Commercial and industrial wiring methods

Lesson 1 sized feeders and services; this lesson is about how conductors are installed between the equipment. Commercial and industrial buildings use a different toolkit from houses: metal and non-metallic raceways, armoured and metal-clad cables, cable trays, busways and wireways, installed in exposed and concealed locations, through fire separations, in wet and corrosive areas, and in places where conductors need mechanical protection. Choosing the right wiring method affects safety, cost, installation time and how easily the system can be changed later. The code sets the conditions of use for each method, how many conductors a raceway may hold, how boxes are sized, how raceways and cables are supported, and how fire separations are maintained. This lesson covers those rules and the calculations an electrician uses every day: conduit fill, box fill, bends, expansion and support.

Choosing a wiring method

Each wiring method is permitted only in the locations and building types its rules allow. The main options in commercial and industrial work:

In the CEC, wiring methods are covered by Section 12, which states for each method where it may and may not be used. Section 2 contains general requirements such as protection against mechanical damage and corrosion, and Rule 2-128 addresses fire spread through fire separations. Non-metallic sheathed cable is permitted in buildings of combustible construction, and in other buildings only in the circumstances Section 12 allows, so in most non-combustible commercial buildings armoured cable or raceways are used instead.

Beyond the code minimums, the specification for a project often restricts wiring methods further: many owners require EMT for all exposed work and feeders, allow armoured or metal-clad cable only for short drops and concealed branch circuits in fit-outs, and require rigid conduit in mechanical rooms and loading docks.

Conduit fill

A raceway must not be packed so tightly that conductors are damaged during pulling or overheat in service. The code limits the cross-sectional area of the raceway that conductors may occupy:

Two conductors get the lowest percentage because two round conductors can jam side by side in a raceway during pulling. The calculation uses the actual cross-sectional area of each insulated conductor, from the code tables or the manufacturer, compared with the internal area of the raceway. Equipment bonding conductors are counted. Short nipples (raceway sections not exceeding about 600 mm or 24 in) may be filled to 60%.

Bends, pull points and pulling tension

Every bend increases the force needed to pull conductors and the pressure on their insulation against the inside of the bend. The code limits the total bends between pull points (boxes, conduit bodies or fittings where cable can be pulled) to the equivalent of four quarter bends, 360° in total, including offsets and kicks. Practical installations also limit run length between pull points, typically 30 to 60 m depending on conductor size and the number of bends, because pulling tension rises exponentially with total bend angle:

where \(\mu\) is the coefficient of friction (about 0.35 to 0.5 with lubricant) and \(\theta\) the bend angle in radians. With \(\mu = 0.4\), a 90° bend multiplies tension by \(e^{0.4 \times 1.571} = 1.87\), and four 90° bends by about 12. For large feeders, engineers calculate pulling tension and sidewall pressure and compare them with the cable manufacturer's limits, and electricians use pulling lubricant, rollers at bends and power pullers with tension meters.

Box and enclosure fill

Outlet and junction boxes must be large enough for the conductors, devices and fittings in them, both to avoid damaging insulation and to dissipate heat. The code assigns a volume allowance to each conductor based on its size, and adds allowances for devices (switches and receptacles), internal clamps, fixture studs and bonding conductors.

Rule 12-3034 gives the method, with Table 22 listing the usable capacity of standard boxes and Table 23 giving the free space required per conductor (for example, 24.6 cm³ for 14 AWG, 28.7 cm³ for 12 AWG and 36.9 cm³ for 10 AWG). A device such as a receptacle or switch counts as two conductors of the largest size connected to it.

Pull boxes and junction boxes for conductors 4 AWG and larger are sized by dimensions rather than volume: for straight pulls, the length must be at least eight times the trade size of the largest raceway; for angle or U pulls, the distance to the opposite wall must be at least six times the largest raceway plus the sum of the other raceways on the same wall. These rules leave room to bend large conductors without damage.

Support and securing

Raceways and cables must be securely fastened and supported at specified intervals and near boxes and fittings, so that they do not sag, pull out of fittings or transfer strain to terminations. Typical requirements are support within about 1 m (3 ft) of each box or fitting and at intervals of about 3 m (10 ft) for EMT and rigid conduit in smaller sizes, with longer spacing permitted for larger rigid conduit, and closer spacing for armoured and metal-clad cables (about 1.5 m or 6 ft) and for PVC, which sags when warm. Supports must be independent of ceiling grid wires and other services unless the support is designed for it; above suspended ceilings, raceways and cables need their own supports or support wires identified for the purpose. In seismic regions, supports for heavy raceways and busways may need seismic bracing (ELEC-210 covered seismic restraint of equipment).

Expansion of PVC conduit

Rigid PVC conduit expands and contracts with temperature about five times as much as steel, so long runs exposed to temperature changes, such as outdoors, on roofs or in unheated buildings, need expansion fittings that allow the conduit to slide.

Cable trays

Cable trays (ladder, ventilated trough, solid bottom and wire mesh) support large numbers of cables in industrial plants, data centres and commercial service areas. A cable tray is a support system, not a raceway: only cable types approved for tray use may be installed (such as TECK90, ACWU90 and tray-rated cables in Canada; Type TC, MC and others in the US), conductors smaller than certain sizes may need to be in multiconductor cables, and single conductors must be large enough and arranged to avoid heating and induced currents. The code limits tray fill (by the sum of cable diameters or cross-sectional areas relative to tray width and depth, depending on cable size and type) and sets ampacity rules, which can be reduced where many cables are bunched and where solid covers trap heat. Metal trays can serve as bonding conductors if they are suitably constructed, marked and bonded at joints.

Cable tray installations are governed by Rules 12-2200 to 12-2210, which set where trays may be used, the cable types permitted, fill and ampacity requirements (with reference to Table 5D for derating cables in trays), and bonding.

Busways and wireways

Busways are prefabricated assemblies of copper or aluminium bars in a metal housing, rated from about 100 A to over 5,000 A. Feeder busway carries large currents compactly between transformers and switchboards; plug-in busway has openings along its length where tap-off units with breakers or fuses can be connected, making it popular in factories and data centres where loads move. Overhead track busway feeds rack power in data halls (DCE-310). Busways must be supported at the manufacturer's intervals, installed only where permitted (generally exposed and accessible, not where subject to severe physical damage or corrosive vapours unless designed for it), and protected at their rating; tap-offs must have overcurrent protection as the code requires for taps. Wireways (metal troughs with hinged or removable covers) serve as gutters above panelboards and motor control centres, with fill limits (typically 20% of the cross-section) and limits on the number of current-carrying conductors before derating applies.

Fire separations and penetrations

Fire separations, the walls and floors that divide a building into fire compartments, must keep their fire-resistance rating where raceways, cables, busways and boxes pass through them. Every penetration must be sealed with a tested firestop system, listed for the specific penetrating item, the wall or floor construction and the required rating, and installed exactly as tested. Boxes in fire-rated walls have spacing and size limits on opposite sides of the wall, or need putty pads. Electricians often install firestopping for their own penetrations, and inspectors and fire officials check it closely, because unsealed penetrations allow fire and smoke to spread between compartments.

Rule 2-128 requires electrical installations to be made so that the spread of fire through fire separations is minimized, and the National Building Code requires firestop systems tested to CAN/ULC-S115 with ratings (F and FT ratings) appropriate for the separation. Section 2 also restricts the combustibility of cables in buildings of non-combustible construction and in plenums (FT1, FT4 and FT6 ratings).

Wet, corrosive and damp locations

Conditions dictate materials. In wet locations, outdoors and below grade, conductors must be rated for wet use (for example RW90 or XHHW-2), raceways must drain and be arranged to prevent water entering equipment, and boxes and fittings must be weatherproof. Underground raceways are assumed to be wet inside. Corrosive locations, such as wastewater plants, chemical plants, swimming pool equipment rooms, food processing with washdown, and fertilizer storage, need corrosion-resistant materials: PVC conduit, PVC-coated rigid steel, stainless steel boxes and fittings, and suitably jacketed cables. Where raceways pass between areas of different temperature, such as from a heated building to a cold room or outdoors, they must be sealed to prevent warm moist air from circulating and condensing inside.