Two buildings: Home and Detached Garage (circa '04 US). Service comes in through a Meter on the Garage to a PVC conduit into the Main box in the home (Single phase 240V with Neutral). In the main box Neutral and GND are bonded and bare ground wire coil heads out somewhere towards the front of the house, presumably to a ground rod.

There is an additional PVC conduit from the main panel back to the garage with 2 Hot, Neutral, and Ground that is used as two 120V runs (opposite phase), one for lights one for outlets and no sub-panel.

I'm planning on putting a 240V outlet in the garage and increasing the conductor side for 50A service. I will replace the two 120V breakers in the main panel with a 240V breaker and pull new wire (6ga x3 and 10ga Gnd) to a new sub-panel. From the sub-panel I'll wire back in the lights/outlets and the new service. Ground and Neutral will not be bonded and I know I need at least one grounding rod on this panel.

So finally I get to my question: There is an existing grounding rod that is being used by the Meter and apparently nothing else. Can I just add a new ground rod clamp to use with the sub-panel? Any code I'm violating?

  • NEC 250.130. (A,B,C) is the code reference that allows this. – Ed Beal Dec 4 '20 at 19:02

As far as code it is allowed to add your clamp at the rod and use that for the new panel. If there are no breakers in the panel just the meter the only folks that could squak about it would be the power company. Sometimes the utility can be funny but if you have breakers there it is required so you would be fine. I would do it and I carry a couple ground rods on the van when I do service calls. Code specifically allows this

  • Because the sub panel is new work, and the grounding electrode system at the detached building has one rod, isn't a supplemental electrode (or a 25-ohm test) also required by 250.53(A)(2)? (2017 NEC numbering) – Greg Hill Dec 4 '20 at 20:59
  • I have never been required to have a second rod on a 4 wire feed , the 1 time the inspector asked I checked it and it was 15ohms he said that makes sense the further your rods are separated the better the ground. With a multi point tester like I have I would just string the electrodes towards the house and probably even pass in the desert. I should mention he knew I had the tester and had not required a second rod previously this was in early 18 after the 17 code was adopted, I believe any 4 wire feed will pass as the code only specified 25 ohms not the direction of the test. – Ed Beal Dec 4 '20 at 21:10
  • Interesting... I happen to be in the midst of a branch-circuit-to-feeder upgrade not unlike the one OP describes. When I filed for the building permit the plan inspector specifically mentioned he'd expect to see two rods installed. I guess you're usually getting off easy with neither the test nor the extra rod, whereas my inspector is going to be very by-the-book. – Greg Hill Dec 4 '20 at 21:39
  • OK, let's face it....not all inspectors are created equal. Much of what gets called out is dependent upon the experience and knowledge of the inspector. When I built my house I did all utility (power, cable, telephone) terminations at a small pump house. It had a sub-panel in it to run the pump, lights, outlets, water softener, irrigation controller, and battery tender and heater for the standby generator. I ran a "tap" from one of the 200 amp lines to the sub-panel. I did it according to code, but had to "school" the inspector on chapter and verse of the NEC that allowed it. – George Anderson Dec 5 '20 at 1:08
  • continuing (sorry, but ran out of space). The inspector didn't initially believe I wired the house bc I'm the owner, not an electrician. But after talking with him he realized I knew what I was doing and only had a couple of minor findings (I missed a couple of AFCIs) and outlet spacing was off by too much in one area. – George Anderson Dec 5 '20 at 1:12

You must tie to the existing grounding rod...

Since there's a rod present there already, you must tie your new subpanel's grounding electrode conductor to it, as per NEC 250.50 and the reference to Part III of Article 250 in NEC 250.30(A).

...and may be required to drive a second

If your existing rod is judged not to meet the 25ohm standard in NEC 250.53(A)(2), or if the AHJ wishes it, you may need to drive a second grounding rod 6-8' away (or more, if you wish) from the first.

You can either tie to the existing grounding electrode conductor with a 6AWG copper wire and an appropriate tap connector, or simply run the new rod's grounding electrode conductor back to the grounding bar on the subpanel, though.

  • I read 250.64(F)(2) to mean that, as an alternative to tying the new rod to the existing grounding electrode conductor with an appropriate tap connector, it's also allowed to install a new grounding electrode conductor from the new rod all the way back to the panel. Do you read it the same way? – Greg Hill Dec 5 '20 at 1:41
  • @GregHill -- the issue is that you'd still have to tie the new rod to the existing rod in that case – ThreePhaseEel Dec 5 '20 at 2:05
  • Maybe I could be more clear - I interpreted that section to mean that 2+ GEC could be independently home-run. Each rod would connect to its own #6 wire; both wires would land on the ground bar in the panel. That's OK, isn't it? A few extra dollars/feet of #6 wire to run the second wire all the way to the panel seems like a no-fuss way of getting the job done - if it's acceptable. – Greg Hill Dec 5 '20 at 3:53
  • @GregHill -- yeah, you could run a GEC to a new rod then a GEC to the existing rod – ThreePhaseEel Dec 5 '20 at 4:37
  • Just to clarify code separation of 8’ or longer driven grounding electrodes is 6’ or more if you want them 12’ apart that is actually better. – Ed Beal Dec 5 '20 at 7:06

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