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To keep this concise, here are the facts:

  • This is an old house, with a lot of 2-prong outlets. The garage is fed off a 20A breaker (which is not grounded).
  • I'm finishing the garage. I don't plan on this being a living space, but want to have outlets for lite project work, etc. This circuit really needs to be grounded, especially with the cement floor, etc.
  • So far, I've replaced the socket that this circuit feeds off with a GFCI (with no ground). I've fed six 3-prong outlets off of this on the walls of the garage, but I still need an actual ground.
  • The garage is on an exterior wall, which contains a metal natural gas inlet line.

I'm considering installing a new ground electrode outside of the exterior wall, running a conductor through the hole and attaching this to the ground wire in the circuit. Is this a viable strategy that will fix my grounding problems on this circuit?

  • What does your main house's breaker panel look like? If you live in the US, the NEC say's it's legal to keep ungrounded outlets in a home that does not have a grounding conductor installed. Keep in mind that if you do this, you can't change the outlets to 3-prong UNLESS you mark them in some way to say that they are actually ungrounded. – tbox Aug 23 '16 at 21:25
  • It looks like it's been partially updated (bathrooms have GFCIs that actually test as grounded) so it seems like somebody ran some grounds, just not this circuit. I could probably run a ground all the way back to the box, but it it actually seems easier to just put in a new electrode for this circuit (since it's literally 2 feet from the actual soil). – Network Effects Aug 23 '16 at 21:41
  • Is this circuit run directly off the main panel, or off a subpanel? – ThreePhaseEel Aug 23 '16 at 23:26
  • Why do you need an actual ground? The GFCI should protect you from any ground-faults, so what exactly do you need the equipment grounding conductor for? – Tester101 Aug 24 '16 at 12:00
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What you need isn't a ground rod -- it's an EGC (ground wire) run back to the rest of the grounding system, so that current can use it as a safety return to the neutral/ground bond in the main panel.

Fortunately, under the 2014 NEC -- you can run your new ground wire to a suitably sized branch circuit ground, as well as to the equipment ground conductor system, or to the main panel. It can also follow any route you wish -- as it's a safety-drain of sorts, it doesn't have to be routed with the other circuit conductors.

  • I agree , a second ground location other than the main panel can really cause bizzare problems if there is a possibility of lighting. But will get flamed if I explain this because most have never seen it. + – Ed Beal Aug 24 '16 at 1:03
  • @EdBeal -- I'd actually expect weird stuff to happen with multiple independent rods and lightning -- the potential gradient across the earth would be quite significant in that case... – ThreePhaseEel Aug 24 '16 at 1:06
  • That is what I was suggesting. Since no measurement of actual grounding value at the main is required by code with 2 rods, most electricians have never connected the line to ground with the utility neutral disconnected at 25 ohms as required by code this will not trip even a 15 amp breaker, less than 5 if memory serves. We used to do this in the 70's and measure the current... This was the basis that got me flamed several times when first trying to help on the site now I only cite code, no matter how wrong it may be.! Maby a topic of a forum or ? On this site not done that yet – Ed Beal Aug 24 '16 at 1:46
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No, no, no!

It's an easy error to make, but you are thinking wrong-headedly about what that green wire is for. We call it "ground" but mainly it's to provide a return path for electrical faults. A path that isn't you. The return path only works if it goes back to the electrical service panel.

As a separate matter, we want this "equipment safety ground" and the neutral to be quite near whatever ambient voltage may exist in the soil around your house. That way equipment safety ground can't shock you if you're standing in a puddle. That is the only reason for the grounding rod.

Now here's where people get confused: thinking these are interchangeable. Actually, earth is a terrible conductor. You would not want to use earth as a ground wire. If you had a fault from hot to ground, the fault would try to "lift" the equipment safety grounding system up to 120V. The high resistance of the earth would mean it would succeed! Not enough currernt would flow to trip the breaker. Your entire grounding system would be energized near 120V which would make even the cover screws on a light switch deadly.

That is why you absolutely need the equipment safety ground to be an actual conductor, which goes back to the service panel. That way, there is a fat, low-resistance grounding path, which assures that a ground fault will flow a lot of current and definitely trip the circuit breaker.

Under the new NEC 2014 rules, you can route the ground wire any way which is practical, back to the panel the breaker is in. The ground wire does not need to run with the conductors. Also, you don't need to home-run all the way back to the panel, if you can reach another circuit whose grounding path goes back to that panel. (And the grounding path must be large enough; if you need to run a 12 AWG ground wire, a path involving a 14 AWG ground wire is not big enough.)

You don't need a separate grounding rod if the building is attached. Water lines are fine, but never use a gas line as a ground rod.

  • OK. That's helpful. Actually my main concerns was that the cement floor of the garage may be a lower resistance ground than a wire back to the panel, but I could always just run it back and test the resistance. – Network Effects Aug 24 '16 at 20:37
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    Coppe is a much better conductor than concrete. You certainly don't want fault current returning through the concrete, because in that case it is likely also returning through you :) Fault current wants to get back to source, not to ground. Current doesn't come from the ground, it comes from the 2-3 wires coming in from the poletop. – Harper Aug 24 '16 at 23:21

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