Electrical Basics: Connecting A Dimmer Switch

By Bruce W. Maki, Editor

This dimmer switch has two wires to connect to the "hot" and "switched" wires.

After I had installed the old work box I prepared the cables by stripping away the jacket.

In this case the ground wires were tied together with wire nuts, and the two neutral wires were connected with wire nuts.

The grounds and neutrals were tucked away in the back of the box. One black wire is "hot-at-all-time" and the other is "switched". It does not really matter which wire connects to which terminal of the switch (unless the instructions say so).

I wrapped the flexible stranded wire of the switch around the stripped end of the hot wire, and installed a wire nut.

With both hot wires connected, the switch is ready to install.

I drove in the screws with a drill-driver, being careful not to over-tighten them.

I installed the cover plate and tested the new circuit.

Notes About Switch Wiring:

There are two basic ways that switches are connected in home wiring. The first method is shown here, where a "feed" line comes into the switch box, the neutral passes through, and the hot is switched. 

I call the second method "Remote Switch", for lack of a better name. In this scheme, there is a "hot-at-all-times" cable at the light's junction box, and a single two-conductor cable goes to the switch. Technically, both the black and white wires to the switch are hot (and in this scheme the white wire is supposed to be marked as hot by wrapping the ends in black electrical tape). Imagine the hot wire that feeds the light on the ceiling... and just before the wire reaches the light, it is re-routed down to a box on the wall where it connects to one side of a switch. When the switch is closed, the current continues along the other wire in the 2-conductor cable and reaches the light bulb. It's as if the hot wire has been s-t-r-e-t-c-h-e-d all the way down to a remote location... a switch on the wall. 

So if you find a light switch with just one cable in the box... it must be this "remote switch" method of wiring.  Also, note that with this scheme there will be live wires in the junction box for the light, regardless of the position of the light switch.

And if you find a light fixture with only one cable in the box, it must be the first scheme. In this case turning off the switch will definitely turn off the power to the light's junction box (but check for power anyway, just for fun).

Copyright © 2001, 2005  HammerZone.com

Written January 30, 2001
Revised January 7, 2005