Finding an Electrical Source for a New Outlet
Also see:This page offers some options for locating a source for a new electrical receptacle or light fixture. Use these links to locate and cut holes for new cable, fish cable through walls and ceilings, run cable from an upper floor and get tips for fishing electrical cable.
Choosing Wire Gauge
Household wiring in the US is color coded as follows: Black and red are hot, meaning the electricity come through these wires first. The white wire is the neutral wire, it carries the electricity back to the service panel. The green wire, and in the case of NM cable, the bare wire, are ground wires. These are safety wires that carry electricity immediately to ground in case of a short. When running new cable pay attention to the amp rating for the circuit you are extending. For a 15 amp circuit, use 14 awg (american wire gauge) cable, for 20 amps use 12 awg, 30 amp: 10 awg, 40 amps: 8 awg, 50 to 60 amps: 6 awg. Most lighting and receptacle circuit will be 15 amp and make use of 14/2 or 14/3 cable.
Depending on the wire gauge and box depth, there is a limit to the number of wires allow inside an outlet box. If the number of wires entering the box would force exceeding the limit to extend the circuit, a junction box can be added. It will always be OK to extend a circuit from an existing box with no more than one cable running to it, as long as the total electrical draw from the other outlets in the circuit doesn't exceed the ampere rating. To determine the load of the circuit, use the chart and formula for measuring amps in a circuit at this link.
Finding an Electrical Source
There will probably be several options for an electrical source for a new outlet in a wall or ceiling. A new receptacle or switch can usually be added in the wall space between studs using an "old work" outlet box, but a new ceiling fixture or ceiling fan will require some support. See Framing for a New Ceiling Fixture for help.
Decide on the location for the new electrical fixture and where the source will be. Determine inside of which walls and ceilings the cable must be run. For example, with a new ceiling fixture using a source from an existing wall receptacle, the cable will have to be run inside the wall cavity, through the wall header and into the ceiling cavity to the new fixture.
Or if you are using a receptacle on an upper floor to serve a ceiling fixture below, the cable would have to be run through the sole plate and possibly some flooring, and into the ceiling cavity below.
Source in an Adjacent Room
In the example here a new wall receptacle can be installed using an existing receptacle in an adjacent room. This can often be the easiest way to access a source for a new receptacle or a switch to a new light fixture.
Add a Junction Box
If you want to use a source with more than one cable currently installed, check the wire splicing page for limits on the number of wires that are allowed in one outlet box. If adding another cable to the box is not possible, it may be possibe to split the source cable before it enters the box. Access to the cable can be cut in wall material and a junction box can be added to splice the new outlet into the circuit. The junction box must remain accessible, so look for a spot where the cover plate will blend. Use a 3 x 2½ inch device box at the same level as wall receptacles to minimize visual impact.
Source at a Ceiling Fixture
Often a ceiling fixture can be used as the source for a new outlet if the wiring at the fixture has a hot wire that is always on. That is, one not controlled by the fixture switch.
Alternately, a wall receptacle from an upper floor can be used as source for a new ceiling fixture below but a switch loop must be run to a wall switch in addition to fishing the cable from the source.
Bury Armored Cable
In a room with old plaster of at least ¾ inch thickness, an armored cable or greenfield can usually be run under the baseboard by removing the baseboard and chiseling a channel along the floor wide enough to bury the metal casing flush with the wall surface. The armored cable will protect the wires from nail punctures.
This method will not work with ½ inch drywall. At least ¾ inch of wall material is needed to bury the armored cable. For drywall or plaster walls less than ¾ inch, the studs can be notched to run a new NM cable and a metal plate used at each stud to protect the soft cable from punctures.
Use these links to locate and cut holes for new cable, fish cable through walls and ceilings, run cable from an upper floor and get tips for fishing cable.