What could be the purpose of my single pole switch in this example?

I have an average understanding of electrical wiring so forgive me if my question might seem vague to you. I have two switches in my house, one is an Insteon SwitchLinc and the other one is a single pole switch (I believe). I'm excluding the ground wires from the explanation.

The Insteon SwitchLinc has 3 wires, a red, black, and white (and a ground attached). The single pole switch has two gold connectors (and a green).

There are two Romex gray wires, one is a 12/3 with ground. The other is a 12/2 with ground.

The red wire from the 12/3 is connected to the bottom terminal of the single pole switch.

The Insteon SwitchLinc red wire is connected to the 12/3 black wire and the black wire is connected the the 12/2 black wire.

Hopefully the diagram below illustrates what I am describing. The Insteon turns on an outside light but I can't figure out what the single pole switch is doing. It has no effect on the light and doesn't turn off any other light in the room. There is an outlet near the baseboard in line with the switch. That particular outlet has 3 sets of black and white wires all connected to it. I also plugged in a light to each outlet (top and bottom) and the switch had no effect.

Can someone explain and help me understand what the purpose could be based on this wiring or could this be incorrectly wired?

• The data for the Insteon SwitchLinc is required plus your diagram needs to name the terminals. Also add the load details. However, with only one wire going to the SP switch, it's highly likely you have drawn it incorrectly. You should also show what or where the grey cables connect to.
– Andy aka
Commented Nov 11, 2022 at 17:54
• As drawn, your single-pole switch does nothing. Commented Nov 11, 2022 at 18:15
• Another "MAGIC"/"MORE MAGIC" switch?
– vir
Commented Nov 11, 2022 at 18:37
• Either you've drawn the circuit wrong, or the switch is miswired. As drawn, it does absolutely nothing but confuse people who see it. Commented Nov 11, 2022 at 18:43
• Probably something else from inside that box connected to the single-pole switch at one point (like the outlet) but someone made it "always-on". Commented Nov 11, 2022 at 18:51

When /3 cable is used going to a light, that is for one of two reasons.

• It is supplying two switched-hot wires so that a light and fan (or two lights) can be used separately.
• Power (always-hot and neutral) are being carried through this cable, either to support other points of use, or to provide the now-mandatory neutral at the switch.

I suspect it is the first case.

• White is neutral
• Red is hot for the fan
• Black is hot for the light (or possibly vice versa)

And commonly this is provided even if the house isn't sold with a fan, because the owner is likely to add that later.

So most likely there used to be a fan here, and someone removed the always-hot supply power to that second switch, since it was no longer used.

• These switches are in a basement with drop ceilings. A fan installation is unlikely but it is strange that a /3 wire was used that only gets 10V supplied to it when the power is on. Do you know if there's a reason for this behavior? Commented Nov 11, 2022 at 22:20
• 10VDC or 10VAC? Commented Nov 11, 2022 at 23:04
• @jpQuick that's most likely a measurement error from using a highly sensitive DVM on a wire vulnerable to capacitive coupling from nearby wires. I would disregard that indication. We can tell from your diagram and photo that the red wire is connected to nothing. Since nothing supplies hot to the switch. Commented Nov 11, 2022 at 23:12

It looks like someone retired that switch in place by lifting the hot wire from its other terminal. As there's no obvious way to center one switch in the box, you might consider getting a blank decora insert.