Electricity: Adding or Extending a Branch Circuit


To install new switches, fixtures and receptacles, first determine the types you want and where you want them. Either run a new circuit from the service panel or extend an underutilized existing circuit. Before adding a new circuit, calculate your power usage to make sure the new circuit won’t overload the service panel. Check to see whether the panel has space for a new breaker or fuse.

Extend an existing circuit only if it’s grounded and not already operating at or near capacity. If an extension is possible, pick an accessible box to serve as its power source. The simplest source is a junction box in which wires from the new and existing cables are spliced together. The power source must always be hot, so don’t use an end-of-circuit switch, a fixture or a switch-controlled receptacle. Junction and device boxes must be replaced with larger ones if the additional wires will exceed the box’s volume limitations.

To estimate cable lengths, plan the shortest route from power source to the new outlet(s). Add a foot for each box plus 25 % for errors. Wire sizes for new wiring must match that of the existing wiring. Check local codes for specific requirements and have your work inspected.

Caution: Cut power to an electrical box before working on it. Use a voltage tester to make sure all wires are dead.

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Tapping into an Existing Circuit

Tapping into an Existing Circuit

An end-of-circuit receptacle is a good place to start a circuit extension, as long as the receptacle isn’t switch- controlled. Connect the new cable’s black wire to the receptacle’s unused brass screw, its white wire to the unused silver screw. Splice grounding wires in the box.

A middle-of-circuit receptacle has wires from two cables connected to it. Adding a third cable may require a bigger box. Use pig- tails to attach new and existing black wires to the brass screw, white wires to the silver screw. Splice all grounding wires.

To tie into a switch, carefully identify the incoming hot wire with a voltage tester. Cut power and disconnect the hot wire. Pigtail new and incoming black wires to the switch terminal. Splice new white and grounding wires to existing white and grounding wires.

To tap a middle-of-circuit fixture , use tester to find incoming hot wire. Shut off power and splice black wires from new and incoming cables to the switch loop’s hot white wire (marked black). Splice white wires from new and incoming cables and fixture.

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Last modified: Saturday, 2007-11-03 1:14 PST