Every now and then an electrician mentions something called a switch loop. An example of installing a new switch loop is when a light fixture originally switched with a pull chain is converted to use a wall switch.
Normally the neutral or return conductor accompanies the hot conductor everywhere.
When the power feed enters the light fixture box and we wish to have a wall switch, it is quite useless for the neutral to go down to the switch and come back up to the light fixture (Diagram 1).
So an exception is made so that the neutral is connected directly to the light fixture and just the hot feed goes down to the switch and just the switch controlled hot wire comes back. This is called a switch loop (Diagram 2).
Premade cables such as Romex are hard to find with both conductors colored. The convention is that a white wire may be used for the hot feed (unswitched hot). Here, both ends need to be marked with a band of black tape or stain. (Actually any color except green can be used.) The wire attached to the light fixture hot lead or hot terminal may not be white.
Sometimes it is desired to continue a power feed past the switch box to somewhere else such as another light fixture. Here we have a combined switch loop and power feed. Just one neutral wire as well as the unswitched hot is needed and a three wire cable is commonly used (Diagram 3). Most dimmer switches require power (and neutral) so you will need to wire it this way to install a dimmer.
Note that it is not permissible to take the unswitched power from a switch loop and use a neutral from some other circuit or branch of a circuit. If there is no neutral coming into a switch box, then an additional branch of a circuit may not be continued from there.
The blue dots in the diagrams show where connections with wire nuts are most likely made. If the light fixture has screw terminals and only one wire needs to be attached, then you do not need a wire nut and a short length of wire leading up to the fixture.
The next edition of the National Electric Code (year 2011) will require that new switch loops include a neutral, namely be wired as Diagram 3 above. In the past, too many home handymen, needing a neutral in the switch box, used the ground wire (not shown in the diagrams) for that purpose, which is improper usage.
Last updated 8/7/10
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