Conductors, Cables, and Wiring Methods
Selection logic
The Conductors, Cables, and Wiring Methods domain tests whether you can choose a conductor material, insulation, sheath and installation method that survive the environment and carry the load after every relevant correction factor has been applied. The key sequence is design current, protective device rating, tabulated current-carrying capacity, then derating for ambient temperature, grouping, thermal insulation and installation method.
Core rules
- PVC insulation is generally rated 70 degrees C and XLPE 90 degrees C. The higher rating is useful only if terminals, accessories and joints are also rated for it.
- The tabulated current-carrying capacity must be adjusted for actual conditions. Typical correction factors include ambient temperature, grouping with other loaded circuits, thermal insulation, buried depth and soil thermal resistivity.
- The protective device rating must not exceed the corrected cable capacity, and the design current must not exceed the device rating.
- Minimum cross-sectional area, bending radius, support spacing and sheath suitability are separate checks from current capacity. Armoured cable suits mechanical risk; MICC suits fire resistance and high temperature.
- Fire performance, low smoke and acid gas behaviour matter in escape routes, tunnels and crowded buildings. Do not substitute ordinary PVC cable where a fire-rated circuit is specified.
Worked example
A 4 mm2 PVC twin-and-earth cable clipped direct has a tabulated capacity of 37 A. It runs in a 40 degrees C ambient with four other loaded circuits, giving Ca = 0.87 and Cg = 0.80. The design current is 24 A. Corrected capacity = 37 x 0.87 x 0.80 = 25.75 A. A 25 A MCB satisfies In is greater than or equal to Ib (25 is greater than or equal to 24) and In is less than or equal to Iz (25 is less than or equal to 25.75). Voltage drop must still be checked: at 11 mV/A/m and 20 m, Vdrop = 11 x 24 x 20 / 1000 = 5.28 V, which is 2.3 percent of 230 V and acceptable for a power circuit. If the grouped circuits were fully loaded and the ambient were higher, the 4 mm2 cable would fail and a 6 mm2 cable would be required.
Traps that cost marks
Applying derating factors to the breaker rather than the cable; using the tabulated capacity without checking the installation method; ignoring harmonic current in shared neutrals; and assuming a 90 degrees C cable can be connected to 70 degrees C terminals without consequence.