Equipment Grounding Large Motors

[Leo1]

I always include an equipment grounding conductor, per table 250.122, in the feeder raceway(ungrounded and equipment grounding conductors in same raceway) that serves each motor. However, I have noticed on large motors, usually, 100hp and above that many designs and contractors also bolt an additional equipment ground to the motor frame and extend it to the counterpoise ground. Is this driven by prudent engineering or is there a Code requirement to have multiple equipment grounds on large pieces of equipment?

 

[Ponchik]

I don't think there is a code reference but rather a design preference.

But others with more experience with large motors will chime in and set me straight.

 

[tom baker]

No code requirement.
Motor frame ground is likely based on some old specification language that dates back generations and never gets updated.
My guess is back before we had 277/480 solidly grounded systems, with delta ungrounded, each frame was grounded as you describe
Best and most code compliant is an EGC with the ungrounded condutors. This will reduce the impedance in a fault. The frame ground could cause issues as its induces additional paths for current