Hot Topics: Connecting Different Light Circuits Together

working with breaker box

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Sometimes new home owners find that the previous owners or builders made a few questionable choices. For example, you may find that you've got a living space with multiple lights all connecting do different circuits. Why did previous electricians think that was best when lights use such little power?

You may never get to the bottom of their logic, but you can fix it fairly simply. Take a look at advice these DIYers give on joining two separate circuits into one.

Original Post: Connecting Different Light Circuits Together

gmoniey Member


I have two different light circuits in the same hallways I'd like to connect:

  1. Lights controlled by two switches
  2. Lights controlled by one switch

I'm not quite sure the right way to connect them. My gut instinct would be to connect one light from each circuit to each other and make sure they are all three-way switches, but I'm pretty sure my gut is wrong.

Would appreciate any insight.


Highlights from the Thread

ray2047 Group Moderator

Do you want to get rid of the single location switch and put them all on the 3-way switches? If so, disconnect the switch and power to the light on the single location switch and connect that light directly to one of the lights on the 3-way circuit.

AllanJ Member

For each light that you want to connect to the other circuit, disconnect the existing switch and power wiring to the light up at the light fixture, and run a cable over to one of the lights on the other circuit, and connect that light directly to one of the lights on the other circuit.

An existing single switch location can become part of the 3 way light circuit. Disconnect it from its existing wires and tape those wire ends. Run a 3 wire cable over to one of the existing 3 way switches. Label the wires of that existing 3 way switch, notably identifying the common wire. Remove that switch and put it where the single location switch used to be. Match the wire colors of the original cable when connecting the new 3 wire cable to the non-common switch terminals. Connect the other wire to the common terminal. Back at the old 3 way switch location you worked on, connect a 4 way switch, putting the original non-common wires on the dark terminals and the new non-common wires (same colors) on the lighter terminals. Connect the other wire of the new cable to the common wire of the original wiring you unhooked.

A fourth switch location can be added, but the cable should be run to a 3 way switch, not the 4 way switch.

gmoniey Member

Thanks for the responses... I think I understand, but I'm still slightly confused. So, let me try and recap, and please correct me if I'm wrong.

Also, I do want the third switch to be able to control the lights (since it's around the corner of the hall).

Lets name the circuits:

  • A: Two switches
  • B: Single Switch

I'm assuming the switches on A are 3-way, so no need to make any changes at those switches.

Steps I'll need to take:

  1. Connect a light from A to B
  2. Change the switch on B to be a 3-way switch

Number two is where I am a little confused, does this mean I need to run a 12/3 or 14/3 down to that switch? It didn't look like the switches on A has 3 wires (excluding ground) running to them, but I can take a closer look now.

ray2047 Group Moderator

You will need two 3-way switches and one 4-way switch, not three 3-way switches.

light switch diagram

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