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This weekend I attempted to install a breaker box into a pre-wired garage. The previous owners took the breaker box with them when they moved, and since I have installed quite a few subpanels, it seemed like an easy job to replace it.

The subpanels I have installed have always used 4-wire connections, so I was surprised to find that only three wires had been run underground through the conduit, and the ground was attached to a separate rod at the garage.

Reading up on the permits for the house and some NEC info lead me to believe that this was a pre-2005 installation, at which point 3-wire was still allowed to connect a sub to a main in a detached structure.

My question is this:

What is the correct setup for neutral and ground in the sub panel? Should they be bonded, and do I need to attach a ground to the buried rod at the garage as well?

Specifically, I need to know if this image shows a correct setup before I proceed. This question has been touched upon in other posts, but not adequately answered:

3-wire feeder diagram

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If you are in a situation where a 3 wire feeder can be used, you will need to bond the grounded (neutral) conductor to a proper grounding electrode system in the detached building. So yes, the diagram is correct.

NOTE: The grounding electrode system, may be required to be made up of more than a single ground rod.

  • Thank you! As I am unable to test the existing ground rod, I will be connecting a second one just to be safe. – droidski Oct 13 '15 at 0:45

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