[Editor's note: In 2014, the National Electric Code was revised, rendering the original diagrams with this article inaccurate. Please see "Neutral Necessity: Wiring Three-Way Switches," JLC 5/15.]

Q. My clients want to add another light and a second switch to one of their existing lighting circuits, a wall sconce controlled by a single-pole switch. Power feeds the light before running to the switch; because of existing conditions, the easiest option would be to replace the single-pole switch with a three-way switch, add another three-way switch at the new location, and connect the two switches with three-conductor cable. Will this work, or will my electrician ask me to open up the walls so he can get access to the entire circuit?

A.Rex Cauldwell, a master electrician in Rocky Mount, Va., responds: The answer depends in part on the location of the new sconce. If the power cable feeds the fixture closest to the switch, you simply wire the most distant sconce into the first sconce and treat both as a single fixture (see illustrations).

But if the power cable goes to the most distant light first, a three-conductor cable between the two lights is required to connect the second light's neutral conductor back to the first light's neutral conductor (in other words, the lights have to be wired in parallel). Without this three-conductor cable, the circuit won't function properly, regardless of whether it's controlled by a single-pole switch or a pair of three-way switches. This means that in addition to running three-conductor cable between the two new switches, your electrician may need to run three-conductor cable between the old and new fixtures.

In general, the first rule of three-way switching is to bring the hot conductor straight to the common terminal of a three-way switch. The second rule is to bring the hot cable's neutral conductor straight to the neutral of each light and then connect the light's hot conductor to the common terminal of the opposite switch.

By the way, it's always better to bring the hot cable to the switch — rather than the fixture — first. If the circuit develops problems later and the incoming power needs to be checked, working at the switch is easier and safer than climbing a ladder, removing a fixture (or two), and then trying to figure out just where the hot conductors are.