Raceways, Boxes, and Fittings
Why raceway design matters
The Raceways, Boxes, and Fittings domain tests the physical installation: whether cables can be drawn without damage, whether boxes have enough room for conductors and accessories, whether supports keep the system serviceable, and whether the enclosure suits the environment. Good electrical calculations can still fail an inspection because the conduit is overfilled or the box cannot be accessed.
Core rules
- Conduit and trunking fill is limited so cables can be drawn and heat can dissipate. A common design limit is 40 percent of the internal cross-sectional area for three or more current-carrying conductors in a straight run, with lower allowances where bends are tight or the run is long.
- Use the overall diameter of the cable or conduit, not the conductor cross-sectional area, when calculating fill.
- Bends must be swept, not kinked. Minimum bending radius depends on the cable type and diameter; armoured and MICC cables have larger minimum radii than flexible cords.
- Boxes must be accessible, correctly sized for the number and size of conductors and devices, and fitted with lids or plates that maintain the stated IP rating.
- Metal raceways and enclosures need reliable circuit protective bonding, especially where joints, expansion sections or dissimilar metals are involved. Plastic raceways do not provide earth continuity.
- Fire-rated walls, floors and ceilings require tested fire-stopping around penetrations. Cable tray and trunking need support spacing suitable for the loaded weight, plus expansion allowances on long runs.
Worked example
A 25 mm conduit has an internal area of about 490 mm2. At 40 percent fill, the usable area is 196 mm2. A 2.5 mm2 single insulated cable has an overall diameter of about 3.9 mm, so its cross-sectional area is pi x 1.95 squared = 11.9 mm2. The maximum number by area is 196 / 11.9 = 16.4, so 16 cables would be the theoretical limit in a straight run. With two 90-degree bends, the practical limit is lower and the pull tension rises, so a larger conduit or a divided route is the better design. The reasoning must use cable overall area, not the 2.5 mm2 copper area.
Traps that cost marks
Using conductor area instead of cable overall area; treating a 40 percent fill as valid for every bend and length; ignoring IP rating after drilling an enclosure; and relying on a metal lid or gland to provide earth continuity without a dedicated bonding conductor.