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I have an older house where I want to turn a single pole switch to a 3 way. The problem is that this light is on older wires which I cannot replace at this time. When I open the switch I only see 2 wires total. I believe (maybe incorrectly) that the neutral line in these old systems may have been embedded higher in the walls, so I cannot reliably say that this is an end run by there only being 2 wires. Is there a way to tell (maybe with a voltmeter) what the wires are doing?

I ask because as far as I know this will make the difference between having to run only 14/3 or 14/3 AND 14/2 (as I can only run wires from the switch down, and not from the light)

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  • Note that with an older switch loop you may not have neutral available at the switch, which would prevent you from solving the to three-way problem with most smart-home equipment too. (There are a few possible exceptions such as the old X10 systems, which powered themselves parasitically rather than having their own neutral... But making those work with fluorescent or LED light fixtures requires some ugly kludging, and connecting them to the rest of a smart home system is also complicated. Different question, though.)
    – keshlam
    Nov 22, 2022 at 8:53

4 Answers 4

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Your existing switch has hot and switched hot. You can figure out which is which with a non-contact tester or a multimeter.

The following advice applies only if extending a circuit to a 3-way switch does not require getting neutral to the second switch. It code requires it then you have no choice but to figure out the total circuit wiring first.

  • Run a 14/3 (if 15A circuit) or 12/3 (if 15A or 20A circuit) cable from this switch box to the new switch box.
  • Install a 3-way switch in this box.
  • Attach the hot wire to the common screw.
  • Attach 2 of the new cable's wires to the traveler screws.
  • Use a wire nut to connect the switched hot wire to the remaining wire of the new cable. This wire in the new cable is now switched hot.
  • Install a 3-way switch in the new box.
  • Attach the switched hot wire to the common screw.
  • Attach the other two wires to the traveler screws.
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  • How to test which with a multimeter? Nov 21, 2022 at 23:17
  • Check from each screw/wire to ground (ground wire, if you have one, metal box if it is in one, otherwise gets a little trickier) - hot wire will show ~ 120V, switched hot will show 0V when off, 120V when on. Nov 21, 2022 at 23:22
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You most likely have an old-style switch loop. Hot and neutral would be in your ceiling junction box at the light. The cable in your switch's junction box would also run to the ceiling junction box. In the ceiling, hot is connected to one wire of the cable running to the switch (most likely black), and comes back on the other wire as switched-hot. The switched-hot wire would be connected to the light fixture, and neutral would go to the light as well.

If the above assumptions are true, and you have hot and the return switched-hot available in your switch junction box, it may be possible to run 14/3 from the current switch to the second switch location, containing two travelers and a return switched-hot. However, the newer electrical codes require neutral to be available in switch junction boxes, and I'm not sure how the rules around grandfathering would apply here.

Adding photos of the wiring in the ceiling and in the switch will help one of the pros here identify what you have to work with.

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If the switch box contains only two wires (not counting safety ground), especially if one is black and the other is either white or (most especially) white whose end has been blackened with marker or tape, you are almost certainly looking at a switch loop.

For confirmation, look at how the light is wired. If there are two cables coming into it's box, there is either a switch loop or something else "downstream" being powered from the same circuit and controlled by the same switch. (Vanity lights, exhaust fan, ...) You can determine this by checking whether the light is connected across wires from different cables (switch loop) or the same cable (switched circuit with switched power being passed along to something else)

To be absolutely certain you should turn off power, disconnect the blacks at the lamp (make sure you keep track of what was connected to what!), Turn the switch OFF, power back on,and carefully prove the wires. If one wire has power in it, you have confirmed a switch loop setup. If nothing at the lamp has power but black at the switch does, it would be a normal switched circuit.

Nut sering only one cable/two wires coming into the switch box, with the switch across those two wires, would make me assume switch loop until proven otherwise. Normally there should be two cables, with whites tied together and the switch between the two blacks.

Now if you see a red wire...

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Most older houses used switch loops if power was at the light fixture, instead of coming to the switch first.

What happen was that they used /2 cables and used the white wire as hot or switch hot.

Power came from the light to one switch screw and return on the other wire attached to the second screw. It should be the only place that has white connected to black in most simple circuits.

No neutral is at the switch or somewhere in the walls or anywhere else except at the light.

Recent code requires you to use /3 cables so neutral is available at the switch, because people want smart switches. You would use the black and red wires instead of only the white(supposed to have black/red marking on it).

If only two wires at the switch(not two cables), then most likely it is a switch loop and no neutral is near it.

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