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I have a back storage unit that has to be reconnected. There are only three wires coming from the main building. One wire has three white stripes on it, and the other two are solid black. How do I reconnect these lines to a new breaker box?

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    With a new panel, you will probably need to add a ground wire and if the unit is detached, two ground rods also. If wired correctly at the main panel(unknown), the two blacks will be the hots and the white stripes wire will be neutral. The size of the wires will need to be known for the feed breaker after a load calculation is done, if they are over 10 gauge(30 amps).
    – crip659
    Commented Feb 15 at 17:15
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    Pics of the wires where they're attached to the main panel, plus pics of the new subpanel would be most helpful in providing you an accurate answer.
    – FreeMan
    Commented Feb 15 at 17:45

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Normally neutral has to be white. However, at large sizes such as typically used for panel feeders, black wire can be marked as white. Which means the three white stripes (redundancy is good - one stripe would technically be enough) marks the neutral and the other two wires are the hot wires.

However, this raises a ton of issues about the existing and new installations. Such as:

  • Where is the ground wire? Old installations don't require it and a straight replacement would not (usually) require adding one. Possibly metal conduit as ground? Can you add a ground wire back to the main panel?
  • If the storage unit is not attached to the main building then ground rods are required.
  • If you do have a ground back to the main panel then the new panel must keep ground and neutral separate.
  • If this is not attached to the main building then you need a disconnect, which can be a "main breaker" in the new panel.

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