Cables and Conductors
Conductor material and sizing
Copper is standard; aluminium needs larger cross-section and careful termination. Cross-sectional area is chosen so that:
- It carries the design current after derating (Ib ≤ In ≤ Iz).
- Voltage drop is within limits (commonly 3% lighting, 5% other for a public supply - check the applicable value in your edition).
- It withstands short-circuit energy.
- The protective conductor is adequately sized.
- It is mechanically robust (minimum sizes apply).
Voltage drop
Voltage drop = (mV/A/m × Ib × L) / 1000, using tabulated values. Traps: using the wrong tabulated column (three-phase vs single-phase), forgetting to include the neutral on a heavily loaded circuit, and applying voltage drop to the design current, not the device rating.
Insulation and sheath
Know the common cable types (thermoplastic vs thermosetting, armoured, flexible) and their temperature ratings, because the tabulated Iz depends on the insulation. A cable with a higher temperature rating can carry more current but may not suit terminations rated lower.
Neutral and protective conductors
Neutral must be sized for harmonic current where non-linear loads are significant. Protective conductor sizing follows the adiabatic equation S = √(I²t)/k, with a minimum size table and a rule that the CPC need not exceed the live conductor size (for conductors above a threshold).
Worked example
A 230 V single-phase circuit, Ib = 25 A, length 30 m, using a cable with tabulated drop 18 mV/A/m: drop = (18 × 25 × 30)/1000 = 13.5 V = 5.9% - exceeds a 5% limit, so upsize the conductor.