motor in addition to the VFD. The VFD is remote in a electrical room. My guess is that mechanical maintenance staff may not have access to the location of the VFD so the local safety switch gives them a place to disconnect the motor. My understanding if the disconnect is not within site but lockable it is acceptable but either way we have one.
Jraef; yes the concern is more about the safety switch bonding, if the recommendation is not to bond the grounds or the shield from the VFD cable in the disconnect switch and carry them directly to the motor terminal box, the grounding of the safety switch is then either from the conduit between the two or another individual grounding conductor installed in the conduit with the drive cable. Just curious if anyone else has come across this and how they handled it??
Jraef; yes the concern is more about the safety switch bonding, if the recommendation is not to bond the grounds or the shield from the VFD cable in the disconnect switch and carry them directly to the motor terminal box, the grounding of the safety switch is then either from the conduit between the two or another individual grounding conductor installed in the conduit with the drive cable. Just curious if anyone else has come across this and how they handled it??
- Location
- Illinois
- Occupation
- retired electrician
You don't have a choice about connecting the EGC within the drive cable to the disconnect enclosure. (it is my opinion that 250.148 applies to the disconnect enclosure)
Interpertation
Interpertation
The wording of 250.148 is a little un-clear it states "If circuit conductors are spliced within a box or terminated on equipment within or supported by a box, all equipment grounding conductor associated any of those circuit conductors shall be connected within the box or to the box with devices suitable for the use accordance with 250.8 and 250.1487 (A) through (E).”
it says "within or to the box"
Interpertation
The wording of 250.148 is a little un-clear it states "If circuit conductors are spliced within a box or terminated on equipment within or supported by a box, all equipment grounding conductor
it says "within or to the box"
Cow
Senior Member
- Location
- Eastern Oregon
- Occupation
- Electrician
We use a fair amount of vfd cable with disconnects in line to the motors. I don't have a problem going from the VFD to a disconnect and terminating the drive cable there, and then taking off with more drive cable to the motor.
I don't see the issue.
I don't see the issue.
Fnewman
Senior Member
- Location
- Dublin, GA
- Occupation
- Sr. Electrical Engineering Manager at Larson Engineering
I agree and have seen it done many times without problem. With respect to the local disconnect, it is often a request from the from the client/owner to avoid having to go and lock out at the MCC. However, I always recommend an auxiliary contact in the local disconnect to let the control system know the motor has been stopped so interlocking can operate properly and to prevent the motor from automatically restarting which the local disconnect is reclosed.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
That auxiliary contact can also be used to inhibit the VFD for it's protection.
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