Installing Wiring (How to Build Additions Guide: Simple Room Additions)

The rough wiring must be completed before the walls are insulated and covered. Your plans will indicate the locations of receptacles, lights, switches, and other fixtures.

One new general-purpose circuit should be sufficient for a simple room addition, unless it has a heater or other built-in electric appliance. Each circuit should be protected by its own breaker in the main service panel or in a subpanel near the addition. You may also need to run low-voltage wires for telephone, cable TV, and stereo speakers.

Do not attempt wiring unless you are completely familiar with basic concepts like grounding, circuits, polarity, and wire sizes. Be sure to consult your local code about any specific requirements, and have all work inspected before covering it up.

Planning the Circuits

Before beginning the actual wiring, do some preliminary planning. Plot the location of all new lights, switches, receptacles, and fixed appliances on your plan, if they aren't already included. Then link them together into circuits. Except for fixed appliances, you should be able to wire the entire addition on one general-purpose circuit, which typically has from 10 to 13 fixtures on it. If the house is wired so that lights and outlets are never on the same circuit, extend one of the light circuits for the addition lights and provide a new circuit for all the outlets.

Plan where you will run the circuit wiring back to the main breaker panel or a subpanel. This is usually done under the floor or in the attic of the house.

Installing Boxes

The first step of rough wiring is to install a box for each fixture. There are dozens of types to choose from. The first decision is whether to use metal or plastic boxes. Plastic boxes are cheaper and don't require direct grounding. Metal boxes are easier to fit wallboard around (they pro vide a wider margin for error), but they have to be grounded and are more expensive.

The shape of the box depends on what you will be using it for. Switch boxes, also called utility boxes, are rectangular and are the most common type. Single boxes can be ganged if they are metal, or bought in large sizes for multiple fixtures if they are plastic. Octagonal and round boxes are used for light fixtures and sometimes for junction boxes. Square boxes are more often used for junction boxes. Specialty boxes are available for mounting on wall surfaces, in existing walls, or outdoors. Some fixtures, such as wall heaters and recessed lights, have their own electrical boxes built into the housing.

You can use any shape and con figuration that suits your needs; the critical factor is size. The National Electric Code is very strict about how many wires and fittings can be safely stuffed into a box. The accompanying box-fill table tells what size box you will need for a particular situation. When using the table, consider the following factors.

~ Each hot or neutral wire (black, white, red) counts as one conductor.

~ All grounding wires together count as one conductor.

~ Each receptacle or fixture counts as one conductor.

~ All internal clamping devices and fixture studs count as one conductor.

The NEC does not specify box heights, but the following are typical for most areas.

~ Switches: 44 to 48 inches above the floor.

~ Receptacles: 12 inches above the floor; over counters, 44 inches above the floor.

~ Boxes for baseboard heaters: Varies (typically 6 inches above floor).

~ Ceiling light: Center of ceiling.

~ Junction boxes: Wherever they will be accessible; they can't be concealed inside walls.

Number of Conductors Permitted in a Box:

Box Size


Maximum Number of Conductors


No.14. No.12 No.10 No.8


Round or Octagonal

4 x ¼


4 x 2 1/8

6 5 5 4

7 6 6 5

10 9 8 7




4x2 1/8

9 8 7 6

10 9 8 7

15 13 12 10

Switch Boxes

3x2x 1½







Junction Boxes

4x2½ x

4x2 1/8x1 7/8

4x2 1/8 x2 1/8


Some Code Requirements for Fixtures

Lights. Every room, hallway, stair way, and outdoor entrance must have at least one permanent light fixture controlled by a wall switch located at each entrance to the room. A switch-controlled wall receptacle may be substituted for the light fixture in any room except kitchen or bath.

Light fixtures in closets must be located on the ceiling or on the wall above the door They must also have sufficient clearance to avoid contact with any combustible materials.

Switches. A switch should be located on the latch side of every doorway into a room

Receptacles. Convenience out lets must be placed so you can plug in an appliance with a 6-foot cord anywhere along a wall with out using an extension cord and without crossing a doorway. This means one receptacle approximately every 12 feet and also that any stretch of wall longer than 2 feet with a doorway on either end must have a receptacle.

Appliances. Any 240-volt appliance or built-in 120-volt appliance such as an electric heater must be on its own circuit.

Running Wire

Most residential wiring is done with nonmetallic cable ( Type NM), widely known by the trade name Romex. This cable has plastic sheathing that encloses hot conductor(s), neutral conductor, and ground wire all together. For a 20-amp circuit use No.12 with two conductors and a ground (12/2 w/G).

The NEC requires that cable be stapled with approved nail-on staples at least every 54 inches. It should also be stapled within 12 inches of any metal box and within 8 inches of any plastic box.

When the cable changes directions, bend it softly—no tighter than a radius of five times the cable diameter. Avoid kinks. Keep cable flat, with out spirals or twists. To run cable through framing, drill holes in the center of the stud or plate so cable will be at least 1¼-inches from either face. If it's closer, cover it with a metal plate.

Your local code will specify what kind of cable to run under the floor if it will be exposed from below. In most areas, NM cable is allowed in crawl spaces, but BX armored cable may be required for certain crawl spaces or basements. Run cable along sides of joists and top plates in a basement ceiling, or drill holes through joists.

Avoid running the cable across the tops of joists in an attic, unless you nail 1-by boards on each side as guard strips. (This requirement is waived for attics with no permanent stairs or ladder, except within 6 feet of the scuttle opening.)

New cable through or over joists: Cable, Joists; New cable through a stud: A box must extend the thickness of the wall covering beyond the edge of the stud, If the hole isn't in the center of the stud, the cable must be protected with a metal plate; Running cable: Continuing circuit

Connect cable to metal boxes with approved metal or plastic connectors. Plastic boxes don't require separate connectors, but many have internal clamps. Leave about 12 inches of cable extending out of box for finish wiring.

If you have any 3-way or 4-way switches you will need to use cable with three conductors and a ground wire to connect them to the fixture or to each other. Cable for this purpose can be 14 gauge (14-3 w/G). Cable with three conductors is also used when a single cable carries two different circuits. This is called a multiwire circuit and requires 12-gauge wire (12-3 w/G). Multiwire circuits are most often used in kitchens and bathrooms.

After running cable between all boxes and back to the source (do not connect circuit breaker yet), strip sheathing from the ends protruding from boxes. Do not install fixtures yet, but make whatever connections can be made independently of fixtures. Blacken any white wires that are part of a switch loop. Use a low-voltage continuity tester to check pathways before having the work inspected and covering it.

Wiring to an Existing Circuit

You can wire one or two extra fixtures to an existing circuit that's not used to capacity and is properly wired itself (grounded, protected by circuit breaker, and having correct wire size). Do not make the actual connection until you have completed all finish wiring for the addition and the circuit is shut off.

The easiest connection is at a junction box, usually in the attic or crawl space. Connect all white neutral wires together and all black hot wires together. Connect ground wires together and , if the box is metal, ground it with a short pigtail of bare or green wire. The box must have a metal cover and be accessible.

Service panel: For clarity, only one circuit is shown; Main disconnect; Service entrance stab or— prong; Power bus A; Power bus B; Knockouts for other circuits; Power wires from meter; Neutral wire from meter; Cable to house circuit; Connecting flange; knockouts for other circuits; Ground wire to cold-water pipe or ground rod; Ground bus bar

Wiring to an existing circuit:

Wiring into an End-of-Run Receptacle

An end-of-run receptacle is an easy place to tie into a circuit, as long as the outlet isn't controlled by a switch. Run the new cable into the box. Then connect the two black wires to a black pigtail, the two white wires to a white pigtail, and the two ground wires to a grounding pigtail. Attach the receptacle to the pigtails. Include a second pigtail for grounding a metal box.

Wiring into a Middle-of-Run Receptacle

A middle-of-run box already has an incoming and an outgoing cable, in addition to a receptacle, so it will probably be necessary to gang an other box with it or deepen it with a box extender.

Run the new cable into the box and connect black wires, white wires, and ground wires to their respective pigtails and add another grounding pigtail for a metal box.

Wiring a Middle-of-Run Switch

If power comes directly to a switch, the box will have two cables coming info it, and you can connect new wiring. Do not tie a new circuit into a switch loop, which usually has only one cable.

Before turning off power, use a voltage tester to establish which black wire is the incoming hot source (be sure switch is off). Shut off power and disconnect that wire from switch. Tie black wire from new cable and short pigtail into it. Attach pigtail to switch. Connect ground wires with pigtail and neutral wires without one. Attach grounding pigtail to switch.

Finish Wiring Techniques

After the finish walls are installed and painted, add the electrical fixtures. Use the same techniques for connecting receptacles, switches, and lights as for any wiring project. See our Basic Wiring Techniques.

Adding an extension to an end-of-run receptacle: Existing cable; Grounding pigtail; Extending a circuit from a middle-of-run receptacle: Grounding pigtail; Extending a circuit from a switch: Power source comes to switch; not fixture; From source; To light fixture;

Next: Insulating the Addition

Prev: Access to the Room Addition

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