Electrical Materials and Wiring
Electrical Materials and Wiring tests conductor and insulation selection, wiring system choice, colour code and conduit practice for domestic and workshop installations.
Core rules
- Copper resistivity is about 0.0175 Ω·mm²/m; aluminium is about 0.028 Ω·mm²/m, so aluminium needs a larger area for the same voltage drop.
- PVC insulation is rated about 70 °C, XLPE about 90 °C; derate cables for ambient temperature, grouping and enclosure.
- Common wiring systems: cleat, casing-and-capping, batten, conduit (surface or concealed) and cable tray. Concealed conduit must run in straight lines, with inspection boxes at bends.
- IS 732 colour code for three-phase: red, yellow and blue for phases; black for neutral; green-with-yellow for protective earth. Older installations may use different phase colours, so test before working.
- Conduit fill should not exceed about 40% of the internal area; wires must be drawn without damage.
Worked example
A 16 A load is 18 m from the distribution board, wired in 2.5 mm² copper, two wires. Resistance per conductor = 0.0175 × 18 / 2.5 = 0.126 Ω. Loop resistance = 0.252 Ω. Voltage drop = 16 × 0.252 = 4.03 V, which is 1.75% of 230 V. If the same load used 2.5 mm² aluminium, drop = 16 × 2 × 0.028 × 18 / 2.5 = 6.45 V (2.8%), so a 4 mm² aluminium conductor would be selected to keep drop near 2%.
Traps
The most common error is using one-way length; the current goes out and back. Do not mix mm² with wire gauge, and do not assume the earth wire can be smaller on a 2.5 mm² circuit: for phase conductors up to 16 mm², the protective conductor is normally the same size.