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I have an existing wire run from my main panel on the house to a detached garage with three wires: Red, Black and White. No ground. I want to install a new subpanel in the garage. I'll be installing grounding rod(s) for the subpanel.

  1. Since my subpanel will have it's own grounding rod, do I still need to keep the neutral and ground wires separate?
  2. Do I run my ground from the grounding rod directly to the ground/neutral bus?
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    Hello user2760221. Is the run to your detached garage in metal conduit?
    – Edwin
    Jan 25 at 1:21
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    What sort of cable was used for the run from the house to the garage, and I take it the run was done underground? Jan 25 at 2:10
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    All sub panels since the 1999 NEC have been required to have an isolated neutral and a detached building requires additional grounding. I have found some jurisdictions that allow a sub on a 3 wire system but you need to check with your building codes inspector.
    – Ed Beal
    Jan 25 at 2:18

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Grounding rods at the sub-panel are not a substitute for a proper ground to the main panel. The earth is generally pretty high resistance and so will not guarantee that a hot-to-ground short will trip a breaker. Thus, such a short will raise the local ground to near 120 volts.

Bonding hot to neutral (the old 3-wire method) avoids this problem but adds a different problem: if the neutral wire back to the main panel fails, the local neutral/ground can rise to 120 volts through a turned-on 120 volt load. This is why it was banned.

To do it properly and to current code, you need a separate ground back to the main panel. Are your current wires running through conduit? If so, it should be pretty easy to add a ground wire. In fact, if you have metal conduit which is properly installed, you should be able to use it as ground.

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  • Thanks for the info! The cable is in a conduit so I'll try pulling a new wire. There are a lot of questionable electrical things goin on with this house so I don't trust that the conduit was properly installed. Jan 25 at 13:23
  • Is there a way to test to see of the conduit (buried) is connected from panel to panel. Jan 25 at 13:27
  • @user2760221 that's a great new question to ask! Down here there just isn't enough room to give it the attention it needs. Feel free to link your new question back to this one for reference if you want.
    – FreeMan
    Jan 25 at 16:19

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