Voltage Drop & Wire Size Calculator

Voltage drop and % for DC (24 V supplies), single- and three-phase from current, distance and wire size (mm² or AWG), plus minimum size and maximum length.

CircuitSupply voltage (V)Current (A)One-way length (m)

Wire sizeCross-section (mm²)ConductorConductor temperature (°C)Allowed drop (%)

Results
Resistance of one conductor
Voltage drop
Percent drop
Voltage at the load
Power lost in the cable
Minimum cross-section for the allowed drop
Maximum one-way length with this size

Results update as you type. Everything runs in your browser; nothing you type is sent anywhere.

Formulas

Conductor resistance R = ρ × L ÷ A (ρ: resistivity Ω·mm²/m, L: one-way m, A: mm²)
DC and single-phase 2-wire: ΔV = 2 × I × R (out and back)
Three-phase 3-wire (balanced, line-to-line): ΔV = √3 × I × R
Temperature: ρ(T) = ρ20 × (1 + α × (T − 20))
Percent drop = ΔV ÷ supply voltage × 100

Copper ρ20 = 1/58 ≈ 0.01724 Ω·mm²/m, α = 0.00393 /°C (IEC 60028). Aluminium ρ20 = 0.028264, α = 0.00403 (IEC 60889). Resistance per length scales with ρ, so the same formulas work in feet once you convert lengths to metres.

What this does not cover

Reactance and power factor: only conductor resistance is counted. For large AC cables the reactive part grows and the real drop can be higher.
Ampacity: a wire must also carry the current safely, which depends on installation method, ambient temperature and grouping. Use your local code's tables (NEC, IEC 60364-5-52, …).
Real cable resistance: IEC 60228 sets maximum resistance per conductor class (solid, stranded, flexible); stranded and tinned conductors are slightly higher for the same mm². Check the cable maker's data for exact values.
How much drop is acceptable: that depends on the load's input voltage range and local rules. For 24 V devices, comparing «Voltage at the load» with the device's minimum input voltage is the surest check.

Resistance by mm² (copper, 20 °C, calculated)

Cross-section	Ω/km (one conductor)	Drop per 1 A per 100 m (out and back)
0.5 mm²	34.48	6.897 V
0.75 mm²	22.99	4.598 V
1 mm²	17.24	3.448 V
1.5 mm²	11.49	2.299 V
2.5 mm²	6.897	1.379 V
4 mm²	4.310	0.862 V
6 mm²	2.874	0.575 V
10 mm²	1.724	0.345 V
16 mm²	1.078	0.216 V
25 mm²	0.690	0.138 V
35 mm²	0.493	0.099 V
50 mm²	0.345	0.069 V
70 mm²	0.246	0.049 V
95 mm²	0.181	0.036 V
120 mm²	0.144	0.029 V
150 mm²	0.115	0.023 V
185 mm²	0.093	0.019 V
240 mm²	0.072	0.014 V
300 mm²	0.057	0.011 V

AWG ↔ mm² (ASTM B258)

AWG	Diameter mm	Area mm²	Ω/km (copper 20 °C)
30 AWG	0.255	0.051	338.6
28 AWG	0.321	0.081	212.9
26 AWG	0.405	0.129	133.9
24 AWG	0.511	0.205	84.22
22 AWG	0.644	0.326	52.96
20 AWG	0.812	0.518	33.31
18 AWG	1.024	0.823	20.95
16 AWG	1.291	1.309	13.17
14 AWG	1.628	2.081	8.286
12 AWG	2.053	3.309	5.211
10 AWG	2.588	5.261	3.277
8 AWG	3.264	8.366	2.061
6 AWG	4.115	13.302	1.296
4 AWG	5.189	21.151	0.815
2 AWG	6.544	33.631	0.513
1 AWG	7.348	42.408	0.407
0 AWG	8.251	53.475	0.322

FAQ

How much does 24 V DC drop over 50 m?

2 A over 1.5 mm² copper, 50 m one way: the 100 m loop is about 1.15 Ω, so about 2.3 V (9.6 %) is lost and the load sees about 21.7 V. With 2.5 mm² it is about 1.38 V (5.7 %).

Do I enter the round-trip or one-way length?

One way — the cable length from supply to load. For DC and single-phase the calculation counts both conductors itself.

How do AWG and mm² convert?

ASTM B258: d = 0.127 × 92^((36 − AWG) ÷ 39) mm and area = π ÷ 4 × d². 18 AWG ≈ 0.82 mm², 16 AWG ≈ 1.31 mm², 14 AWG ≈ 2.08 mm².

For signal wiring with a tight supply budget, such as 4–20 mA loops, use the 4–20 mA Loop Load Calculator to check the voltage left at the transmitter.