Base ampacity tables (found in standards like IEC 60364-5-52 or NEC Table 310.16) assume specific reference conditions (e.g., 30°C ambient, single cable in air). Real-world installations rarely match these ideals. Derating factors are applied to lower the allowable current:
If the heat generated exceeds the heat dissipated, the conductor temperature rises. The calculation ensures this temperature remains below the limit of the insulation material (e.g., 70°C for PVC, 90°C for XLPE). cable calculation size
A less frequent but more violent design condition is the short circuit. When a bolted fault occurs, the current can rise to thousands of amperes for a brief period (milliseconds until a protective device operates). During this interval, the cable undergoes near-adiabatic heating—meaning the heat generated has no time to dissipate. If the conductor is too small, its temperature will exceed the insulation’s short-circuit rating (e.g., 160°C for PVC, 250°C for XLPE), causing immediate breakdown. Base ampacity tables (found in standards like IEC
Cable sizing is a balance between safety and cost. While "oversizing" a cable is generally safe, it is expensive and difficult to install. "Undersizing," however, is a hazard. Always consult local wiring regulations (such as BS 7671 or the NEC) and, when in doubt, consult a qualified electrical engineer. The calculation ensures this temperature remains below the
Where:
Is the cable in a hot loft or a cool basement? Grouping: Are there ten other cables bundled next to it?