This guide follows and British Standard BS 7671 (which aligns with many other national codes). For US/Canada (NEC), the process is similar but uses different tables (AWG, 60/75/90°C columns, and NEC ampacity tables).
The choice of conductor metal fundamentally shifts the sizing requirements due to differing resistivity ( ) properties. Electrical Conductivity Tensile Strength Sizing Impact Requires smaller cross-sectional area Aluminum Moderate (~61% of copper) calculating cable size
S ≥ √(I² × t) / k Where:
| Load (kW) @ 400V 3-phase | Approx. Cu size | Max length (5% VD) | |--------------------------|----------------|--------------------| | Up to 7.5 kW (15A) | 4mm² | 80m | | 11 kW (22A) | 6mm² | 90m | | 15 kW (30A) | 10mm² | 100m | | 22 kW (42A) | 16mm² | 120m | | 37 kW (70A) | 25mm² | 150m | This guide follows and British Standard BS 7671
The base current rating of the cable ($I_t$) is calculated using the formula: calculating cable size
While Copper is the standard for most internal wiring, Aluminum is common for utility distribution and large feeders due to cost.