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I’m updating the switches throughout my house. I have an old rotary style single-pole dimmer controlling the just-as-old chandelier in my dining room. This dimmer shares a double-gang box with a 3-way switch that controls half-hot receptacles in a connecting room. (For context: both switches are on the same circuit but unrelated, as the 3-way switch is the dead end of the 3-way.)

So...back to the dimmer wires. There are two black wire pigtails pre-attached and coming out of the back of the old dimmer. There are no screw terminals. Each black wire is attached to a separate single black wire coming out of the box. The new dimmer I’m replacing it with is a rocker switch with a small slider next to it. The new dimmer can be used in a single-pole/1-way or a 3-way application. (More specifically, Legrand Radiant RHCL453PTC.)

The old rotary dimmer is a no-name brand from 1986 (even though this house was built in ‘96), so any identifyable markings are nonexistent. I know for an ordinary single pole single throw switch, wire placement is typically interchangeable (with the exception to the grounding conductor, obviously). Is this the same for a dimmer switch? How about if the dimmer switch can be used in both a single pole and 3-way application, separately?

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Unless the new dimmer is specifically marked with LINE and LOAD or similar, it doesn't matter which wire goes to which screw/pigtail in a simple two wire switch or dimmer. Since your new switch can be used as a three way, the instructions should say which screw should not be used.

Some electricians will follow a standard of the hot wire going to the top screw and the switched wire going to the bottom, but that's just so they personally know what they did and not a "rule" or "code".

Edit based on comments:

Here is a diagram that I believe is for your dimmer:

Legrand dimmer

They do specifically call out a screw for Line and a screw for "single pole" operation. Now, the dimmer might work regardless of how you hook up the two black wires, but, they warn than the dimmer needs to be wired this way, so we really should listen to them.

With the old dimmer removed you can separate the two black wires in the box and make sure they are not touching anything then turn the power back on. Using a voltmeter you can check the voltage between the two exposed black wires and ground or neutral. The LINE wire will be the one with voltage (~120v). The wire going to the lamp will be dead. DO NOT touch any wire until the power is turned back off, or call an electrician if you are not comfortable making this measurement.

Based on the diagram, the wire that had voltage goes to the "common" screw and the wire to the lamp (that had no voltage) goes to the "single pole" screw.

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  • "Unless the new dimmer is specifically marked with LINE and LOAD..." This is where the instructions for the new dimmer are unclear to me. The instructions say, "For 3-way installations: Identify the “Common” wire (wire connected to the terminal marked common or odd colored terminal). For new installation identify wire connected to power source or to the load." It's not clear to me if the part about identifying the wire for new installations is referring to single-pole or 3-way applications.
    – butterjellytoast
    Mar 25, 2019 at 15:56
  • Since this switch can also be used in a 3-way, there are three terminals: the black (or common, if using in 3-way) which is by itself on one side and two terminals on the load side, plus the ground screw. On an ordinary single pole switch, both terminals are on one side (so no common) but when using this in a single pole application, the instructions say to use the top screw terminal on the load side and the single black (or common, if used in 3-way) screw on the opposite side (the source), which is across from the bottom terminal of the load.
    – butterjellytoast
    Mar 25, 2019 at 16:06
  • @butterjellytoast Added a diagram from an install manual I found online with description.
    – JPhi1618
    Mar 25, 2019 at 16:07
  • Yes, that's the diagram I have. Originally, I was just going to test it with my voltmeter to identify the wires, but unfortunately there's no power to this part of the circuit right now, regardless of the breaker being on, due to everything upstream being disconnected while locating/troubleshooting a different problem on this circuit, which has been identified but a new cable will need to be ran for that problem (or an approved in-wall splice kit) so I was trying to get the unaffected downstream devices buttoned back up. Since this dimmer does specify, perhaps I should wait until I can test.
    – butterjellytoast
    Mar 25, 2019 at 16:44
  • @butterjellytoast if there is no power at all, you could short the light bulb socket and check for continuity between each black wire and neutral. The black wire with the hard short would be from the lamp (remove the short on the socket and re-test to make sure). Or if close enough, plug an extension cord into the light socket with an adapter and check continuity that way.
    – JPhi1618
    Mar 25, 2019 at 17:19

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