The two constraints that decide cable size
Cable sizing per IEC 60364-5-52 is a dual-constraint problem. The cable must be large enough to carry the design current without overheating its insulation, AND it must be large enough that the voltage drop across its length stays within the regulated maximum. Most spreadsheet calculators handle one or the other. Picking the larger of the two is the only correct answer — and which one bites depends on the run length, the power factor, and how aggressively the site has been derated.
Ampacity, then derated ampacity
The IEC 60364-5-52 Method C table lists base ampacity for each standard mm² size, copper, single-core, clipped direct, in still air at 30 °C. Two corrections derate that base figure: Ca for ambient temperature (sub-30 °C boosts ampacity, hot sites cut it sharply — at 50 °C, PVC is at 71% of its 30 °C rating), and Cg for grouping (touching cables in the same conduit derate each other — 3 circuits cut ampacity to 70%, 6 circuits to 57%). The required ampacity from the table is the design current divided by (Ca × Cg). For aluminium, multiply the copper ampacity by 0.78.
Voltage drop: the second constraint
Voltage drop scales linearly with cable length and current, and inversely with cross-section. For cables under 25 mm², resistance dominates and the simplified formula Vd = k × ρ × L × I × cos φ / S gives the right answer. Above 25 mm² (and above 16 mm² when power factor drops below 0.9), reactance becomes a meaningful share of the total and the formula expands to include the X sin φ term. National codes set the limits: 5% from origin to load is the IEC baseline, 2.5% for DEWA final circuits, 3% is common practice for sub-mains and feeders.
Why the regional defaults matter
Indoor ambient in a chilled office in northern Europe is 20–25 °C; outdoor in summer in the Gulf is 45–50 °C. Sizing the same load for both sites gives different answers — and a cable that's ampacity-correct in a temperate site can be one or two size steps short when installed in a hot region. The region defaults in this calculator preset Ca and the voltage- drop limit to the typical values for KSA, UAE, and Egypt, and fall back to a generic IEC baseline (30 °C ambient, 5% drop limit) for everywhere else. Override the defaults whenever your site differs.
Standards behind the math
Ampacity values from IEC 60364-5-52 Ed. 3.1 (2024-11) Method C, copper, single-core in air. Aluminium ampacity is the IEC convention of 0.78 × copper. Ca tables from IEC 60364-5-52 Table B.52.14; Cg from Table B.52.17 (touching, single layer). Resistivity from IEC 60228 with the standard temperature coefficient applied at the insulation rating (70 °C PVC, 90 °C XLPE). Reactance simplified to 0.08 mΩ/m, the Schneider EIG cross-vendor value for typical LV cable.