Installing cable, boxes and devices

Most of the hours on a residential job are spent installing cable and boxes during rough-in and terminating devices at finish. The code sets the minimum requirements, but workmanship decides whether an installation is safe for decades: a cable stapled too tightly, a box with one conductor too many, or a loose terminal can each start a fire years later. This lesson covers the practical rules and good practice for non-metallic sheathed cable, boxes and their fill, protecting cables in the structure, making connections and terminating devices, and the sequence of rough-in and finish work.

Non-metallic sheathed cable

In the US, type NM-B cable is used in most one- and two-family dwellings. Its conductors are rated 90 °C, but its ampacity is limited to the 60 °C column, and it is permitted only in dry locations; damp and wet locations need type UF cable or conductors in raceway. Some jurisdictions restrict NM cable in larger or taller buildings.

Cable is identified by its conductor size and number: 14/2 means two insulated 14 AWG conductors plus a bare bonding (grounding) conductor; 14/3 has three insulated conductors (black, red and white) plus the bare conductor, used for three-way switching, split receptacles and multiwire circuits. Jacket colors help identify sizes on the job, with white often used for 14 AWG, yellow for 12 AWG and orange for 10 AWG, but the printing on the jacket is what counts.

Supporting and protecting cable

NM cable must be secured within 12 in of every box and at intervals of not more than 4½ ft, with staples or straps that do not damage the jacket. Where it runs through bored holes, the edge of the hole must be at least 1¼ in from the nearest edge of the framing member, or a steel plate at least 1/16 in thick must protect the cable from nails and screws.

• Staples hold the cable flat, never on edge, and are driven snug but not tight; a crushed jacket or nicked conductor can fail later.
• Cables are not stapled in bundles under one staple unless the staple is listed for it, and bundles of many current-carrying cables in insulation may need ampacity adjustment (ELEC-120).
• Cables are protected where they are exposed to damage, for example run along the face of basement walls below joists or in garages, using conduit or by running them through framing instead.
• Cables passing through metal studs need grommets to protect the jacket from sharp edges.
• Bends are gradual, with a radius of at least five times the cable diameter.

Boxes

Every splice, device and luminaire connection must be in a box (or a fitting or enclosure listed for the purpose), and every box must remain accessible after the building is finished, never buried behind drywall.

• Device boxes hold switches and receptacles: single-gang and multi-gang, plastic (non-metallic) or metal, new-work (nailed to studs) or old-work (clamped into finished walls).
• Octagon and round boxes hold ceiling and wall luminaires; boxes for ceiling fans, or for luminaires heavier than a set limit, must be listed for that support.
• Square boxes with device rings are used where more volume is needed, for example where several cables meet.
• Vapor-barrier and airtight boxes are used in exterior walls and insulated ceilings (Lesson 2).

Boxes are mounted so that their front edge will be flush with the finished surface of combustible materials such as wood panelling, and set back no more than a small distance (typically 6 mm, or ¼ in) in non-combustible surfaces such as drywall, so that heat and arcs stay contained. Box extenders fix boxes set too deep.

Box fill

A box must be large enough for its conductors, devices, clamps and fittings, so that conductors are not crushed and heat can dissipate. ELEC-120 (Lesson 6) explained the calculation; in practice electricians count before they choose the box:

The NEC (314.16) counts each conductor originating outside the box as one, each device yoke as two conductors of the largest size connected to it, all equipment grounding conductors together as one, internal clamps as one, and luminaire studs or hickeys as one each; the total is multiplied by the volume allowance per conductor (2.00 in³ for 14 AWG, 2.25 in³ for 12 AWG).

Making connections

Most residential faults that cause fires begin at connections. Good connections are tight, use connectors and terminals listed for the conductors, and do not stress the conductors:

• Strip only the length the connector or terminal needs, without nicking the conductor.
• Twist-on wire connectors: pre-twist conductors if the manufacturer recommends, choose the connector for the number and sizes of conductors, and turn until tight; no bare copper should show below the skirt.
• Lever-type and push-in connectors are listed for specific conductor sizes and types; follow their markings.
• Leave enough free conductor in each box to make connections and replace devices: at least about 150 mm (6 in) of free conductor, with some extending beyond the box opening.
• Fold conductors neatly into the box, with bonding conductors connected together and to every metal box and device yoke as required.

Terminating devices

• Screw terminals: wrap the conductor clockwise (the direction the screw tightens) about three-quarters of the way around, and tighten firmly. Only one conductor goes under a screw unless the device is marked for two.
• Back-wire clamps (pressure-plate terminals) accept a straight stripped conductor under a plate tightened by the screw, and are reliable for one or two conductors as marked.
• Push-in ("backstab") terminals, where allowed, accept only solid 14 AWG copper on most devices and have a higher rate of failure under load than screw terminals; many electricians avoid them.
• Torque: where the manufacturer specifies a torque, use a calibrated torque screwdriver; this is required at breakers and panels and good practice everywhere.
• Pigtails: rather than using a receptacle to pass current to the next outlet, splice the circuit conductors together with a short pigtail to the device. A failed or loose receptacle then does not interrupt or overheat the rest of the circuit, and multiwire circuits must use pigtails for the neutral so that removing a device does not open the shared neutral (ELEC-120).

The NEC requires terminals to be identified for the conductor material; aluminum branch-circuit conductors may connect only to devices marked CO/ALR (for 15 A and 20 A receptacles and switches) or with listed connectors (Lesson 11).

Rough-in and finish

Residential electrical work is done in two main stages around the other trades:

• Rough-in: after framing, plumbing and ductwork are in place and before insulation and drywall, the electrician installs boxes, runs and staples cables, makes up grounds and splices in boxes (often called "making up"), installs the panel and services where scheduled, and calls for the rough-in inspection. Low-voltage cabling (data, coaxial, alarms, Lesson 12) is usually roughed in at the same time.
• Finish (trim-out): after drywall and paint, the electrician installs devices, cover plates, luminaires, fans, appliances connections and breakers, labels the panel, tests everything (Lesson 9), and calls for the final inspection.

Between the two, the electrician protects work from damage: marking box locations so drywall installers do not cover them, and checking that insulation installers have not pulled cables from staples or packed insulation around recessed lights not rated for contact (IC rating).

Luminaires, fans and recessed lights

Lighting outlets have their own installation details:

• Ceiling boxes must support the luminaire. Light luminaires hang from the box's mounting strap; heavier ones and all ceiling fans need boxes or braces listed for the weight or for fan support, fastened to framing, because a fan's motion loosens ordinary boxes.
• Recessed luminaires (pot lights) in insulated ceilings must be rated IC (for contact with insulation) or kept the required distance from insulation; in cold climates they should also be airtight (often marked IC and AT), or they leak warm moist air into the attic. Many current LED recessed fixtures are thin "wafer" lights with a remote driver box, which must remain accessible.
• Luminaires in damp or wet locations (bathroom showers, covered porches, outdoors) must be marked for that location.
• Closet luminaires must keep set clearances from storage areas, and incandescent lamps without enclosures are generally not allowed in closets.
• Under-cabinet and strip lighting often uses low-voltage LED tape with a Class 2 driver; the driver location and the cable from it are planned at rough-in.