Wiring methods, boxes and raceways

Wiring methods are the physical means of carrying conductors through a building: cables, raceways, cable trays and busways, together with the boxes and fittings that join them. Exam questions on wiring methods test where each method may and may not be used, how it must be supported and protected, how many conductors fit in a box or raceway, how many bends are allowed between pull points, and how methods change in wet, corrosive, buried and fire-rated locations. Calculation questions on box fill and conduit fill are common and reward a careful counting method. This lesson reviews the main wiring methods and their uses, support and protection, boxes and box fill, raceway fill and bends, underground installations, and fire separations.

Choosing a wiring method

Every wiring method has a defined scope: the occupancies, locations and conditions where it is permitted, and those where it is prohibited. Non-metallic sheathed cable is the standard method in houses but is restricted in other building types and prohibited in some locations; armored and metal-clad cables are flexible and widely used in commercial work; rigid metal conduit gives the best mechanical protection; electrical metallic tubing (EMT) is the common commercial raceway in dry indoor locations; rigid PVC conduit resists corrosion and is common underground and in wet locations; and flexible conduits connect vibrating equipment. Exam questions often describe a location, such as a parking garage, a farm building, a hazardous area or a wet location, and ask which method is permitted.

Support and protection

Cables and raceways must be secured and supported at intervals and near boxes, so that they do not sag, pull out of connectors or transfer strain to terminations. Cables run through framing must be protected from nails and screws, typically by keeping them back from the edge of studs and joists or by installing steel plates. Cables and raceways exposed to physical damage need extra protection or a more robust method. Conductors entering boxes and enclosures through openings must be protected from sharp edges with bushings or connectors.

For example, NM cable is generally secured within 300 mm (12 in) of every box or fitting and at intervals not exceeding 1.4 m (4.5 ft), and cable through bored holes within 32 mm (1-1/4 in) of the edge of framing must be protected by a steel plate at least 1.6 mm (1/16 in) thick. EMT must be secured within 900 mm (3 ft) of each box and at least every 3 m (10 ft).

Boxes and box fill

Every splice, switch, receptacle and luminaire connection must be in a box or enclosure that remains accessible, and the box must be large enough for the conductors and devices it contains. Overfilled boxes cause damaged insulation, loose connections and overheating. Box fill rules assign a volume to each conductor according to its size, and count devices, clamps and other items as equivalent conductors.

In the NEC, 314.16 sets box fill. Table 314.16(A) gives volumes and maximum conductor counts for standard boxes, and Table 314.16(B) gives the volume allowance per conductor: 2.00 in³ for 14 AWG, 2.25 in³ for 12 AWG, 2.50 in³ for 10 AWG, 3.00 in³ for 8 AWG and 5.00 in³ for 6 AWG. Each conductor originating outside the box counts once; conductors that do not leave the box, such as pigtails, are not counted; all equipment grounding conductors together count as one, at the size of the largest; internal cable clamps together count as one; each device or yoke counts as two conductors of the size connected to it; and luminaire studs and hickeys count as one each.

Raceway fill

Raceway fill limits make conductors possible to pull without damaging insulation and allow heat to escape. For conductors of different sizes, the total cross-sectional area of the conductors, including insulation, is compared with a percentage of the raceway's internal area: 53 % for one conductor, 31 % for two and 40 % for three or more. For conductors all of the same size, tables in the code give the maximum number of conductors per raceway size directly, which is faster on exams. Conductor areas depend on insulation type and thickness, so a raceway that holds four conductors of one type may not hold four of another type with thicker insulation.

In the NEC, Chapter 9 Table 1 gives the maximum fill percentages, Table 4 gives internal areas of raceways, Table 5 gives conductor dimensions by insulation type, and Annex C gives maximum numbers of conductors of the same size for each raceway type and size.

Bends and pull points

Pulling conductors through conduit becomes harder with every bend, because tension multiplies at each change of direction. Codes therefore limit the bends between pull points, such as boxes, conduit bodies and fittings, to the equivalent of four quarter bends, a total of 360°, including offsets and kicks. Exam questions may describe a run with a 90° bend, two 45° offsets and a 30° kick, and ask whether a pull point is needed. Bends must be made without kinking or reducing the internal diameter, within minimum radius limits, and conductors must not be installed in a raceway until it is complete between pull points.

Underground installations

Underground wiring must be buried deep enough to avoid damage from digging and traffic, with depth depending on the wiring method, the voltage, the location (such as under a driveway, a road, a building slab or a lawn) and whether additional protection such as concrete encasement or a protective board is provided. Direct-buried cables need a cover of sand or screened fill to avoid sharp stones, and many installations require warning tape above the cable. Where conductors leave the ground and rise up a building or pole, they must be protected against damage to a specified height. Raceways entering buildings from underground must be sealed to prevent water and gas from entering, and allowance must be made for frost heave and settlement with expansion fittings.

In the NEC, Table 300.5 gives minimum cover requirements for 0 to 1,000 V, by wiring method and location, for example 600 mm (24 in) for direct-buried cables in general, 450 mm (18 in) for rigid nonmetallic conduit and 150 mm (6 in) for rigid metal conduit, with different values under driveways, buildings and streets, and for GFCI-protected residential branch circuits.

Fire separations and plenums

Wiring that passes through fire-rated walls, floors and ceilings must not reduce the fire resistance of the assembly. Penetrations are sealed with listed firestop systems that match the wiring method and the assembly rating. Spaces used for environmental air, such as ceiling plenums and air-handling spaces, are restricted to wiring methods that will not spread fire or produce excessive smoke, such as metal raceways or cables with low-smoke, flame-resistant jackets rated for plenum use. These requirements link the electrical code with the building code, and exam questions may ask which cable types are permitted in a return-air plenum or how penetrations must be treated.

Cable trays and busways

Cable trays support cables in industrial and large commercial installations, allowing large numbers of cables to be installed and changed easily. Only cables rated for tray use may be installed, and fill rules depend on cable sizes and tray type. Ampacities of cables in trays have their own rules because heat dissipation depends on spacing and covers. Busways, prefabricated assemblies of bus bars in an enclosure, carry large currents along buildings with plug-in or bolt-on taps for loads; they have rules about where they may run, how they are supported and how taps are protected.

Conductors in parallel raceways and cables

Large feeders often use several raceways in parallel, each carrying a complete set of phase and neutral conductors. Each raceway is sized for its own set, and the grouping factors apply to the conductors within each raceway. Using one raceway per phase is not permitted for AC circuits in metal raceways, because the unbalanced magnetic field heats the raceway by induction. The same principle requires all conductors of a circuit to be in the same cable, raceway or trench, so that their magnetic fields cancel.

Expansion, drainage and corrosion

Raceways must allow for thermal expansion, particularly rigid PVC conduit, which expands about five times as much as steel for the same temperature change; long outdoor runs need expansion fittings. Raceways that may collect condensation, such as those passing from warm to cold spaces, must be arranged to drain, sealed against air movement, or both. Metal raceways in corrosive locations, such as farm buildings, swimming pool equipment rooms and chemical plants, need corrosion-resistant materials or coatings. Questions may describe a situation and ask which material or fitting is required.

Boxes for luminaires and fans

Boxes that support luminaires and ceiling fans must be rated for the load. Boxes for ceiling fans must be listed for fan support, because fans produce vibration and dynamic loads, and the box must be supported directly by the building structure. Heavy luminaires above a stated weight must be supported independently of the box. Boxes in walls must be set flush with or slightly behind combustible and non-combustible finishes, within limits, and gaps around boxes in drywall must be repaired. Exam questions on these points often test the weight thresholds and the setback distances.

Raceway seals and pulling

Conductors must be pulled into raceways without exceeding their allowable pulling tension and sidewall pressure, using lubricants approved for the insulation. Long or complex pulls are planned using calculations of pulling tension, which grows exponentially with each bend. Raceways are cleaned and inspected before pulling, and conductors are protected from abrasion at the raceway entrance. These practices are examined as common occupational skills.

Exam strategy for wiring method questions

Wiring method questions are often answered fastest by going to the article or rules for the specific method mentioned, which typically list permitted uses, prohibited uses, support, bending and fill in a consistent order. Learn this common structure, because it lets you jump to the right part of any wiring method's rules without reading from the start. When a question asks which method is permitted for a location, check both the method's own rules and any rules for the location itself, such as hazardous, wet, corrosive or fire-rated areas, since either can exclude a method.