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I was replacing an old Leviton 15 Amp Single Pole Dual Switch and noticed all wires were black. This was for a bathroom that one switch would power the light and the bottom switch would just power the fan.
Instead of wiring the same way as they were which was, the hot from the source was on the common side and the other two were on the hot side. (I used a voltmeter and read which one was hot), I decided to try to put the hot wire from the source to the brass hot screw and the other two black on the common screws.
Obviously this did not work and only the top switch worked to power both fan and light and the bottom switch did nothing. I am trying to figure out why this is … why would I put the hot from the breaker panel to the common and the other two black wires on the hot side?

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    Share a picture of how you've got it wired now and one of how it was wired originally. Put pieces of different colored tape on each wire so we can tell them apart.
    – FreeMan
    Mar 26 at 20:58
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    I always wonder why folks try to change what works and wire things differently then wonder why it doesn’t work. You probably do not have a complete understanding of the circuit without that in the forms of a schematic and or all the connections in both locations we won’t have enough information.
    – Ed Beal
    Mar 27 at 16:49

1 Answer 1

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We start with a simple single switch. Two screws - one used for line (incoming power) and one for load (switched hot). It does not actually make any difference whatsoever which wire goes on which screw. You could make an arbitrary convention such as "switch is flipped up to turn on, so use the top screw for load and the bottom screw for line" but it makes no difference whatsoever.

Next we have the 3-way switch. Which is actually a pair of switches. (And possibly 4-way switches in between, but we'll ignore that for now). With a 3-way switch, it is not exactly line and load. Instead, one switch is line and 2 travelers and the other switch is load and 2 travelers. So while functionally the common screw means something, that depends on where/how the switch is installed. The only thing you can really do is call it common. And, in fact, that screw must be truly common - i.e., a single screw on one side (electrically, physically the screws could be located in any arrangement, e.g., common on left, travelers on right, common on top, travelers on bottom, etc.) because you must have a single incoming hot line wire on one switch and you must have a single outgoing switched hot load wire on the other switch or the circuit won't work properly.

Which brings us finally to the "2 switches in 1" which you have. These switches can actually be set up, potentially, 3 different ways:

  • 2 line, 2 load
  • 1 line ("common"), 2 load
  • 2 line with a removable jumper, 2 load

The first choice lets you have two switches on separate circuits. Why? If they are high power devices (not much of an issue for residential lighting in the age of LEDs, but still important for bathroom heat fans and occasionally other things) then this lets you use more power than you can use with an ordinary single 15A or 20A circuit. Note that there may be additional restrictions - I know this OK (if done properly) with an MWBC with a double-breaker (or handle-tied), but it is not an MWBC it may be against code.

The second choice matches your description. It forces you to use only one circuit, but for typical lights and fans, and even a heat fan if it is sized appropriately, this works great.

The last choice is actually quite common too. It is really the same setup as a typical duplex receptacle where a removable tab lets you separate the two receptacles for switching purposes (not applicable here - you are installing switches!) or for more power using an MWBC, but the manufacturer only has to supply one device to serve both applications.

The end result is that the common screw is splitting internally to provide power to one side of both switches. Unlike the single switch, this means it must be line, not load. The other screws are, by definition (as opposed to by installation/usage) load, and therefore separate for each of two switched devices.

Another way to look at it: If you replaced the double switch with two single switches, you would add two pigtails to the incoming line wire and put one on each of the single switches, and put the load wires on the other screws of the single switches. The common screw is in place of the two pigtails.

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