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Long story short, I want to branch off an existing 100 amp, three-wire subpanel to a new workshop I'm building. Running a whole new wire would be expensive as the house is finished and from what I've been told pulling the existing wires out of the conduit after 30+ years isn't going to end well. So, one of the electricians suggested that we leave the existing three-wire setup and just run a smaller (I think he said 8awg) ground wire from the main panel to the shed to separate the neutrals and grounds (they're bonded in the shed right now). I tried searching for if this is to code and if it would be allowed but couldn't find anything. Is this a bad idea?

Summary: Running a single smaller AWG ground wire to an existing 3 wire subpanel is much easier and cheaper than trying to run an entirely new four-wire circuit.

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  • Think running a separate ground wire okay, but not an 8 gauge. Have seen mention of 2,2,2,4 with the ground being a size smaller than the hots. Running the ground from the main panel?
    – crip659
    Dec 16, 2021 at 20:35
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    @crip659 #8 cu is good for conductors rated up to 100A (NEC Table 250.122). The table says overcurrent device, but text says if conductors are oversized you need to adjust the ground too. Never understood 2224 AL, the #4 seems oversized except the color identification for #4 is less restrictive allowing the #4 as a 3ph neutral?. The Code allows "not less than 83%" for residential service feeders. A 110A service would require conductors rated for 91A and #2 is only good for 90. so it seems like the maximum service #2 could feed would be 100A, requiring only #6 AL. Dec 17, 2021 at 1:38
  • What type of conduit is being used for the run? Dec 17, 2021 at 2:43

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If the workshop is detached, you will need to drive a ground rod at the location of the subpanel in addition to running a new ground wire. Your main panel should be grounded and bonded to the neutral and tied back to the neutral-ground of the utility.

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    Only on NEC 2005. A ground rod can't do what a ground wire does. The point of a ground wire is to return large quantities of fault current. Dirt can't do that... it has 25 ohm resistance on a good day, so a 120V bolted fault will only flow 5 amps. That won't trip a breaker. Dec 16, 2021 at 19:51
  • @Harper-ReinstateMonica But won't that ground wire just go back to a ground rod at the main panel? (maybe I'm working too hard). I thought the subpanels had to have a driven ground.
    – JACK
    Dec 16, 2021 at 21:10
  • Subpanels in separate buildings need ground rods. All subpanels need ground back to main. Dec 16, 2021 at 21:42
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    But at the main panel is a neutral-ground bond, which will assure fault current completes the loop back to source (utility neutral). There's another reason I forgot to mention: the dryer/range problem. With a 3-wire feed, a simple break of the neutral wire results in all grounds being energized at a floating voltage as high as 120V. (the usual lost neutral situation + energized grounds). Of course the ground rod tries to substitute as a neutral wire, but it's really not up to the task. Dec 16, 2021 at 22:00
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    @Harper-ReinstateMonica Ah yes, the neutral-ground bond back to the source. Hate getting old.
    – JACK
    Dec 16, 2021 at 23:14

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