Cable Size Calculator Guide

The voltage drop (Vdrop) for a DC or single-phase AC copper cable is calculated as: Vdrop = 2 × K × L × I / CM, where K = 12.9 Ω·cmil/ft (or 22.5 mΩ·mm²/m for metric), L = one-way length (m), I = load current (A), and CM = circular mils (or mm²). Our calculator uses the more precise metric form: Vdrop = (2 × ρ × L × I) / A, with ρ = 0.0172 Ω·mm²/m (20°C, annealed copper). This assumes sinusoidal steady-state, no skin effect (valid ≤ 300 mm² at 50/60 Hz), and ignores reactance—appropriate for short to medium LV runs per IEC 60364-5-52 and NEC Article 215.2(A)(1) Fine Print Note No. 2.

IEC 60364-5-52 recommends ≤ 3% for lighting circuits and ≤ 5% for other final circuits (e.g., motors, sockets) under normal operating conditions. Similarly, the NEC (Article 210.19(A) FPN No. 4 and 215.2(A)(1) FPN No. 2) treats 3% for branch circuits and 5% for feeder + branch combined as design guidelines—not enforceable ‘requirements’ but essential for efficiency, equipment performance, and compliance with energy codes like ISO 50001. Exceeding these thresholds risks dimming, motor overheating, and non-compliance in audits. Our calculator flags values above 3% (lighting) or 5% (power) per these widely adopted benchmarks.

The value 0.0172 Ω·mm²/m corresponds to annealed copper at 20°C per IEC 60228 and ASTM B3, representing the most common conductor material in general-purpose power cables. Higher values (e.g., 0.0175–0.018) apply to hard-drawn copper or elevated temperatures (e.g., 70°C operating temp raises ρ by ~22%). Our tool defaults to 20°C for conservative baseline sizing; users should manually adjust ρ if ambient or operating temperature exceeds 30°C (using ρT = ρ20[1 + α(T − 20)], where α = 0.00393/°C) — critical for accuracy in hot climates or buried ducts per IEC 60287-2-1.

This calculator models only DC resistance (R = ρL/A) and assumes negligible reactance — valid for single-core or multi-core copper cables ≤ 50 mm² at 50/60 Hz over distances < 100 m, where XL contributes < 5% to total impedance (per IEC 60364-5-52 Annex G). For larger cables (>70 mm²), parallel runs, or long three-phase feeders, AC impedance (including skin and proximity effects) becomes significant. In such cases, engineers must use IEC 60287–1–1 methods or software like CYMCAP. Our tool includes a warning tip advising manual impedance verification when cable size exceeds 120 mm² or length exceeds 200 m.

This calculator is explicitly calibrated for annealed copper (ρ = 0.0172 Ω·mm²/m). Aluminum has ~56% higher resistivity (ρ ≈ 0.0283 Ω·mm²/m at 20°C), so direct input of aluminum cable size will underestimate voltage drop by ~1.65×. To size aluminum correctly: either increase the input size by ~65% (e.g., 16 mm² Cu ≈ 25 mm² Al for equivalent R), or manually scale the output voltage drop by multiplying by 0.0283/0.0172 ≈ 1.65. Always verify against IEC 60228 Class 2/5 stranding tolerances and NEC Table 8 adjustments for aluminum’s lower ampacity and higher thermal expansion.

Installation method does not directly alter voltage drop calculation, since Vdrop depends only on conductor resistance and current—but it critically affects allowable current and operating temperature, which indirectly impact ρ and long-term drop. For example, cables in thermally insulated conduits run hotter → higher ρ → increased drop over time. Per IEC 60287 and NEC Table 310.16, derating factors reduce ampacity, potentially requiring larger conductors to carry the same current without exceeding temperature limits. Thus, while our calculator outputs instantaneous drop at 20°C, engineers must cross-check selected size against ambient, grouping, and installation derating to ensure real-world performance stays within 3–5% limits.

Yes — for a balanced three-phase system, voltage drop is calculated per phase as Vdrop = √3 × K × L × I / A (vs. 2× for single-phase DC/AC), because line-to-line drop relates to phase current via √3 geometry. Our calculator assumes single-phase or DC by default (hence the factor of 2), as stated in the spec ('cable length: one way'). For three-phase, users must either divide the nominal voltage by √3 to get phase voltage and interpret % drop relative to phase, or multiply the output Vdrop by √3/2 ≈ 0.866 to approximate line-to-line drop. IEC 60364-5-52 Annex G clarifies this distinction, and misapplication is a common cause of undersized feeders.

The formula Vdrop ∝ L is mathematically linear — but real-world error compounds nonlinearly due to secondary effects: (1) Longer runs increase likelihood of undetected high-resistance joints (per IEEE 400.2), (2) Ambient temperature gradients become non-uniform, invalidating constant-ρ assumption, and (3) Voltage regulation at the source degrades, causing current to rise to maintain load power (P = V×I), further increasing drop. Per CENELEC TR 50480, field measurements show ±15% typical deviation beyond 150 m due to termination quality alone. Hence, the calculator’s linear model remains valid for design, but engineers must add 10–20% margin for >100 m runs and validate with FLIR thermography or loop impedance testing per IEC 61557-4.