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I'm rewiring my garage from in my opinion is an odd setup. In my house panel (200A split phase) there is a 20A breaker there is a junction somewhere ( I haven't traced it out yet which branches this circuit to two. The net result is a 3 way switch for the garage lighting and an always hot circuit. I haven't quite figured out how this is done with three wires. Both circuits are run over 3 conductor overhead wiring to the garage. At the service drop, the cable is connected to two 12/2 cables. The neutral is shared, grounds are tied together and just left hanging there at the drop.

Not liking this setup at all I worked backward and wired up all the new stuff in the garage to a small secondary panel. The next step is to connect to the existing wiring, but I'm trying to figure out what the previous guy did. The voltages to the wires coming in are strange. I'm pretty certain this is partially because they are not grounded. This brings me to my first question.

I've done lots of work inside homes but never from building to building. Since overhead cable doesn't have a ground, and neutrals and grounds are generally on the same bus within the breaker panel, this this mean I should have a grounding rod at both ends of this cable?

The other question I'm sure I can figure out. I haven't closely examined the previous guys work other than at the service drop. Since the voltages are so strange it's difficult to tell what is going on. If someone has a quick answer, I would appreciate it. Like I said the garage lighting can be controlled from my kitchen and a switch in the garage. How did they wire a three way switch in addition to a always hot switch on three wires.

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  • Are you sure that is true in this case? I'm not an electrician, but my understanding of the reasoning behind this is to avoid getting neutral current at a ground wire which goes back to the main panel. The problem is that there is no ground back to the main panel. This is why I asked if I would need a grounding rod.
    – mreff555
    Aug 22, 2015 at 14:35
  • You will need a grounding electrode system installed at the garage, which the electrical system will have to be bonded to. You keep mentioning "strange voltage", but you never clarify what that means. You haven't really provided enough information for us to help you, maybe a few diagrams would help. If you're upgrading the system, I'd consider installing a 4 wire feeder instead of the 3 wire. And if you're installing a panel, you might want to upgrade the feeder to something larger than 12 AWG.
    – Tester101
    Aug 22, 2015 at 14:49

2 Answers 2

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The sub panel will need the ground to be separated from the neutral. The only place they are joined is inside the main breaker panel. This prevents open neutrals between the main and sub panel from sending current over the ground, and possibly electrocuting someone if a better path to ground can be found.

For the 3 way switch, I'm guessing you need to trace more wires, or you have a smart switch of some kind. You'll need to check the switches first, and then start tracing the cables on each end to see where they go. I'm guessing a second line is running just for the 3 way switch that you haven't found.

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  • 1
    Are you sure that is true in this case? I'm not an electrician, but my understanding of the reasoning behind this is to avoid getting neutral current at a ground wire which goes back to the main panel. The problem is that there is no ground back to the main panel. This is why I asked if I would need a grounding rod.
    – mreff555
    Aug 22, 2015 at 14:44
  • If the grounded (neutral) and grounding conductors are bonded, current will always flow on the grounding conductor as well as the grounded (neutral). It won't take an open neutral to put current on the grounding conductor.
    – Tester101
    Aug 22, 2015 at 15:06
  • @mreff555 The grounding electrode system at the secondary building is used for equipotential bonding, so that the reference to ground is the same throughout the entire system. With a 3 wire feeder, the grounded (neutral) will have to be bonded to the electrode. This is because in this case, the grounded (neutral) is also serving as the grounding conductor. It's not an ideal situation, but is allowed with existing 3 wire feeders is some situations.
    – Tester101
    Aug 22, 2015 at 15:15
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Thanks guys,

I spoke with an experienced electrician who validated my concerns and gave me two options after I explained to him that I am not concerned about a three way switch.

  1. Replace the 20A breaker and goofy three way switch wiring in the house with a 40 or 60A double pole breaker. Continue to use the 100A three conductor (SEC cable). Install a grounding pole at the garage. In this case neutral and ground could be on the same bus but it would be better if they were not.

  2. Replace the 20A breaker and goofy three way switch wiring in the house with a 40 or 60A double pole breaker. Replace the SEC cable with 4 conductor SER cable and run my grounds back to the house.

from my understanding both options are correct, albeit the second may be a bit more correct and the first is much cheaper, I'm thinking about going the first route.

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