I am trying to add a new timer switch and standard single pole light switch in my bathroom. Both of these are in the same metal switch box. I drew a diagram to help myself understand what I am looking at. I omitted the bare ground wires which are attached to the back inside of the box.

Here is my understanding so far:

1.) The light switch on the right seems to be using a 3-way switch given that there is a brass screw and two other unused screws (that don't appear to be for grounding screws due to their silver color).

2.) I assume the power for the light comes in through one of the cables at the bottom right of the diagram and the blacks are pigtailed together to provide power to the other cable at bottom right and to go to the light switch. When the switch is flipped power continues to the light through the black wire at top right.

3.) To change the switch I should be able to connect the two black wires currently connected to the switch to the two screws of the new switch. Which black wire I attach to the top and bottom screws shouldn't matter. I am swapping out the old switch for a Leviton single pole switch.

Is that right?

The left side of the box is a puzzle to me. The fan is marked "Line" and "Load" on the screws on the right side of the fan timer. Can someone explain how the power flows and is controlled with the fan timer?

wiring diagram


I opened things back up and took some pictures. I have not changed anything since originally taking off the cover plate. I believe my diagram was basically accurate, though the unused screw on the light switch was on the right side of the switch and not the left as I had previously drawn.

picture of light switch box

I also noticed that on the fan timer the wires pass through the timer and connect to screws on the other side. So "LOAD" and "LINE" based on what the timer said are actually revered from my original diagram. It's hard to get a good picture but the black wire passes through the timer box and connects to the screw on the top left side of the box marked "LOAD 1".

enter image description here


Also, having looked closer I believe the the fan is controlled by two switches. There are two bathrooms right next to each other and I think the switch in either bathroom will control that shared fan.

  • 3
    I'm not doubting your artistic skills but how about just taking a few pictures of the box without disconnecting anything.
    – JACK
    Oct 13, 2022 at 0:18
  • 4
    That diagram left side is incorrect. The circuit as-drawn would be impossible. Oct 13, 2022 at 0:41
  • 2
    @Harper-ReinstateMonica agreed. The neutrals should be just be connected to themselves and the black wires (w/o the wire nut) should be connected to the timer. But worse, is the diagram on the left would result in a full blown short, flipping breakers, mit sparken shooting und ger snappen der springerverk. Nicht so guud. Oct 13, 2022 at 1:22
  • 2
    Thank you all. I am going to open things back up and have a closer look and try to get some clear pictures. Will update soon Oct 13, 2022 at 2:31
  • 1
    well I wouldn't be changing anything until you fully understand why it's the way it is now. Also, life goes better if you use the rotary-knob type timers with a clock motion. Those are powered by YOU, so they don't need neutral. Oct 13, 2022 at 23:57

1 Answer 1


You have a classic switch loop with a twist

The twist is you have TWO switches, either of which will make the fan run. Thus the two switches are wired literally in parallel. And that is fine.

Because this is a classic switch loop with no neutral, the white wire must be re-marked with paint, shrink tube or tape to be black or a color.

Also, you do not have neutral available. It will not be possible to use electronic "turns the knob for you" type switches, unless those switches are UL-listed to work without a neutral. (a small minority are). Neutral cannot/must not be poached from unrelated wires from another switch in the box.

  • If I am dead set on having an electronic timer for the fan in each bathroom, what (re)wiring would need to be done to make that possible? Would parts of the circuit need to be run with 3 wire cable? Oct 16, 2022 at 21:18
  • @donkey_tooth Then my advice would be to seek out the small minority of touch switches which are UL Listed to be used without neutral. (they bootleg their power from ground, which UL will allow them to do is it's inconsequential (0.5mA) and it is built to be impossible to leak a dangerous amount. Or, if drywall is no object, run /3 from fan to all timers. Oct 16, 2022 at 21:40
  • Just for my own understanding of my original question: I now understand this all to mean that, as far as the fan is concerned, one of the two cables in the box is powered and is coming from the fan. If this was the only switch wired in a switch loop fashion there wouldn't be a second cable for the fan. However, there is a second cable, and this second cable is coming from the second switch in the other bathroom. If either switch is on, the fan will get power. Is my understanding correct? Oct 17, 2022 at 0:16
  • @donkey_tooth You got it. The two wires are "always-hot" (should be white with marking) and "switched-hot". Oct 17, 2022 at 0:27

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