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I'm trying to replace 2 conventional on/off light switches with smart dimmer switches.

This is what the wiring looks like. enter image description here

The middle black is live. The black wire on the left was the load line connected to one switch and red wire on the right is the load line connected to the 2nd switch. Tested with a voltage sensor.

Was surprised to see the switch box have a neutral wire because our house is old. Why would there be 2 neutral wires? I'm assuming one runs back to the panel, but what about the 2nd one?

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  • Surely the load needs a neutral to work too?
    – Justme
    Commented Jul 19, 2023 at 20:45
  • Each live/hot wire should/must be matched to a neutral in a two wired cable. Just because the house is old, does not mean some wire circuits have been redone. Be glad someone was nice and gave you neutral in the switch box.
    – crip659
    Commented Jul 24, 2023 at 16:12

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In this setup, the /3 cable is coming from power via a lamp. It is carrying supply power on black, supply neutral on white, and switched-hot for one lamp is doubling back on the red wire. That lamp gets its neutral at its end of the cable.

The other cable carries switched hot #2 on black and neutral on white, to your other lamp. The yellow wire nut contains your neutrals.

A core rule of AC electrical is that currents in a cable or conduit must be equal and opposite (cancel each other out) between so the AC magnetic fields cancel each other out. Otherwise you get eddy current heating, vibration/metal fatigue and other bad effects.

To comply with this, a smart switch must choose its neutral carefully, and only take neutral from a cable whose hot wire also goes to the smart switch. That is not a problem here, but it is often an irreconcilable problem in other boxes.

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