Conductors and Accessories
This domain tests conductor resistance, current rating, terminations and jointing - the details that decide whether a joint runs cool for twenty years or fails in a year.
Conductor basics
- Resistance R = ρL/A, with ρ ≈ 0.0175 Ω·mm²/m for copper at 20 °C and about 0.028 for aluminium; aluminium therefore needs a larger cross-section for the same drop.
- Current density in house wiring is normally kept around 1.5–3 A/mm². Typical copper PVC single-core ratings in conduit: 1.0 mm² ≈ 11 A, 1.5 mm² ≈ 14 A, 2.5 mm² ≈ 19 A, 4 mm² ≈ 25 A.
- Solid conductors suit fixed wiring; stranded and flexible conductors suit portable appliances and panel doors that move.
- Copper is preferred at terminations. Where aluminium is used, fit bimetallic lugs or use anti-oxidant paste to prevent electrolytic corrosion.
Accessories and terminations
- Cable glands (PG or double-compression) give mechanical grip and IP sealing at panel entry.
- Lugs must match conductor and stud size; crimp with the correct die and pull-test the result.
- Ferrules identify cores; heat-shrink and tape finish the joint; cable markers match the wiring schedule.
- Crimped joints are preferred; Britannia and married joints belong to older installations. Joints must sit in junction boxes, never buried.
Worked example
A single-phase 20 A load is 30 m from the board. For 4 mm² copper, drop = 2ρLI/A = 2 × 0.0175 × 30 × 20 / 4 = 5.25 V, about 2.3% of 230 V, comfortably inside a 4% budget. With 2.5 mm² the drop is 8.4 V (3.65%) - inside the limit but with no margin for the rest of the run, so 4 mm² is the defensible choice.
Traps
- Putting diameter instead of area in mm² into the resistance formula.
- Omitting the factor 2 (go and return) in single-phase voltage drop.
- Undersized lugs and stray strands creating hot spots.
- Aluminium conductor clamped under a plain brass terminal without treatment.