Cable Sizing: Ampacity, Voltage Drop, and Short-Circuit Withstand

Priya Menon, MIEEE12 Jul 20260 comments2652 views

Three tests must all pass: continuous ampacity with derating, voltage drop within regulation limit, and short-circuit thermal withstand.

Ampacity per IEC 60364-5-52

Apply factors for grouping (Cg), ambient temperature (Ct), and installation method. A 4-core 95 mm² XLPE Cu cable in a tray with 4 circuits: I_base = 250 A × 0.75 × 0.94 = 176 A.

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Voltage Drop

Rule of thumb: ΔV% ≤ 3% for final circuits, ≤ 5% total from origin. Formula: ΔV = √3·I·L·(R·cosφ + X·sinφ).

Short-Circuit Withstand

Adiabatic equation: S = √(I²·t)/k, where k = 143 for Cu-XLPE. If your protection clears in 100 ms at 20 kA, minimum cross-section = 44 mm².

cable sizing ampacity voltage drop

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