Copper Conductor Cable Sizing — Method
Copper is the default conductor material for cables up to ~120 mm² in branch circuits, sub-mains and most industrial wiring. It has lower resistivity (16.78 nΩ·m vs 26.5 nΩ·m for Al), better creep resistance at terminations and longer service life. The trade-off is cost and weight — for cables ≥ 95 mm² and long runs, aluminium becomes economically competitive even with the larger cross-section it needs.
Where:
- Ib — design current of the circuit (A), from the load calculation
- Ca — ambient temperature correction (1.00 at 30 °C reference)
- Cg — grouping / bunching factor (1.00 for a single circuit)
- Ci — thermal-insulation factor (1.00 if the cable is in free air; 0.50 if fully buried in insulation)
Then pick the smallest cable cross-section in IEC 60364-5-52 Table B.52.4 (copper, XLPE, Reference Method B1) whose tabulated ampacity Iz ≥ It.
Related cable sizing calculators
Other standard- and method-specific cable-sizing calculators in the same series — same procedure, different reference tables and defaults:
- Cable Sizing Calculator (universal) — the seed page covering all standards in one tool
- Aluminium Cable Sizing Calculator — Aluminium · Al / AAC / AAAC
- IEC 60364 Cable Sizing Calculator — IEC 60364-5-52 · International
- NEC Cable Sizing Calculator — NEC NFPA 70 · USA
- All Electrical Engineering Calculators →
Frequently Asked Questions
Below ~50 mm² aluminium becomes mechanically fragile, harder to terminate reliably, and the size penalty (~1.5× larger Cu cross-section for the same current) costs more in conduit and labour than the copper saves in metal. Most national codes (NEC 110.14, BS 7671 526) require AL-rated terminations and anti-oxidant compound, adding labour cost. For cables under 50 mm² copper is almost always the lower total-cost choice.
Aluminium has roughly 60–65 % the ampacity of copper for the same cross-section. Examples (IEC 60364-5-52 Method B1 XLPE): 16 mm² Cu = 85 A, 16 mm² Al = 67 A; 50 mm² Cu = 168 A, 50 mm² Al = 132 A. To match the same current as Cu, aluminium typically needs the next or next-but-one IEC standard size (e.g. 50 mm² Cu ≈ 70 mm² Al).
At 20 °C, hard-drawn copper has resistivity 17.24 nΩ·m, giving a per-metre resistance: 1.5 mm² = 12.1 mΩ/m, 2.5 mm² = 7.41 mΩ/m, 6 mm² = 3.08 mΩ/m, 16 mm² = 1.15 mΩ/m, 50 mm² = 0.387 mΩ/m. At 70 °C operating temperature, multiply by 1.20 (the temperature coefficient). Reactance for single-core XLPE is typically 0.08–0.10 mΩ/m.
No — Cu and Al follow the same Cg grouping factors from IEC Table B.52.17 (0.80 for 3 circuits, 0.70 for 6 circuits, etc.). The thermal model for grouping derating depends on insulation and air gap, not conductor material. The Iz starting point is different for Cu vs Al, but the multipliers are identical.
Only with bi-metallic compression connectors (CuAl, listed for the application) and inhibitor compound. The dissimilar-metal galvanic risk and the differential thermal-expansion creep make a direct Cu-Al connection a top cause of overheating fires (NEC 110.14, BS 7671 526.2). Cu-only or Al-only joints with the correct CSA are always preferable.
Per IEC 60364-5-52 Table B.52.4 (XLPE/EPR, 30 °C, no grouping): 25 mm² Cu carries 99 A in Method A1, 112 A in B1, 119 A in C, 121 A in D1. PVC (70 °C) values are ~10 % lower. After typical derating (Ca 0.87 × Cg 0.80) the working ampacity drops to roughly 78 A in B1 — adequate for a 63 A protective device.