Wiring 12/2 or 12/3
12/2 cable (Romex, UF, etc) consists of two insulated #12 and one uninsulated #12 conductor or wire. The two insulated wires have a black and a white colored insulation. Number 12 copper is rated for 20 amps. This cable can carry a single circuit with ground.
12/3 cable is identical to 12/2 with the exception being that it has a third insulated conductor that is colored red. This third wire allows one cable to supply 2 circuits (one on black and one on red with both sharing the white and ground wires) with one cable run. It is much cheaper to buy and install a single 12/3 cable than two 12/2 cables to get two circuits into the same general area.
Twenty amp 120 volt circuits in dwelling units are required for kitchens, dining rooms, washers, disposals, and other appliances that require more than 12 amps (but less than 16 amps) to operate. Twenty amp 240 volt circuits are typically for specialty appliances and devices such as electric heaters, pumps, etc. Generally, 20A/240V appliances devices do not need a 12/3 cable as they only require connections for Line1, Line2 and ground. One insulated conductor would be unused in a 12/3 cable serving such a device. A 12/2 cable is run instead and the white wire is taped red (any color other than gray or green, to indicated that it is no longer a neutral) at each location it is accessible, such as wiring compartments, panels and junction boxes.
Circuits fed by 12/2 cables will connect to single pole circuit breakers and those fed by 12/3 cables must be connected to double pole circuit breakers. Check the National Electrical Code (NEC) to determine which locations require GFCI (Ground Fault Circuit Interrupter) supplied circuits. The latest (2012) NEC requires most new circuits in dwelling units (residential) to be protected by AFCI (Arc Fault Circuit Interrupter) type circuit breakers. Consult your local building department to find out if the NEC has been adopted in your locality before installing.
Lastly, a 12/3 cable would be run between 3 way & 4 way light switches and hard wired smoke detectors; but only if they are on a 20 amp circuit. Most circuits in the home are 15 amp capacity, especially for lighting circuits. A 14/3 cable would be run on these circuits as there is no need for the additional expense of a cable with the larger #12 wires and the increased labor to handle, install and connect the wires to device terminals.
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Jan 18, 2012 |