Trying to install motion sensing light switches at the top and bottom of my stairs. I read that I need 3 way units but was not sure if there are specific ones required.

Should I remove the old ones, take pictures and post them here for advice?

Other 3 way switches in the home seem to have 3 wires which I assume are positive, negative and common.


  • Please do take pictures of the existing configuration -- turn the circuit off, pull the switches out, and post photos of the switch and inside of box without removing them. – ThreePhaseEel Dec 28 '15 at 16:28

A three way switch is a single pole, double throw (SPDT) switch:


The common terminal corresponds to the one on the left in the picture, the two throws on the right are the traveler terminals. There are a number of different ways to wire them, but this diagram is a typical configuration that illustrates how they work:

wiring diagram - three way switches

The travelers go switch to switch - they are the red in that diagram and the black running along the red. So they are both switched hot - one or the other will be hot depending on the position of the switch the feed from the panel comes to, the switch on the left in the diagram.

They are wired so that whenever you throw either switch, if the lights are on they'll turn off, or if they're off they'll turn on. (Study the diagram a minute and that should make sense.) Note that neutrals are not involved in the switching, and in normal house wiring you're dealing with AC so there's really no positive and negative.

When replacing three way switches with occupancy sensor switches things work a little differently. The exact instructions will depend on the switches you select. For example the Lutron Maestro is more or less typical:

Lutron Application Note 435

Lutron makes a switch with an occupancy sensor and a companion switch to replace the two 3-way switches. The wiring is pretty much like regular 3-way switches, getting the common right is the main thing, but there's one big difference: the occupancy sensor switch has a neutral connection.

Since the switch itself is a powered device, it needs a neutral; regular switches do not. Until recently electrical code did not require a neutral in switch boxes. If there is a white wire in one of the boxes that is not connected to the switch, it is probably a neutral.

If you put the sensor switch in the box on the left in the diagram, it's simple - there is an available neutral. If you want to put it in the box at the right, you have a bit of a challenge. Whether it is easy or challenging in your case depends on the specifics of the wiring.

Photos never hurt; clear, well lit photos that make it easy to see all the way to the back of the box are particularly helpful.

  • +1 But note that in the illustration, there is no white wire to the switch on the right. In former practice, a 14/3 cable was often used and the white wire was converted to the common black and should be marked with tape or a marker. Current practice would use 14/4 to wire the right switch with the white reserved as neutral. – bib Dec 28 '15 at 16:43
  • @bib, yes that should say left rather than right, I will edit the answer. – batsplatsterson Dec 28 '15 at 17:07
  • One problem -- he wants two motion sensors, not a motion sensor + companion mechanical switch. It's still doable, but requires different wiring... – ThreePhaseEel Dec 28 '15 at 17:51
  • Old Light switches: goo.gl/photos/8uQJrWP9UK2ye5GR6 Wiring from wall; bottom wire is common: goo.gl/photos/jXp5sf65K6HYwbzP7 New light switch: goo.gl/photos/TiQgZ7EbbeDetj9Z9 Manual for new light switch: goo.gl/photos/CirNK6vUrLNwqEVn7 – Nicholas Gerasimatos Dec 29 '15 at 0:59
  • The photos linked in the comment shows one of the switch boxes, only the three switch wires, no neutral. The motion detector can't go in this box without running wire. The other switch box may or may not have a neutral so you can install the motion detector. – batsplatsterson Dec 29 '15 at 10:51

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