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I've searched the forum and haven't found an answer for this exact situation. Here's the configuration: Main 200A service panel at one end of house (exterior), and 100A sub panel at opposite end of house (interior access on exterior wall). EMT running in crawlspace to connect panels. Both panels in one building. I know that the ground bus and neutral bus are not bonded at the sub panel, and their respective conductors run back to the main service panel.

Question: What are the thoughts on adding additional grounding electrodes at the sub panel side of the house, and connecting those to the sup panel's ground bus (again, not bonded to neutral) as an additional redundant ground? Does this increase grounding safety, grounding performance? Does it interfere with the purpose of the ground back to the main panel? Is it a waste of money, since the main service panel has its own grounding electrodes? Most of all, what does the NEC say, if anything, on this situation?

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  • You know the EMT itself is a valid grounding path right? You are already doing belt and suspenders by running dual grounds.
    – Harper - Reinstate Monica
    Jul 10, 2018 at 18:58
  • Yes. However, the EMT run uses a mix of set screw, compression, and some insulated raintight fittings and runs about 80 feet. The crawlspace can get damp, there may be corrosion over time on some fittings. I feel better with a separate grounding conductor in the EMT, rather than relying entirely on the EMT in this case. So, about that separate grounding electrode (two actually) at the sub panel ... overkill?
    – sc_props
    Jul 10, 2018 at 19:38
  • I should add that my goal for the EG is safety, service life, and low impedance. Money is not a the primary concern.
    – sc_props
    Jul 10, 2018 at 19:59

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What you want is a redundant grounding conductor

In your situation, provided that conduit fill allows, pulling a redundant equipment grounding conductor (bare 8AWG copper is fine) through the conduit between the panels is A-OK -- simply land it on the ground bars at both ends of the run.

The electrodes you describe would violate the NEC though

Within a structure, grounding electrodes are connected together by grounding electrode conductors, not equipment grounding conductors, though, and trying to use an EMT to do both jobs at once violates NEC 250.121:

250.121 Use of Equipment Grounding Conductors. An equipment grounding conductor shall not be used as a grounding electrode conductor.

So, forget the ground electrodes, pull the redundant EGC, and be happy.

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  • Thanks for answering. Maybe something wasn't clear though - I have no intention of using the EGC as the grounding electrode conductor. Nor do I have any intention of relying on the EMT as any sort of grounding conductor. The EMT is protecting the wire, that's all. I'm going to run 6 or 8 AWG as you say from the sub panel 's ground bar back to the ground bar at the main service panel. The question is really about whether to connect the sub panel's ground bus to grounding electrodes outside, in the immediate vicinity, with bare copper IN ADDITION to the EGC back to the main service panel.
    – sc_props
    Jul 11, 2018 at 2:53
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    @sc_props -- the redundant EGC is all you need, and the EMT will become a GEC if you put redundant grounding electrodes in at the subpanel for a single structure, whether you like it or not
    – ThreePhaseEel
    Jul 11, 2018 at 3:02
  • Cool, thank you. To clarify, I don't like it or dislike it. I'm totally okay with the EMT being GEC.
    – sc_props
    Jul 13, 2018 at 17:09
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    @sc_props -- but the NEC isn't OK, that's the point
    – ThreePhaseEel
    Jul 13, 2018 at 22:04
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    @sc_props One reason you don't want to add the extra grounding electrode as you described is that it tends to cause current to flow between the electrodes. When lighting gets involved, either via a nearby ground strike or a strike on/near a power line, that current can get awfully high. If you get enough current going through your house wiring and/or EMT, bad things happen.
    – mrog
    Sep 20, 2018 at 18:41

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