You are here:

Electrical Wiring in the Home/Adding motion sensor to existing light circuit


I have a 20-year-old home with three exterior lights at the garage that are controlled by a single switch in the foyer. I also have a non-switched floodlight in the garage gable controlled by a motion sensor. The garage lights and floodlight are on two separate circuits.

I would like to replace the floodlight with a motion sensor and wire it so that it feeds the three exterior garage lights. My goal is to be able to turn the lights on and off manually from the foyer as well as have the lights turn on by the motion sensor when activated.

As the motion sensor and manual switch are fed from two different circuits, would there be a problem if the manual switch was turned on at the same time the motion sensor was on?

First make sure the the circuit with the three lights can handle the load of the floodlight.

If so, you will want to disconnect the power to the floodlight.
You do not want two circuits feeding these lights.

Go into the box holding the floodlight (after turning the breaker off) and disconnect the wires feeding the floodlight and cap them individually.

Then you need to run romex or conduit from the floodlight to the first light being fed from the switch. You will need three wires and a ground wire. Run black, red, white, and ground.

At the light box of the first light:
Disconnect all the wires in that box. (again, with the breaker off).
Connect power from the switch to black going to the floodlight.
Connect the neutral to the neutral going to the light at that box, the floodlight, and the one going to the next light.
Connect all grounds together.
Connect the black of that light in that box to the black of the next light, and then connect the red going to the floodlight to those two blacks.

At the floodlight:
Connect the black to the wire feeding the motion sensor.
Connect the red to to the wire coming from the motion sensor, and connect both of those to the black feeding the floodlight.
Connect the whites together.
Connect the grounds together.

The sequence of operation:

Power comes from the light switch and goes immediately to the motion sensor.
From the motion sensor, power goes to the floodlight and to back to the first light, where it feeds that first light and then the rest of the lights.

If you turn off the switch, it removes power to the motion sensor, which in turn removes power to all the lights.

Hope this helps.  
About Electrical Wiring in the Home
This site answers questions related to home electrical wiring, home wiring, general electrical help,and other electrical questions related to aleternating current (AC). You can find help on the National Electical Code, home electrical issues, wiring electrical outlets, installing lighting, electrical grounding, and general electrical help for do-it-yourself projects not require an electrician. If you do not see your home electrical wiring question answered in this area then please ask your electrical wiring question here

Electrical Wiring in the Home

All Answers

Answers by Expert:

Ask Experts


Greg Hughes


Troubleshooting electrical including appliances and HVAC.


34 years of experience.

Both college and tech schools in this field.

©2017 All rights reserved.