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 CEC, wiring methods are covered in Section 12, which is organized by method: general requirements first, then rules for conductors in free air, non-metallic sheathed cable, armoured cable, mineral-insulated cable, raceways of each type (rigid metal conduit, rigid PVC conduit, electrical metallic tubing, flexible conduit and so on), cable trays, and boxes and fittings. Section 12 rules apply in addition to the general rules of Section 2 and the conductor rules of Section 4.

Cables

• Non-metallic sheathed cable (NMSC, such as NMD90): insulated conductors and a bare bonding conductor in a PVC jacket. It is the standard method for dwellings and permitted in many other buildings of combustible construction, but it must be protected from mechanical damage and is not permitted in some locations (for example where exposed to damage, or embedded in concrete).
• Armoured cable (AC90, Rules 12-600 series): conductors with an interlocking aluminum armour. The armour protects against mechanical damage and serves as the bonding path (with a bonding strip). Common in commercial work and wherever exposed cable needs protection.
• TECK90 cable: armoured cable with an overall PVC jacket, suitable for wet locations, direct burial and industrial use, terminated with special connectors.

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 NMSC, the cable must be secured within 300 mm of a box and at intervals of not more than 1.5 m, and where it runs through holes in wooden framing it must be kept back from the face of the framing or protected with steel plates (Rule 12-500 series and related rules).

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): heavy-wall threaded steel or aluminum, for maximum protection, outdoors and in industrial and hazardous locations.
• Electrical metallic tubing (EMT): thin-wall steel tubing joined with set-screw or compression fittings, the most common raceway in commercial buildings.
• Rigid PVC conduit: non-metallic, corrosion-resistant, used underground, in concrete and in wet or corrosive areas; it expands with temperature and needs expansion joints on long exposed runs. Because it is not conductive, a separate bonding 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 CEC, these percentages are in Table 8, and the dimensions of conductors and the internal areas of raceways are given in the code's tables.

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° centre 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 CEC, box fill is calculated by allowing a certain usable volume per conductor according to its size, given in a table, and comparing the total with the box capacity marked on the box or listed in the code. Conductors that pass through without a splice, devices, and the conductors connected to them all count; check the counting rules in the box fill rule of Section 12 before calculating.

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 CEC sets minimum cover depths in Rule 12-012 and its table, depending on the wiring method and whether it is under a driveway or roadway; direct-buried cable typically needs more cover than conduit, and cables must be protected with a warning tape or planking above them in many cases.

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.