General wiring method rules in Section 12

Section 12 of the CE Code is the longest general section and the one candidates find hardest to navigate. It covers every common wiring method: open wiring, non-metallic sheathed cable, armoured cable, mineral-insulated cable, raceways of all kinds, cable trays, busways, boxes and cabinets. The good news is that it is organized in a predictable way. A block of general rules at the start applies to every wiring method. Then each method has its own block of rules, numbered in a series, usually in the same order: where the method may be used, where it may not, how it is installed and supported, and how it is terminated.

This lesson covers the general rules and the habit of choosing a wiring method by location. The next three lessons cover raceways, cables, and boxes in detail. As always, the descriptions here are ours, not the Code's, and the location summary is illustrative. Read each rule in your 2024 copy as it comes up.

How Section 12 is organized

The general rules at the start of Section 12 deal with matters that every wiring method shares: the scope of the section, conductors in general, protection from mechanical damage, installation in damp, wet and corrosive locations, underground installations, wiring in hollow spaces and through framing, the effect of fire separations, and the connection and splicing of conductors. After the general rules, the numbered series for each method follow. Learn where the series start in your copy: for example, the 12-500 series for non-metallic sheathed cable, the 12-600 series for armoured cable, the 12-1000 series for rigid metal conduit, the 12-1100 series for rigid PVC conduit, the 12-1400 series for electrical metallic tubing, and the 12-3000 series for boxes and cabinets.

Priya tabs the start of each series with a small green tab and writes the method name on it. Her test is simple: Denise calls out a wiring method and a question type, such as ‘armoured cable, support spacing’, and Priya has to open the right series and find the support rule within thirty seconds. By week seven she can do it for every method on the list.

Protection from mechanical damage

A recurring requirement in Section 12 is that conductors and cables be protected where they are exposed to mechanical damage. Each wiring method has its own rules, but the general principle is the same: if the cable could be struck, crushed, cut or abraded, it must either be of a type that can withstand that exposure or be protected by a raceway, a guard or its location. Exam scenarios often describe a cable run along the floor of a shop, down a column in a warehouse, or through an area where vehicles move. The question is usually whether the chosen method is acceptable there.

At Hartley Farms, the grain-drying shop has a skid steer that runs through the building several times a day. Any wiring on the walls below head height near the door is exposed to damage. Gord's preference is to run rigid PVC conduit up the walls in that zone, or to use TECK90 with additional guarding. The group checks the rules for each method and the general rules on protection and agrees that NMD90 is not acceptable in that location in any case.

Damp, wet and corrosive locations

Section 0 defines dry, damp and wet locations, and the definitions matter. A damp location is one subject to moderate moisture, such as some basements and partially protected outdoor areas. A wet location is one where liquids may drip, splash or flow on the equipment, or where the installation is outdoors and exposed to weather, or underground. Corrosive locations are those where the atmosphere or substances present may damage wiring, such as some agricultural buildings with manure gases, chemical storage areas and car washes.

Every wiring method has limits on these locations. Table 19 tells you which insulation and cable types are suitable for which conditions, and each method's rules repeat or add to those limits. For example, conductors in a raceway in a wet location must be of a type suited to wet locations, because water collects inside raceways even when the raceway itself is weatherproof. Cables with a non-metallic outer jacket over their armour are generally needed where a bare metal armour would corrode or where the cable is buried.

Use the figure as a memory aid, not as an answer key. The real answer always comes from Table 19 and the rules for the method. Questions about agricultural buildings are a favourite because they combine dampness, dust and corrosion, and because Section 12 and the farm building rules elsewhere in the Code add specific requirements. Hartley Farms' livestock barn, which Parkland Valley wired last year, is the example Denise uses to show how many rules can apply at once.

Underground installations

Wiring run underground must be of a type suitable for direct burial or be in a raceway suitable for burial, and it must be buried at least the minimum depth set out in the Code. Section 12 contains the general underground rules, and Table 53 sets minimum cover depths for different situations, such as under driveways, under lawns, and with or without protection. Other underground requirements include protection where the conductors emerge from the ground, allowances for frost movement, marking tape or warning means in some cases, and sealing of raceways that enter buildings to prevent moisture and gas entry.

The Hartley Farms bin yard feeder runs underground for most of its length, partly under a gravel lane that grain trucks use. The group finds the underground rules in Section 12, reads Table 53, and identifies three decisions: the burial method (direct burial cable or cable in conduit), the depth for each part of the route (the lane section needs more cover or protection than the field section), and the protection where the cable rises up the bin yard pole. On the exam, read the scenario for words like driveway, vehicles, under a building and riser, because each changes the depth or protection required.

Hollow spaces, framing and fire separations

Cables run through walls and ceilings must be protected where they pass through framing members, so that nails and screws driven into the wall finish do not reach them. The rules for each cable type set minimum distances from the edge of studs and joists or require protective plates where those distances cannot be kept. Cables run in hollow spaces such as plenums and return air paths have additional restrictions, because a fire in the cable insulation there would spread smoke through the building. Read the rules on hollow spaces and on air-handling spaces before you allow any cable in a ceiling space used for air return.

Wiring that passes through a fire separation must not reduce the separation's fire resistance. The Code's rule on fire spread, together with the building code, requires that openings around cables and raceways be sealed with a fire stop system that restores the rating. On the exam, questions on fire stopping usually test whether you know that the electrical installer is responsible for keeping the separation intact, and which rule in Section 2 states the general requirement.

All conductors of a circuit together

Another general rule appears again and again in Section 12: where a circuit is run in a metal raceway or metal-armoured cable, all the conductors of that circuit, including the neutral and the bonding conductor where present, must be run in the same raceway or cable. If the phase conductor runs in one conduit and the neutral in another, the magnetic fields do not cancel, the metal raceway heats by induction, and the circuit can behave unpredictably. The same principle drives the rule for parallel conductors from Lesson 1, where each raceway must contain one conductor of every phase.

Marcus remembers an industrial job where a single conductor was pulled through a steel box knockout and the box became hot enough to discolour the paint. Denise uses the story to show that this rule is about physics, not paperwork. On the exam, any scenario that separates the conductors of one circuit into different metal enclosures is likely to be wrong.

Sunlight, temperature and expansion

Outdoor and rooftop wiring faces conditions that indoor wiring never sees. Ultraviolet light degrades many plastics, so cable jackets and non-metallic raceways exposed to direct sunlight must be of a type marked or listed as sunlight resistant. Rooftop runs also see high temperatures: a raceway lying on a dark roof in summer can be far hotter than the outdoor air, and the ampacity of the conductors inside must be corrected for that higher ambient. The 2024 edition adds attention to these conditions, and exam writers like rooftop photovoltaic and HVAC scenarios because they combine sunlight, heat and wet locations in one question.

Temperature change also makes raceways grow and shrink. Rigid PVC conduit in particular expands and contracts much more than metal conduit for the same change in temperature, so long straight runs exposed to temperature swings need expansion fittings. The rules for rigid PVC conduit address this, and the conduit manufacturer's data gives the expansion rate. A typical exam question gives a run length, a temperature range and an expansion rate, and asks how much movement the fitting must allow.

Splices, connections and support

Section 12 also contains general rules on joints and splices: they must be made with approved connectors suitable for the conductors and conditions, they must be accessible unless the method used is approved for concealment, and they must be enclosed in boxes or fittings. Each wiring method then has rules on support spacing and securing near boxes. Questions on support usually give you a distance and ask whether it complies, or give you a method and ask for the maximum spacing. These are straightforward lookups once you have the series tabbed.

A method that Priya finds useful for any wiring method question is to read the method's series in this order: permitted uses, prohibited uses, installation and support, protection, terminations. If the scenario fails at the first step, you do not need to read the rest. If it passes the first two steps, the question is usually about the installation details.