Light switch with five wires controlled by two circuit breakers: What is this?

Question

I have a living room light switch. It's a one-way switch that controls eight built-in LED lights. This was redone recently (in the last 3-4 years), but the house is new to me, so I don't know anything about how it was done.

There are two things I don't understand:

1. There are five wires going into this switch. One is obviously the ground, but why are there two wires going into each of the other terminals? (There's another terminal on the other side of the switchbox where you could hook up wire(s) to make it a three-way switch, but that's currently unused. It's not operating as a three-way switch.)

2. Turning off these lights requires simultaneously turning off two different circuits in the breaker box. If I leave either of the two breakers on, the lights stay on. (This was very confusing when I was trying to figure out which breaker controlled the lights, before I resorted to just flipping all the breakers labelled "lights" at once.)

Is there a sensible explanation for this?

(The context is that I was hoping to replace this switch with a Lutron Caseta smart switch, but the weirdness here has me unsure if that's even possible or sensible.)

Answer 1

It looks like you have an improperly wired accidental ring circuit.

One of those switch terminals, the hot one, has two wires on it because it is part of a daisy chain of such loads. (The other terminal has two load wires because they each run to several lamps, probably arranged in two daisy chains.)

A daisy chain of loads is acceptable practice, but your chain is improperly connected at both ends, to separate breakers. This violates code and common sense, and makes the wiring easily overloaded and unsafe to work on.

The fix is mechanically simple -- you just disconnect a hot wire from a terminal screw and cap it off with a wire nut -- but you should first decide where to divide the chain. This means identifying everything on the chain, so you can split it roughly in the middle leaving similar loads on each breaker.

EDIT: Well, almost that simple. As @Harper points out, the neutral half of the ring must also be disconnected at the same place where the hot wire is disconnected. So you have two wires to cap off with wire nuts. Just make sure that all the loads powered by one breaker share the same neutral.

Answer 2

Two totally separate issues.

2 wires per screw

First, 2 wires per screw. A. I. Breveleri explains why this is perfectly reasonable, and not per se anything to worry about.

Attaching 2 wires per screw is another story - usually this is bad news. However, in this case, the device uses a "screw-and-clamp" scheme which is specifially listed for 2 wires per screw. Tightening the screws clamps the wire(s). Generally, back-stab wiring is flaky, but when you must tighten the screw to clamp the stabs, that is fine.

Two breakers, one circuit

This is deadly dangerous. My usual "quick fix" is to go into the service panel, pull all the hot wires off the breakers, and pigtail em to the smallest breaker until I have time to sort it out properly. This quick-fix is still paralleling, which is bad, but better than the overload condition you now have.

The proper repair is to follow each set of wires from the panel from box to box and figure out how they were meant to be, and where they were inadvertently looped -- and sever it. Just as important is making sure the neutrals stay with their respective hots: if you have a 1500W heater in a receptacle with hot A and neutral B, and another one in a receptacle with hot B and neutral B, neutral B will be overloaded, and the breakers on hots A and B will not detect this.

It may be difficult to distinguish a normal daisy-chain from the place the circuits were inadvertently linked.