The ambient temperature also affects the cable size. A higher ambient temperature requires a larger cable size to prevent overheating.
Cable sizing is not arbitrary; it must comply with local and international regulations.
Example : 2.5 mm² PVC copper (30 A tabulated), ambient 45°C (k₁=0.79), 4 circuits (k₂=0.65) → effective = 30×0.79×0.65 = 15.4 A. For a 16 A load, this cable fails. Increase to 4 mm².
Example : Isc = 3 kA, t = 0.1 s (breaker trip), Cu/XLPE, k=143. $S_min = \sqrt(3000^2 × 0.1) / 143 = \sqrt900,000 / 143 = 948 / 143 = 6.6 mm²$. Minimum = 6.6 → choose 10 mm² (next standard size). how to size a cable
Sizing an electrical cable correctly is critical for the safety, efficiency, and longevity of any electrical system. An undersized cable can overheat, causing insulation damage, fire hazards, and voltage drops that impair equipment performance. An oversized cable results in unnecessary cost and installation difficulties. This report outlines the standard methodology for determining the appropriate cable cross-sectional area based on current carrying capacity, voltage drop, and short-circuit rating.
The first step is calculating the full-load current ($I_b$).
| Copper, XLPE, 90°C, 30°C ambient, free air | 1.5 mm² → 24 A | 2.5 mm² → 32 A | 4 mm² → 42 A | | Aluminum, PVC, 70°C, buried | 16 mm² → 70 A | etc. | The ambient temperature also affects the cable size
Sizing a cable for a three-phase motor.
Based on this calculation, a 10mm² PVC cable would be suitable for this application.
Formula (for non-PVC): $k_1 = \sqrt\frac\theta_max - \theta_amb\theta_max - 30$, where θ in °C. Example : 2
$$K = k_1 \times k_2 \times k_3$$
$$S = \frac\sqrtI^2 \times tK_sc$$