MV motor frame grounded; necessary to connect the ground terminal at inside the box?

[vagojr]

Good morning,

I have a question about complying the NEC for the grounding of 4000V rated MV motor.
At this time we have the frame grounded at two points as it could be observe in the below sketch as per NEC 250.112(C) and 430.242.
The supply cable is 3P3W without equipment grounding conductor.
The connection to the frame, if it is not indicated by supplier, it has been done by experienced on-site people.

However, our client is claiming us to explain why is not connected any grounding conductor to the grounding terminal installed at inside the terminal box. This terminal box is factory mounted and it is keeping permanent metallic contact with the motor frame, so I understand that it is not necessary to connect the grounding terminal since the motor is already being grounded as per NEC 250.112(C).

The case is that the NEC 430.12(E) seems to require also the connection to the grounding terminal of the motor terminal box.

Question: How could I defend the current design by means of the NEC, which is considering no connection to the grounding terminal but to motorframe (see below sketch)?

Thank you very much in advance.

 

[don_resqcapt19]

...
The supply cable is 3P3W without equipment grounding conductor.
...

In general 300.3(B) requires the supply cable to contain an EGC. It is my opinion that this rule applies ever where a cable is installed in cable tray that is suitable for use as an EGC.

 

[vagojr]

In general 300.3(B) requires the supply cable to contain an EGC. It is my opinion that this rule applies ever where a cable is installed in cable tray that is suitable for use as an EGC.

Hello don_resqcapt19,

In this regards, you could see in the sketch a grounding cable installed all along the cable tray. This cable, properly sized, is those used to ground the motor frame, but it is not entering into the box and therefore there is no connection of grounding cable to the grounding terminal.

I have checked the NEC and I have not been able to find any section that says that if the motor terminal box (metal) is permanently attached to the motor frame (metal), and the motor frame is grounded as per the NEC 250.112(C), then the box is considered grounded.
The most similar case is those indicated in NEC 250.136(A) for "equipment secured to grounded metal supports" [equivalent to OSHA 1926.404(f)(8)(iii)].

Question: Accroding to your experience, could the NEC 250.136(A) be used to defend my current grounding system?

NEC 250.136 - Equipment considered grounded: (A) Electrical equipment secured to and in electrical contact with a metal rack or structure provided for its support and connected to an equipment grounding conductor by one of the means indicated in 250.134

Thanks!

 

[Smart $]

vagojr,

The finer detail of the section mentioned by Don [300.3(B)] is that you can run a cable without an EGC in cable tray, where the cable tray itself qualifies as an EGC or supplemented by a wire- or bus-type EGC. The problem is when the cable w/o EGC leaves the cable tray. See highlighted below...

(B) Conductors of the Same Circuit. All conductors of
the same circuit and, where used, the grounded conductor
and all equipment grounding conductors and bonding conductors
shall be contained within the same raceway, auxiliary
gutter, cable tray, cablebus assembly, trench, cable, or
cord, unless otherwise permitted in accordance with
300.3(B)(1) through (B)(4).

...

(2) Grounding and Bonding Conductors. Equipment
grounding conductors shall be permitted to be installed outside
a raceway or cable assembly where in accordance with
the provisions of 250.130(C) for certain existing installations
or in accordance with 250.134(B), Exception No. 2,
for dc circuits. Equipment bonding conductors shall be permitted
to be installed on the outside of raceways in accordance
with 250.102(E).

As you can see, 300.3(B)(2) limits your options for the EGC to be outside the cable assembly. In your case, an external EGC as depicted is not compliant IMO, assuming your cable is not in an EGC-qualifying raceway or cable-sheath assembly from tray to motor terminal housing.

In regards to 250.136(A), your motor would have to be designed for and secured, i.e. mounted directly to the cable tray for compliance to be obtained that way.

 

[vagojr]

Dear Smart,

The EGC in my design is being laid in the same run of the circuit because every cable tray is having the EGC which is being connected to ground electrode every 30m, so in my understanding the 300.3(B)(2) is being complied through what stated in 250.134(B). What is your opinion?



Then in regards to the NEC 250.136(A), the grounded motor frame could not be considered as a grounded support for the box?

Thanks a lot!

vagojr,

The finer detail of the section mentioned by Don [300.3(B)] is that you can run a cable without an EGC in cable tray, where the cable tray itself qualifies as an EGC or supplemented by a wire- or bus-type EGC. The problem is when the cable w/o EGC leaves the cable tray. See highlighted below...

As you can see, 300.3(B)(2) limits your options for the EGC to be outside the cable assembly. In your case, an external EGC as depicted is not compliant IMO, assuming your cable is not in an EGC-qualifying raceway or cable-sheath assembly from tray to motor terminal housing.

In regards to 250.136(A), your motor would have to be designed for and secured, i.e. mounted directly to the cable tray for compliance to be obtained that way.

 

[Smart $]

300.3(B)(2) is not being complied with. Only times EGC's are permitted outside the cable [and outside the cable tray] are for 250.130(C) existing installations (which is regarding non-grounding receptacles and [no-EGC] branch circuit extensions) and under 250.134(B) Exception No. 2, for dc circuits (I'm assuming it is not a dc motor).

At best, your grounding conductor is a bonding jumper, not an EGC, with respect to the motor.

Regarding 250.136(A), I think you are using a play on words to qualify it. The electrical equipment being referred to in your case is the entire motor, including its frame.

The only way I can see to getting around an EGC within the cable is to run it in a raceway, likely flexible metallic [250.118(5), (6), or (7)], from tray to motor terminal box, and properly bond both ends.

 

[vagojr]

Thank you Smart, I am understanding...
one last question; if the metallic trays and conduits are being grounded by this "bonding" external cable... then, could these trays/conduits be considered as EGC according to NEC?
thanks

300.3(B)(2) is not being complied with. Only times EGC's are permitted outside the cable [and outside the cable tray] are for 250.130(C) existing installations (which is regarding non-grounding receptacles and [no-EGC] branch circuit extensions) and under 250.134(B) Exception No. 2, for dc circuits (I'm assuming it is not a dc motor).

At best, your grounding conductor is a bonding jumper, not an EGC, with respect to the motor.

Regarding 250.136(A), I think you are using a play on words to qualify it. The electrical equipment being referred to in your case is the entire motor, including its frame.

The only way I can see to getting around an EGC within the cable is to run it in a raceway, likely flexible metallic [250.118(5), (6), or (7)], from tray to motor terminal box, and properly bond both ends.

 

[Smart $]

Thank you Smart, I am understanding...
one last question; if the metallic trays and conduits are being grounded by this "bonding" external cable... then, could these trays/conduits be considered as EGC according to NEC?
thanks

Yes, provided all the requirements for each circuit EGC is met. That is, a qualifying EGC for each circuit will have continuity from supply equipment to load equipment. Only one EGC is required where multiple circuits are run in the same raceway, cable, cable tray, etc. and its size is based on the largest overcurrent device rating of the circuits involved.

Another issue which hasn't been elaborated on is this EGC in the cable tray, bonded to grounding electrodes every 30m, cannot serve as a GEC (grounding electrode conductor) also unless it complies with all the requirements of both under 250.121 Exception. Note: 250.121 was added in 2011 edition, the Exception was added in 2014. Prior to 2011 it was essentially the same as 2014 but unwritten.

 

[vagojr]

Thank you very much Smart for your clarifications.
Best regards