Cables and Installation Practice
Cables and Installation Practice
This domain tests cable construction, current-carrying capacity, derating, voltage drop and correct installation methods for PVC, XLPE, armoured and flexible cables.
Core rules
- Conductor: annealed copper or aluminium. Insulation: PVC (70 °C) or XLPE (90 °C). Sheath: PVC or LSZH. Armour: steel wire or tape for mechanical protection.
- Tabulated current ratings assume a reference ambient (often 30 °C in air), one circuit and a stated installation method. Apply derating factors for ambient temperature, grouping and thermal insulation.
- Voltage drop = mV/A/m × current × length ÷ 1000 for single-phase, giving volts. Keep total drop within about 5% or the supply authority limit.
- Bending radius for fixed PVC cable is commonly 8–12 times the overall diameter; tighter bends crack insulation.
- Conduit fill should not exceed about 40% so cables can be drawn.
- Terminate with correct lugs, crimped and insulated; glands must maintain armour continuity and IP rating.
- Insulation resistance is measured with a 500 V megger before energising.
Worked example
A 2.5 mm² PVC copper cable with a tabulated drop of 18 mV/A/m runs 25 m and carries 16 A. Drop = 18 × 16 × 25 / 1000 = 7.2 V. As a percentage of 230 V: 7.2/230 × 100 = 3.1%, within a 5% allowance. A 40 m run at the same current would give 11.5 V (5.0%), so the cable size would have to increase.
Traps that cost marks
- Using the tabulated rating without derating for a hot roof space or a bundled conduit.
- Forgetting to divide by 1000 when using mV/A/m tables.
- Applying the single-phase formula to a three-phase circuit without the √3 relationship.
- Confusing cable overall diameter with conductor cross-section when checking bend radius.