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I am installing a sub panel in my garage this weekend. The previous owner had a subpanel in the same location, but when they moved they took the panel with them.

The wires from the mains panel in the house to the garage are still there, but since they are all black (except for the ground, obviously), I am not sure which is neutral and which 2 are hot.

Is there a way to test these wires without hooking them up to a grounded panel first? They are connected to the mains via a 220v 100A breaker.

I have done quite a few breaker installations over the years, and have installed many romex-fed outlets, but this is a new situation for me. Any help is appreciated!

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  • The wires are currently connected to the main panel at the other end?
    – Tester101
    Oct 7, 2015 at 15:43
  • Yes, they are currently wired to a 100A 220v breaker
    – droidski
    Oct 7, 2015 at 15:47

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Voltage

If the wires are hooked up in the main panel, then all you'll need is a multimeter/voltmeter.

  • Carefully position the wires so that there's no chance of them touching each other, or anything else.
  • Turn on the breaker in the main panel.
  • Carefully measure the voltage between each set of wires.
  • Turn the breaker back off.

When measuring voltage between the two ungrounded (hot) conductors, you'll measure about 240 volts. When measuring between either ungrounded (hot) conductor and the grounded (neutral), you'll measure about 120 volts.

Continuity

Alternatively, if you're not comfortable working with live wires (and really, who is).

  • Make sure the breaker in the main panel is OFF.
  • Using a multimeter set to measure resistance or continuity, check for resistance/continuity between the grounding conductor and the other wires.

You'll measure infinite resistance (or open) between the grounding and ungrounded (hot) conductors, but you'll measure a small resistance (or closed) when measuring between the grounding and grounded (neutral) conductor.

Visual inspection

Examine the wires carefully. If they are part of a cable assembly, one of the wires may have stripes on the insulation or some other markings. If these are individual wires, there's likely no markings unless the installer marked them with paint or tape.

Once you find the grounded (neutral), mark it with white paint or white electrical tape.

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  • UPDATE: The installation is nearly complete. The continuity test won the debate in the end as I had to disconnect my hot wires at the main due to a bad breaker anyway. It worked perfectly and took all of 5 minutes to complete. Thank you all for your assistance!
    – droidski
    Oct 12, 2015 at 4:27
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First off, as with any electrical work, ensure that the breakers are off and you've verified the wires you are working on are not energized.

Assuming you can safely access the wires at the other end and you have a multi-meter that can measure continuity (resistance around zero). What you would do is at one side, connect two of the wires together. Then at the other side, find the two wires that have a resistance near zero. Switch the two wires and repeat the process to identify the other two. Now do it a third time and you should be able to identify all three wires individually by identifying the three "pairs".

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  • To be safely recommending a continuity test you should caution that that breakers at the main panel are OFF and the wires being tested by the means you describe should be fully disconnected from the breakers.
    – Michael Karas ♦
    Oct 7, 2015 at 15:54
  • I was aware of this, but that is an excellent tip for someone who doesn't quite understand what Steven was talking about....so they don't die (lol...but not lol)
    – droidski
    Oct 7, 2015 at 15:56

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