I'm replacing a traditional light switch for a smart one (S1). This switch is connected in a 3-way with another traditional switch (S2) which won't be replaced. So in the end it's one smart and one traditional switch in a conventional 3-way connection.

The wiring instructions include the following diagram: enter image description here

I understand most of it except this: if S1 is powering the red/black wire how can S2 turn the light off? Is that because the electricity would go through the jumper and then through the blue/red wire back to S1, kind of a "shortest path"? If so, would that consume electricity when the light is off?

Thank you for your inputs.

PS: please pardon my terminology, I'm not familiar with the subject.

  • \$\begingroup\$ We don't know which smart switch that is and how it works internally. It might have some detection circuitry to monitor the state of remote switch if it is close or not. \$\endgroup\$
    – Justme
    Sep 28, 2022 at 4:19

1 Answer 1


S2 doesn’t power the light.

The blue wire provides a signal to the smart switch (S1) telling it to switch on/off the light when S2 changes position.

This approach uses existing wiring if converting from a conventional three-way switching and minimizes the hardware in the smart switch by only requiring a single device to control the load current.

  • \$\begingroup\$ When S1 detects the switch on S2 and turns the light off, does that mean that both red and blue wires won't have electricity at the same time? \$\endgroup\$
    – Renan
    Sep 28, 2022 at 4:35
  • \$\begingroup\$ both will have electricity, just not enough electricity to light the lamp. \$\endgroup\$ Sep 28, 2022 at 6:48

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