Cables: NMD90, armoured and others

Cables are factory assemblies of conductors with a common covering. They save labour compared with pulling conductors into raceways, and they dominate residential and much light commercial work. Each cable type has its own series of rules in Section 12, and Table 19 lists the conditions in which it may be used. This lesson covers the cable types a master electrician meets most: non-metallic sheathed cable (NMD90), armoured cable (AC90 and ACWU90), armoured cable with an overall jacket (TECK90), and mineral-insulated cable. For each one, the exam questions follow the same pattern as for raceways: where it may be used, where it may not, how it is supported and protected, and how it is terminated and bonded.

The descriptions of permitted uses below are general summaries. Building types, construction classes and locations interact with the cable rules in detail, and provincial amendments sometimes change them, so treat every statement here as a pointer to the rule you must read in your own copy.

Non-metallic sheathed cable (NMD90)

NMD90 is the familiar house wire: two or three insulated copper conductors and a bare bonding conductor inside a non-metallic outer sheath. Its rules are in the 12-500 series. It is used mainly in buildings of combustible (wood-frame) construction, concealed in walls, floors and ceilings or run exposed in some places where it is not subject to damage. It is generally limited to dry locations, and it may not be used where it would be exposed to mechanical damage, in wet or corrosive locations, embedded in concrete, or in hollow spaces used for air handling, among other prohibitions. The 2024 edition updates the requirements for non-metallic jacketed cables, so check the current wording rather than relying on older notes.

The installation rules focus on protecting the cable from the building's finishes and fasteners. NMD90 must be supported at intervals and near each box, protected where it passes through framing close to the edge of a stud or joist, protected where it runs along the side of framing or across the underside of joists in unfinished basements, and kept away from heating ducts and other heat sources. Questions on NMD90 usually test one of these installation details or one of the prohibited locations.

Support questions come up often enough that Priya writes a small function to check her counts. It takes the run length, the end distance and the maximum spacing, and returns the minimum number of supports. Change the illustrative values to match the rules in your copy and try runs of different lengths.

import math

def supports(run_m, end_m, spacing_m):
    middle = run_m - 2 * end_m          # distance between the two end supports
    spaces = math.ceil(round(middle / spacing_m, 9))
    return spaces + 1

for run in (2.0, 6.6, 9.6, 12.1):
    print(f'{run:>5} m run: {supports(run, 0.3, 1.5)} supports (illustrative spacing)')

  2.0 m run: 2 supports (illustrative spacing)
  6.6 m run: 5 supports (illustrative spacing)
  9.6 m run: 7 supports (illustrative spacing)
 12.1 m run: 9 supports (illustrative spacing)

Watch the rounding: a middle distance that is an exact multiple of the spacing needs no extra support, but anything even slightly longer needs one more. The 12.1 m run shows this: its middle section is 11.5 m, which needs eight spaces rather than seven and a bit, so nine supports. Exam answers often include the number you would get by rounding down.

Cables and building construction

Whether a cable may be used in a building often depends on the building itself as well as on the location within it. The building code classifies buildings by occupancy and by construction type, combustible or non-combustible, and the CE Code's cable rules refer to those classifications. Historically, NMD90 was tied closely to combustible construction, and armoured cable or raceways were expected in non-combustible buildings, though the rules have changed over successive editions and provincial amendments add their own conditions. Read the current 12-500 and 12-600 series carefully for the construction and occupancy conditions, and note any Alberta amendments next to them.

Cables in hollow spaces raise a related issue: combustibility of the cable jacket. In some non-combustible buildings, cables with combustible jackets may need to meet flame-spread ratings or be enclosed, and cables in spaces used for air handling face stricter limits. The Code refers to flame-spread ratings marked on cables, such as FT1, FT4 and FT6, which describe how a cable behaves in a fire test. On an exam, a question that mentions a return air plenum, a high-rise or a non-combustible building is often testing whether the cable's flame-spread rating is suitable.

Priya notes a trap here. A cable type can be permitted by its own series in Section 12 and still be unacceptable because the building code or a flame-spread requirement rules it out. When a question describes the building in detail, the detail is there for a reason: check the construction type, the occupancy and any air-handling spaces before you approve the cable.

Armoured cable (AC90 and ACWU90)

Armoured cable has insulated conductors wrapped in interlocking metal armour, usually aluminum, with a small bare bonding strip or conductor in contact with the armour. Its rules are in the 12-600 series. AC90 is used widely in commercial and some residential work where its mechanical protection is valued, generally in dry locations. ACWU90 adds a moisture-resistant overall jacket and conductors suited to wet locations, which extends its use to wet and underground installations as Table 19 allows.

Two installation details are unique to armoured cable and appear on exams. First, where the armour is cut, an anti-short bushing must be inserted between the conductors and the cut edge so that the sharp armour cannot damage the insulation. Second, the armour and its bonding strip together form the bonding path in many armoured cables, so the connector at each box must be of a type that bonds the armour properly. The rules also set minimum bending radii, support spacing, and the distance from boxes within which the cable must be secured.

TECK90

TECK90 is the industrial workhorse in Canada: insulated conductors with a bonding conductor, inner jacket, interlocking armour and an overall PVC jacket. The overall jacket protects the armour from corrosion and moisture, so TECK90 can be used in many wet, underground and corrosive locations, in cable trays, and in some hazardous locations with the right connectors. Because the armour is not relied on as the only bonding path, the connector must still bond the armour, but the internal bonding conductor carries fault current.

Marcus's industrial background makes TECK90 his default choice, and at Hartley Farms it makes sense for the motor feeders in the dusty dryer room and for the bin yard feeder, where an ACWU90 or TECK90 rated for direct burial avoids digging a conduit trench. The group checks Table 19 for TECK90's conditions of use, the connector requirements for the damp, dusty area, and the bending radius where the cable turns down into the motor control centre.

Mineral-insulated cable and fire-rated circuits

Mineral-insulated (MI) cable has copper conductors embedded in compressed magnesium oxide inside a seamless copper or alloy sheath. It withstands very high temperatures, which makes it useful for circuits that must keep working during a fire, such as some fire pump and emergency circuits, and for high-temperature locations. Its weakness is moisture: the mineral insulation absorbs water from the air if the ends are left unsealed, so each termination needs a sealing pot or a factory seal. Exam questions on MI cable usually test its use for fire-rated circuits, the moisture sealing requirement, or where its bare sheath may be used.

Other cables appear in the Code too, such as flat conductor cable, service entrance cable and cables for specific systems like fire alarm and communications. You do not need to know them all in detail, but you should know that each has its own rules and limits, and that the system sections (such as those for fire alarm and Class 2 circuits) may require specific cable types. When a question names a cable you do not recognise, look it up in Table 19 first.

Cable connectors and terminations

Every cable entering a box or enclosure must be secured with a connector suitable for the cable type and the location. A connector for NMD90 in a dry location is different from a connector for TECK90 in a wet or hazardous location, and the wrong connector is a common deficiency on inspections. Connectors for armoured cables also provide the bonding connection to the box, so their suitability for bonding matters. Where cables enter equipment with factory knockouts, the connector size must match the knockout and the cable.

Denise tells the study group that, in her years as an inspector, connector problems outnumbered almost every other deficiency on commercial jobs: wrong type for the location, missing anti-short bushings on armoured cable, loose connectors that broke the bonding path, and dry-location connectors used outdoors. On the exam, any scenario that mentions a connector is checking whether you know the connector must suit the cable, the location and the bonding role.

Choosing between cable and raceway

The decision between a cable and a raceway system is partly technical and partly economic. Cable is faster to install and cheaper for most runs. Raceway allows conductors to be replaced or added without opening walls, gives better mechanical protection in some forms, and may be required in certain locations or buildings. On the exam, the technical side is what matters: is the chosen method permitted in the location, and does it meet the protection, support and bonding rules? Economic preferences only appear in business questions, which MELC-106 covers.

At Hartley Farms, the final wiring plan uses NMD90 only in the dry office, AC90 in the dry electrical room for small circuits, TECK90 for the motor feeders and the bin yard feeder, rigid PVC for the dusty and damp dryer area branch circuits, and liquid-tight flexible conduit for motor connections. Each choice traces back to a rule the group read in Section 12 and a line in Table 19.