Wire Resistance & Voltage Drop Calculator

Understanding the wire resistance and voltage drop calculator — concept and principle

Every cable has some resistance, so part of the supply voltage is lost along the wire before it reaches the load. This calculator estimates the round-trip resistance of a two-wire circuit, the voltage drop, the drop as a percentage of the supply and the power wasted as heat. It helps you size cable for long runs.

Count both wires: 60 m of copper drops 6.60 V, 2.87% of 230 V.

How it is calculated

Resistance: R = ρ_T · 2L / A, with ρ_T = ρ₂₀ · (1 + α · (T − 20))
Voltage drop: ΔV = I · R
Drop (%): ΔV / V × 100
Power loss: P = I² · R

L is the one-way length in metres (doubled because current flows out and back), A the cross-section in mm², I the current in A and V the supply voltage. ρ₂₀ is 0.0172 Ω·mm²/m for copper or 0.0282 for aluminium at 20 °C, and α is 0.00393 /°C (copper) or 0.00403 /°C (aluminium).

The principle

A conductor's resistance grows with its length and falls with its cross-section, because a longer path means more collisions for the electrons and a thicker wire offers more parallel paths. The constant ρ, resistivity, is a material property. Metals become more resistive as they warm, roughly linearly near room temperature, which the α term accounts for.

Worked example

Inputs: L = 30 m, A = 2.5 mm², I = 16 A, V = 230 V, copper, 20 °C.
Temperature factor: 1 + 0.00393 × (20 − 20) = 1, so ρ = 0.0172.
R = 0.0172 × 2 × 30 / 2.5 = 0.0172 × 24 = 0.4128 Ω.
Voltage drop = 16 × 0.4128 = 6.6048 V.
Drop % = 6.6048 / 230 × 100 ≈ 2.87%.
Power loss = 16² × 0.4128 = 105.68 W.

Each point is the calculator re-run with only “Cross-section (mm²)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Cross-section (mm²)	Voltage drop (V)
0.625	26.4192
1.25	13.2096
1.88	8.8064
2.5	6.6048
3.12	5.28384
3.75	4.4032
5	3.3024

How to read the result

Many wiring rules limit voltage drop to about 3–5%. For example, IEC 60364-5-52 suggests 3% for lighting and 5% for other uses in installations supplied directly from a public low-voltage network, and Korea's KEC uses the same figures for that case. At 2.87% this circuit is just within a 3% lighting limit. Because drop is proportional to length and current and inversely proportional to area, doubling the cross-section halves it. At a typical operating temperature of 70 °C, copper's resistance is about 20% higher than at 20 °C, so check the hot condition too.

Common mistakes

Using the one-way length without doubling, which halves the true drop.
Forgetting that three-phase circuits use a different factor (√3 for line-to-line drop on balanced loads).
Ignoring temperature: conductors carrying their rated current run much warmer than 20 °C.

Keep in mind: This is a simplified DC estimate. Cable sizing must also meet current-carrying capacity, protective device and fault-loop requirements of your local electrical code, and installations should be designed and checked by a qualified electrician.

Frequently asked questions

What cross-section halves the voltage drop?

Twice the area. Going from 2.5 mm² to 5 mm² (in practice 6 mm²) cuts the drop to about half or less.

Why does low-voltage wiring suffer more?

The same power at 12 V needs about 19 times the current of 230 V, and the percentage drop grows with current divided by voltage.

Sources: IEC 60364-5-52, Low-voltage electrical installations — Selection and erection of electrical equipment — Wiring systems (Annex G, voltage drop) · IEC 60228, Conductors of insulated cables · 한국전기설비규정(KEC) 232.3.9 수용가 설비에서의 전압강하

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

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