Cable Sizing FAQ

Use the motor’s full-load current from the nameplate, not calculated current. Apply a 1.25 multiplier for continuous duty per IEC 60364. Select cable size from ampacity tables for your installation method, then verify voltage drop stays below 5% and the cable withstands the motor starting current duration using the adiabatic equation.

PVC cables are rated 70°C continuous; XLPE cables are rated 90°C. The higher temperature limit means XLPE cables carry approximately 15–20% more current for the same conductor size. XLPE also has better short-circuit withstand (250°C vs 160°C limit) and lower dielectric losses. For long runs or high ambient temperatures, XLPE is often more economical despite higher purchase cost.

Cable length does not affect current-carrying capacity directly but critically affects voltage drop. Longer cables have higher impedance, causing greater voltage drop under load. If the calculated voltage drop exceeds the standard limit (typically 3–5%), a larger cable size must be selected even though the smaller size has adequate current capacity. Always check both criteria.

A continuous load operates at maximum current for three hours or more. Under NEC, multiply the continuous load current by 1.25 to determine the minimum conductor ampacity and overcurrent device rating. Under BS 7671 and IEC, cables are already rated for continuous duty, so no additional factor is needed unless the protective device is not rated for 100% continuous loading.

BS 7671 specifies 2.5mm² copper conductors for ring final circuits serving 13A socket outlets, protected by a 32A device. The ring arrangement means current splits between two paths, so each leg carries approximately half the total load. A 2.5mm² cable in Reference Method C carries 27A per leg — adequate for the divided current.

Yes, aluminium conductors are widely used and often more economical for larger sizes. Aluminium has approximately 61% of copper’s conductivity, so aluminium cables require larger cross-sections for the same current rating — typically 1.5 to 2 sizes larger. Special termination techniques are needed to prevent galvanic corrosion and accommodate aluminium’s thermal expansion.

Higher ambient temperatures reduce the temperature differential available for heat dissipation from the cable. At 40°C ambient with PVC insulation, the correction factor is approximately 0.87, reducing capacity to 87% of tabulated values. At 50°C, it drops to about 0.71. These factors multiply with other derating factors, potentially requiring significantly larger cables.

The grouping factor reduces cable capacity when multiple loaded circuits share close proximity. For three circuits bunched together, the factor is approximately 0.70, meaning each cable carries only 70% of its single-circuit capacity due to mutual heating. Apply it whenever cables are in the same conduit, trunking, tray, or clipped together.

PV string cables must carry 1.25 times the short-circuit current continuously. Apply derating for elevated roof temperatures (often 60°C+), which severely reduces capacity compared to standard 30°C ratings. Voltage drop should be below 1–2% to minimise energy losses. Use double-insulated UV-resistant cables rated for the string voltage, typically up to 1500V DC.

A 7.4kW (32A) Mode 3 EV charger requires at minimum 6mm² copper cable for short runs under BS 7671. The circuit is continuous duty — no diversity applies to a single charger. A 6mm² cable in Reference Method C carries 46A, providing adequate margin. For longer runs, voltage drop may require 10mm² cable. Type A RCD protection at 30mA is mandatory.

After selecting cable size for current capacity and voltage drop, check the adiabatic equation: k²S² must be greater than or equal to I²t. The k value depends on conductor and insulation material (115 for copper/PVC, 143 for copper/XLPE). S is the cross-sectional area in mm², I is the fault current in amperes, and t is the protective device clearing time in seconds.