Wiring methods

A wiring method is the system used to carry conductors from one point to another and protect them on the way: a cable, a raceway with conductors pulled into it, a busway, or a cable tray. Choosing and installing the right method is most of an electrician's daily work. The code permits each method only in certain locations and uses, and sets rules for how it is supported, protected, filled and terminated. Most installation deficiencies that inspectors write up are wiring method violations: unsupported cable, overfilled boxes, missing bushings, or a cable used where it is not permitted.

In the NEC, wiring methods are covered in Chapter 3. Article 300 gives general requirements for all methods; Article 310 covers conductors; Article 312 and 314 cover cabinets, boxes and fittings; and each method has its own article, numbered in a standard pattern (for example 334 for NM cable, 344 for rigid metal conduit, 352 for PVC conduit and 358 for EMT), with sections .10 (uses permitted), .12 (uses not permitted), .22 (number of conductors), .26 (bends), .30 (securing and supporting) and so on.

Cables

• Nonmetallic-sheathed cable (Type NM-B, Article 334): insulated conductors and a bare equipment grounding conductor in a PVC jacket. It is the standard method for dwellings and permitted in some other buildings of combustible construction, but it must be protected from physical damage and is not permitted in some locations (for example in wet or damp locations, embedded in concrete, or in many commercial buildings over three floors).
• Armored cable (Type AC, Article 320) and metal-clad cable (Type MC, Article 330): conductors in an interlocked metal armor. AC uses the armor plus a bonding strip as the equipment grounding path; MC usually includes an insulated green equipment grounding conductor. Both are common in commercial work.
• Underground feeder cable (Type UF, Article 340) is permitted for direct burial and wet locations; service entrance cables (Types SE and USE, Article 338) are used for services and some feeders.

Cables must be supported and secured at intervals and close to boxes so that their weight does not pull on terminations, and they must be protected where they pass through framing so that nails and screws driven into the wall cannot reach them.

For NM cable, the cable must be secured within 300 mm (12 in) of a box and at intervals of not more than 1.4 m (4.5 ft) (334.30). Where it passes through bored holes or notches in wood framing less than 32 mm (1.25 in) from the face, it must be protected by a steel plate at least 1.6 mm (1/16 in) thick (300.4(A)).

Raceways

A raceway is a channel designed to hold conductors: conduit, tubing, wireway or surface raceway. Conductors are pulled into the raceway after it is installed, so wiring can be changed later. Metal raceways also serve as a bonding path when properly installed. The main types are:

• Rigid metal conduit (RMC, Article 344) and intermediate metal conduit (IMC, Article 342): threaded steel conduit for maximum protection, outdoors and in industrial and hazardous locations.
• Electrical metallic tubing (EMT, Article 358): thin-wall steel tubing joined with set-screw or compression fittings, the most common raceway in commercial buildings.
• Rigid PVC conduit (Article 352): non-metallic, corrosion-resistant, used underground, in concrete and in wet or corrosive areas; it expands with temperature and needs expansion fittings on long exposed runs. Because it is not conductive, a separate equipment grounding conductor must be pulled in with the circuit.
• Flexible conduit (metal or liquidtight) for connections to motors and vibrating equipment and short runs where rigid raceway is impractical, with limits on length and on its use as a bonding path.

Conduit fill

Conductors must fit into a raceway with enough free space to be pulled without damaging their insulation and to dissipate heat. The codes limit the percentage of the raceway's internal cross-sectional area that conductors may occupy:

• One conductor: 53%.
• Two conductors: 31% (two round conductors tend to jam).
• Three or more conductors: 40%.

In the NEC, these percentages are in Chapter 9, Table 1, with raceway areas in Table 4 and conductor areas in Table 5; Annex C gives ready-made tables of the maximum number of conductors of one size in each raceway.

Bends and pull points

Every bend adds friction when conductors are pulled. Both codes limit the bends between pull points (boxes, conduit bodies or fittings where the conductors can be accessed) to the equivalent of four quarter bends: a total of 360°. Offsets and saddles count too: a pair of 30° offset bends adds 60°, and a three-bend saddle with a 45° center bend adds 90°. When a run needs more, a pull box or conduit body is added.

Raceway installation

Raceways must be complete between boxes before conductors are pulled in, reamed so no sharp edges damage insulation, fitted with bushings or insulated throats where conductors of 4 AWG and larger enter boxes, supported at specified intervals and within a short distance of each box, and protected against corrosion where needed. Metal raceways used as the bonding path must have tight, continuous joints, because a loose coupling can break the fault return path.

Boxes and box fill

Every splice, device and connection must be in a box (or other approved enclosure) that remains accessible. Boxes must be large enough for their contents. A crowded box makes connections hard to do well, damages insulation when devices are pushed in, and traps heat. The codes therefore limit box fill: the total volume required by the conductors, devices, clamps and fittings in a box must not exceed the box's volume.

In the NEC, box fill is calculated with 314.16: each conductor that originates outside the box and terminates or is spliced inside counts as one volume allowance for its size (2.0 in³ for 14 AWG, 2.25 in³ for 12 AWG, 2.5 in³ for 10 AWG). All equipment grounding conductors together count as one allowance (with a quarter allowance for each beyond four); internal cable clamps count as one; each device yoke (receptacle or switch) counts as two, based on the largest conductor connected to it. The box volume is marked on the box or given in Table 314.16(A).

Other box rules appear constantly on the job: boxes in walls must be set so that their front edge is flush with (or only slightly recessed from) a combustible finish; boxes supporting light fixtures must be rated for the weight; ceiling fans need boxes listed for fan support; and every box must remain accessible without removing any part of the building structure or finish.

Underground and wet locations

Underground wiring must be buried deep enough to avoid damage, protected where it emerges from the ground, and suitable for wet locations, because conduits underground fill with water over time. Conductors in underground raceways must be rated for wet locations, and raceways entering buildings from underground must be sealed to keep water and gases out.

The NEC sets minimum cover depths in Table 300.5: for example 600 mm (24 in) for direct-buried cable, 450 mm (18 in) for PVC conduit and 150 mm (6 in) for rigid metal conduit in general locations, with different values under driveways, roads and residential areas with GFCI protection. A warning ribbon must be placed above underground service conductors.

Wiring methods for damp, wet and corrosive locations, and the limits on flexible cords and temporary wiring, are covered in Lesson 10; low-voltage and communications cabling is covered in Lesson 11.