A.Rex Cauldwell responds: Whether or not this
is an acceptable wiring scheme depends on where the wire
leads.
Partially switched receptacle. Three-conductor
wire has two hots — black and red — and a white
neutral. Though normally used for three-way switching,
three-conductor wire is commonly used for duplex receptacle
wiring as well. For a partially switched receptacle, for
example, you would break off the tab on the brass side of the
receptacle. The black and red wires are then connected to the
now independent brass screws, one wire to each; it makes no
difference which wire is on which screw. The neutral side of
the receptacle is left intact. This makes the top of the duplex
receptacle independent from the bottom, while they both share
the same neutral (see Figure 1). The black or the red can go to
a permanently hot feed, while the other goes to a wall switch,
creating a switched outlet. This layout works well for
switching on a table lamp in a room with no overhead lighting,
but it doesn’t require heavy 10-gauge wire.
If you find a red wire and a black wire from the same
three-wire cable coming into the panel, then going to separate
single-pole breakers, you’ll need to check the
installation. This scenario isn’t automatically wrong,
but to be safe I would assume it was until I traced the wiring
back from the panel. It could be one of several different
things, but it’s usually a multiwire circuit or a
220-volt circuit.
Multiwire circuits. Three-conductor wire can
be used to power a single circuit that would otherwise require
two 2-wire circuits. For example, the black might feed a line
of receptacles, while the red feeds a line of recessed light
fixtures in the same area. The white is common to both (Figure
2). If the loads on each wire are nearly balanced, the neutral
wire carries only the unbalanced current, resulting in less of
a voltage drop and wattage loss than the two 2-wire circuits
would have. In this situation, the black and the red
must be connected to the opposite phases in the panel to
prevent overloading the neutral (white) wire.
This type of wiring is called a multiwire branch circuit.
Though common in many areas, I do not recommend it. This system
is governed by specific code regulations. A multiwire branch
circuit must be able to disconnect both phases (that is, both
branches) at one throw of the breaker. Therefore, these
circuits must be ganged together. If someone does split
the branches apart, and they both lead to the same phase on the
panel, the neutral currents would be added together. Such an
overload could have disastrous results. In addition, the
receptacles, switches, and other devices on multiwire branch
circuits must always be pigtailed, as shown in Figure 2,
on page 14. This is done to ensure that the neutral is not
interrupted by a poor mechanical connection on the device (a
wirenut is a much more reliable connection). Because of these
requirements and the problems associated with multiwire
circuits, my advice is not to use them for residential wiring
where do-it-yourselfers are likely to muck around with
them.
220-volt circuits. A similar scheme (red and
black to opposite phases of the panel) is often used by
do-it-yourselfers to power electric baseboard heaters or some
other 220-volt circuit. But do-it-yourselfers often don’t
know to use a double-pole breaker or don’t remember to
connect the tabs between the breaker handles. This might also
explain the 10-gauge wire: Someone incorrectly assumed the
heaters needed the heavier wire. Do-it-yourselfers often
don’t know that the breaker protects the wiring.
It would be impossible to list every conceivable situation,
correct or incorrect, that you might have found. The bottom
line is: Trace the wires. See where they’re going and
what they’re connected to. Or throw the breakers and see
what doesn’t work. Then trace it down.
Rex Cauldwell is a master electrician and owner of Little
Mountain Plumbing and Electric in Copper Hill, Va.