Adding a separate EGC to a three wire subpanel. NEC rules. Best practices.

[brycenesbitt]

Given a three wire subpanel, what considerations are there for adding a dedicated ground wire?
The subpanels in question have 1940's cable bundles with black, red and white wires, in a single non-metallic bundle.
There's no chance of re-running the feeder bundles, and they are adequate other than the lack of the fourth wire.

1. Is there any need for a new grounding conductor to follow the same physical path as the ungrounded conductors?
   
   
2. May the grounded route back to the main panel involve a mix of existing conduit and new wire, with the new wire bonded to the nearest conduit serving other subpanels? How many subpanels could a single conduit serve in such a manner?
   
   
3. Is there any specific protection rule for an insulated external grounding wire... is there any requirement the grounding wire be run inside conduit?

I'm reading NEMA technical bulletin No. 97 Feb. 2009 (Rev. 6/15) on the capacity of existing conduit https://www.nema.org/docs/default-s...onduit-and-emt-proven-to-meet-the-nec-reg.pdf to try and determine if two subpanels could share one conduit as the EGC path to the main.

 

[tom baker]

1. You are referencing ground, do you mean EGC? Please see definition of Ground
2. You mention conduit, is it RMC or EMT?
3. Your questions 1,,2,3 is there a specific rule that requires the EGC to be in the raceway with circuit conductors, yes there is.
See Section 300.5 I
If the EGC is not in the raceway, in a ground fault, the high magnetic fields do not cancel resulting in high impedance and the OCPD will not open.
So run the EGC in the raceway (unless its a raceway meeting requirements of 250.118), or don’t bother running it

 

[infinity]

The subpanels in question have 1940's cable bundles with black, red and white wires, in a single non-metallic bundle.

Are these SE cable? Sounds like it might be. The only solution is to run a new feeder with the proper EGC.

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