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In my office the light switch box has 3 sets of 3 wires, 3 black, 3 white and 3 copper. All 3 white are connected/tied together as well as all 3 copper wires. The 3 black wires all are connected to the switch, 1 wrapped around the screws on the side and 2 stuck in holes on the back of the switch.

Can you explain whats going on, why are there 3 sets of 3 wires?

Currently I have 1 light fixture which I want to change to a ceiling fan, and I want to change the regular light switch to a smartswitch that can control the fan.

Thank you!

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  • Might be a good time to review NEC 314.27(C) Boxes at Ceiling-Suspended (Paddle) Fan Outlets. Outlet boxes or outlet box systems used as the sole support of a ceiling-suspended (paddle) fan shall be listed, shall be marked by their manufacturer as suitable for this purpose, and shall not support ceiling-suspended (paddle) fans that weigh more than 32 kg (70 lb). For outlet boxes or outlet box systems designed to support ceiling-suspended (paddle) fans that weigh more than 16 kg (35 lb), the required marking shall include the maximum weight to be supported. – NoSparksPlease Jun 10 at 13:56
  • Sorry to sound negative and if you can't figure out the wiring through a couple of search engines you should either stop digging and call a qualified contractor, or first check your insurance policies… – Robbie Goodwin Jun 12 at 22:48
  • ...well isn't this website on the internet?!? 🤔 – Clark Jun 14 at 0:12
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Most likely (because anything is possible), the two black wires going into one end of the switch (back stab and screw next to it) are "incoming hot" and "pass-through hot". All neutrals (white) are together (which is good) and all groups (bare copper) are together (which is required).

If your new smart switch has screws:

  • Line/hot/incoming power = connect to the two black wires. If the new switch is designed to have two wires under the screw (clamp rather than just a screw) then you can use that. If it does not, then even if it has a backstab option, connect a short piece of black wire to the existing two wires with a wire nut and connect the other end of that piece (pigtail) to the new switch.
  • Load = connect to the single (top in the picture) black wire.
  • Neutral = connect to the white wires. You will need to add a short piece of white wire to the existing bundle of neutrals. You may need a new and/or larger wire nut to do that (now might be a good time to pick up a pack of wire nuts - you never know when you will need one).
  • Ground = use a short piece of green or bare wire and add it to the existing bundle of grounds.

If your new smart switch has wires:

  • Wire nut the line wire (likely black) to the two existing black (bottom) wires.
  • Wire nut the load wire (likely red) to the top black wire.
  • Wire nut the neutral wire (white) to the bundle of neutrals.
  • Wire nut the ground wire (green) to the bundle of grounds.

Older simple switches (like yours) often don't have a ground connection, which is OK.

New, especially smart, switches can (generally) ground directly to the box instead of using a separate wire. However, that only works with a metal box. You have a plastic box, so connect ground to either to a ground wire (if supplied) or ground screw (should be clearly marked, and probably green).

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Elementary, My Dear

One cable comes from the upstream source (the service panel or another device box).

One cable is the light loop, in which the black (hot) is switched and the white (neutral) is the return. This is the only cable you can identify with any certainty without a voltage test since its path is broken by the switch.

One cable simply passes through to another device or circuit branch. It picks up hot at the shared switch terminal and returns neutral to the source via the bundle.


An unlikely alternative possibility is that both outbound cables are switched, such as for a light fixture and switched outlets. In this scenario the single black (hot) wire would indicate the source cable. You'll know if that's the case, I'm sure.

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