How to ground equipment that has supply power from (2) subpanels

[hughesgh]

I have a custom "box" with multiple pieces of rack mounted equipment inside it (DC power supplies, step-down transformer, fans, etc). The box is supplied with (2) different types of power; (1) 115VAC power input and (1) 208VAC single-phase power input. The metallic rack that houses these pieces of equipment needs to be grounded in case of any shorts or electrification of the rack.

I cannot guarantee that the (2) power inputs will be fed from the same subpanel within the building. How do I ground the metallic rack inside this box? Should I only use the grounding conductor from the 115VAC input, or should I only use the one from the 208VAC input? Or should I tie both grounding conductors together to the rack inside the box?

If the power inputs to the "box" were fed from the same subpanel, then this would not be an issue, correct? Since I cannot guarantee this, I am not sure what the NEC says about this type of situation.

Thanks for any help you can give me!

 

[LarryFine]

Welcome to the forum.

I'd say connect both circuits' EGCs. If either is interrupted, the other remains.

 

[Eddie702]

If both circuits the 208v and the 120v appear in the box, then I would use both EGCs and tie them together (although they don't have to be tied together) they could be terminated separately but together is best practice.

 

[jaggedben]

This is the same question as here:

Utility and UPS grounds to a control panel

We have control panels that have dual 120vac feeds, one commercial power and one UPS power. Should the grounds for each be run independently back to the plant ground grid, or can they be bonded together in the control panel, and run one ground wire back to plant ground grid? Each feed is 20...

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