Is this a three-way or single-pole switch?

Question

Can someone explain why the following older switch is labelled a three-way switch and how it would have been used as such?

It recently stopped working so I want to replace it. Based on the appearance of the switch, age of house, and age of wiring entering the switch box, my guess is it's a minimum of 40 years old, maybe 60+. It was in use as a shut off for a furnace so I expected it to be a basic single pole switch.

However, as you can see in the second picture, it's labelled "THREE WAY 5A-250V-10A-125V" on one side of the switch's front metal plate, and "SINGLE POLE 5A-250V-10A-125V" on the other. The toggle itself has ON and OFF labels, contrary to most modern three-way switches. There are only two terminal screws. One screw is kind of brassy. The other looks a bit coppery, but that might just be tarnish.

So I'm confused about whether this is three-way or single pole. All the three-way switches I see for sale now have three screws: one common and two travelers, and do not have ON/OFF labels on the toggle.

Could it be that that as a cost-saving measure, Leviton manufactured one metal plate that they used for both three-way and single-pole, and the location of the screws indicate which label applies? I.e. in my case, the screws are on the single-pole labeled side of the switch, so this is a single pole switch, and on the three-way model of this vintage, all the screws (or maybe just both traveler screws) would be on the three-way labeled side.

Can a switch be both single-pole and three-way? If so, how would one use this as a three-way switch?

Thanks.

Answer 1

It's a single pole (SPST) switch.

Probably there's one stamping for the front plate, labeled for both types. Cost savings at the factory.

Without the third terminal it can't be a three-way (SPDT) switch.

You know which of the two descriptions apply by looking at the switch terminals.

Answer 2

tl;dr: A single-pole switch can only be open or closed. It cannot be used as a 3-way switch.

How 3-Way Switches Work

The way a 3-way switch works is by always having a closed connection between the common screw (which I've marked as blue in the picture) and one of its two traveler screws. In combination with another 3-way switch you can open or close the circuit from two different sides of the room.

In our picture here, each switch can be flipped to follow the BLACK path or the RED path.
^{Black and red are common colors to use when wiring these kinds of things, but here they just help the illustration.}

We see that the light can only be ON if both switches are using the same path: RED or BLACK. If they are using different paths then the circuit is not closed (does not connect hot and neutral), and the light will be off.

There may also be a ground wire connecting the switches (and possibly the fixture being controlled by the switches), but I left that out.

Wiring it up

Observe that it is entirely possible to cross-connect the traveler screws between the two switch boxes — It just comes down to whether the (light) fixture is ON or OFF when both switches are in the same position.

^{A good electrician will wire it so that both switches are up or both switches are down for OFF, IMHO.}

Also of note is that real world manufacturing is somewhat inconsistent: the screws may be arrayed anywhere around the switch body. The two most common scenarios are:

• the common screw near the bottom on one side and the two traveler screws on opposite sides on the top
• the common screw by itself on one side and the two traveler screws on the opposite side.

Whatever the case, always read the box the switch comes in. The common screw is usually a darker color and the traveler(s) a lighter color metal. (And ground usually green.) If you are ever unsure, you can use a multimeter to find continuity with the switch first in one position, and then the other. The screw that had a lead on it both times is the common screw.

Single Pole Switches

So, we can see that a physical switch with only two terminal screws cannot be wired up as a 3-way switch.

As for why that thing you have is labeled for both, Ecnerwal has already opined in his answer that it was probably just a way the manufacturer shaved production costs. I think that likely as well, but do not know it for fact.

You could wire up a 3-way using two single switches as a replacement for each 3-way (that’s four single-pole switches total), but the user must always make sure that every time he flips a switch on one side of the room he also flips the one next to it, otherwise the switches fall into the why isn’t this thing working category of frustration when simply trying to turn the light on or off.

Answer 3

It helps to know what "SPST" stands for. It's Single Pole, Single Throw. This means you have one circuit (Pole) and one place you can connect it (Throw). So you can turn one circuit on or off. The switches in the diagram posted by Dúthomhas are known as SPDT, or Single Pole, Double Throw. You have one circuit (Pole) and two places it can be connected (Thrown). It's anyone's guess why the switching "destination" is called a "Throw", but I'll bet someone who knows will read this and post the answer.

Answer 4

The best way to tell for the future if you were wondering if the switch was a single pole switch or a 3 pole switch doesn't require you to take anything apart. A single pole switch will always say on/off on the toggle. A 3 way switch will say nothing.