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 illustration).
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.