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I was recently working on some wiring in my bathroom, and I noticed a 3rd load wire that I couldn't quite explain.

There were 4 Black Wires coming in (with 4 corresponding White Neutral wires). One of the Black wires was for the light, one was for the fan, one was the source (hot) wire, and there was a 4th one that I couldn't figure out.

What was really weird, is that the 4th wire seemed to be hooked directly to the source/hot wire. The entire setup was like this:

Switch 1 toggled fan
Switch 2 toggled light

Black/Hot/Middle -------> Top of switch 1 via Screw (middle of the wire was stripped, wrapped around screw and continued to switch 2)
Black/Mystery/End ------> Top of switch 1 via Insert
Black/Fan/End ----------> Bottom of switch 1 via Insert
Black/Hot/End ----------> Top of switch 2 via Insert
Black/Light/End --------> Bottom of switch 2 via Insert

If that list doesn't make sense, I can try and make a better diagram. Really I'm just trying to figure out what that last black wire was for, and why it was directly wired to the source line.

Thanks for any help/advice!

  • 4
    It sounds like the always-hot supply to another device. Are there any other devices (such as outlets) on the same circuit? – Miles Budnek May 3 at 0:02
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    Are there any receptacles on this circuit, or lights in other places for that matter? – ThreePhaseEel May 3 at 0:02
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    If the outlet is on a different breaker then it's not being fed by your mystery wire. Keep in mind that it may go to a different room as well. For instance, the light and fan in my upstairs bathroom are on the same circuit with the lights and outlets in my upstairs hallway, office, and spare bedroom. – Miles Budnek May 3 at 0:08
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    No, assuming that is what's going on you should just leave your mystery wire connected to the incoming hot exactly like you found it. – Miles Budnek May 3 at 0:12
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    The extra hot could go to the fan as well. Some fans have an incoming always "hot" AND a switched "hot". Especially the versions that have a delayed shut-off that keeps them running for another 10-15 minutes after you switch them off. The always "hot" is what feeds them in that delayed shut-off period and the switched "hot" is only the on/off trigger. – Tonny May 3 at 14:12
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This is an onward feed to some other stuff somewhere else in the house

While in the US, bathroom receptacles need to be on a dedicated bathroom receptacle circuit (that can feed any number of bathrooms), with the exception that you can feed both receptacles and lights for a bathroom off a circuit that services only that bathroom, there is no such requirement generally for bathroom lighting. In other words, once the dedicated bathroom receptacle circuit is provided, the bathroom lights can be on a general lighting branch circuit. So, you'll want to put that "mystery" wire back with the incoming always-hot feed, lest you wind up inadvertently disabling some lights or outlets somewhere else in the house.

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  • my bathroom-adjacent laundry room light and fan are powered in this exact manner – Richie Frame May 3 at 15:33
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I had a similar experience except it wasn the bathroom and they used the stab in connection and wire nuts. I discoveted ias t was already suggested to you, it was feeding an outlet on the same wall.

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