Voltage Drop Calculator – Copper Wire

Q: What is the voltage drop formula for copper wire?

For single phase copper: VD = (2 × L × I × ρ_cu) ÷ A. Where ρ_cu = 1.72×10⁻⁸ Ω·m (copper resistivity), L = one-way length (m), I = current (A), A = cross-section (m²). As a percentage: VD% = VD ÷ V × 100.

Q: Why is copper preferred over aluminium for wiring?

Copper has about 40% lower resistivity than aluminium, meaning smaller conductors for the same current and voltage drop. Copper is also more flexible, easier to terminate, and doesn't corrode as readily at connections. For the same cross-section, copper carries more current and has less voltage drop.

Q: What is the maximum acceptable voltage drop for copper wiring?

Most standards allow 3% for lighting circuits and 5% for power circuits from supply point to final outlet. NEC allows 3% for branch circuits and 5% combined (feeder + branch). IEC 60364 recommends 4% maximum for industrial, 3% for lighting.

Copper is the default conductor for most fixed wiring — lower resistivity than aluminum means less drop for the same cross-section. Use this calculator to check whether your cable run meets the 3–5% voltage drop limit required by NEC, IEC 60364, and AS/NZS 3000. Covers single phase and three phase circuits in mm².

This calculator provides estimates only. Always verify against local wiring regulations and consult a qualified electrician.

Calculate Voltage Drop (Copper)

Current (Amps)

One-Way Length (metres)

Wire Size (mm²)

Supply Voltage (V)

Circuit Type

Material

Voltage Drop Results

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Voltage Drop (V)

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Drop (%)

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Voltage at Load (V)

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Status

Formulas

Single phase copper:

VD (V) = 2 × L × I × R_per_metre
VD% = VD ÷ V × 100

Three phase copper:

VD (V) = √3 × L × I × R_per_metre

Copper resistivity ≈ 1.72 × 10⁻⁸ Ω·m (at 20°C). Resistance increases with temperature — at 75°C operating temperature, multiply by approximately 1.25.

Copper Wire Resistance Reference

Wire Size (mm²)	Resistance (mΩ/m)	Ampacity (A)
1.5 mm²	12.1	16 A
2.5 mm²	7.41	25 A
4 mm²	4.61	32 A
6 mm²	3.08	41 A
10 mm²	1.83	57 A
16 mm²	1.15	76 A
25 mm²	0.727	101 A
35 mm²	0.524	125 A

Frequently Asked Questions

What is the voltage drop formula for copper wire?

Single phase: VD = 2 × L × I × R (factor of 2 for return conductor). Three phase: VD = √3 × L × I × R. Where R is resistance per metre in Ω/m. For 2.5mm² copper at 25A over 40 metres single phase: VD = 2 × 40 × 25 × 0.00741 = 14.82V. At 230V: 14.82 ÷ 230 × 100 = 6.4% — too high, upgrade to 4mm² or 6mm².

Why is copper preferred over aluminium for wiring?

Copper resistivity is about 1.72 μΩ·cm vs aluminium at 2.82 μΩ·cm — copper conducts 64% better per unit cross-section. A 2.5mm² copper cable has the same resistance as a 4mm² aluminium cable. Copper is also easier to terminate (aluminium requires anti-oxidant compound and special lugs to prevent connection failure from oxide layer formation).

What is the maximum acceptable voltage drop for copper wiring?

IEC 60364 recommends maximum 4% for most circuits, 3% for lighting. NEC suggests 3% for branch circuits and 5% cumulative (feeder + branch). For sensitive equipment (computers, medical devices, variable speed drives), keep drop under 2%. Higher voltage drop causes equipment to draw more current to compensate, generating heat and potentially shortening equipment life.