Wiring, Cabling, Raceways and Enclosures
Selection and installation
Conductor selection starts with the environment: temperature rating (RW90, T90 Nylon for dry locations, ACWU90 and Teck90 for wet or direct-buried industrial work), insulation type, voltage rating, and mechanical protection. Ampacity comes from the Code tables and must then be adjusted for ambient temperature and for the number of current-carrying conductors in a raceway or cable. Raceway fill for three or more conductors is limited to 40% of the cross-sectional area (Table 9); a nipple of 600 mm or less may be filled to 60%. Box fill is computed by counting conductors, devices, and grounds against the volume allowance for the box size.
Supporting and protecting the installation
Support intervals, bending radius (Table 11/12), expansion allowance, and protection from physical damage all matter. In hazardous locations, seals, classified enclosures, and wiring methods change dramatically - expect at least one question on seal-off placement. Cable tray fill, derating for stacked trays, and firestopping of penetrations round out the topic.
Traps
Forgetting that the neutral counts as a current-carrying conductor in some circuits but not others; counting a device as two conductors in box fill; applying 60% fill to a full-length run rather than a short nipple; and using the cable's overall ampacity without correcting for bundled cables in a tray or in insulation.
Worked example
A 90 A Table 2 ampacity conductor is installed with eight current-carrying conductors in one raceway at 40 °C ambient. Conductor count adjustment from Table 5C for 7–9 conductors is 70%: 90 × 0.70 = 63 A. Ambient correction (say 0.87 for the insulation class) applies as well: 63 × 0.87 = 54.8 A. The conductor can now carry only about 55 A, so the load and the overcurrent device must be checked against that value - a step candidates routinely skip.