Voltage Drop Calculator — pick the gauge you need

Voltage lost over a cable run, as volts and as a percentage, with a recommended gauge.

Example: Carrying 10 A over 10 m at 12 V within 3% needs AWG 7 or thicker — giving 318.5 mV of drop.

Inputs

Load current(A)

Run length (one way)(m)

The calculation doubles this — current has to return.

Supply voltage(V)

Max acceptable drop(%)

3% is the usual design limit.

Conductor

Result

Minimum gauge

AWG 7

Actual drop: 318.5 mV
Allowed drop: 360 mV
Voltage at load: 11.68 V
Loop resistance: 31.85 mΩ
Cross-section: 10.55 mm²
Power lost in cable: 3.185 W

At 12 V, 3% is only 360 mV. Low-voltage runs need thick cable for exactly this reason — the same percentage at 240 V would allow 7.2 V.
3.185 W is lost heating the cable — 2.7% of the delivered power.
Ampacity figures here are general guidance, not a code-compliant rating. For installed wiring, consult NEC, IEC 60364 or your local code.

Drop by gauge for this run

AWG	mm²	Loop resistance	Drop	% of supply	Meets target
4	21.15	15.89 mΩ	158.9 mV	1.32%	yes
8	8.37	40.16 mΩ	401.6 mV	3.35%	no
10	5.26	63.86 mΩ	638.6 mV	5.32%	no
12	3.31	101.5 mΩ	1.015 V	8.46%	no
14	2.08	161.5 mΩ	1.615 V	13.46%	no
16	1.31	256.7 mΩ	2.567 V	21.4%	no
18	0.82	408.2 mΩ	4.082 V	34.02%	no
20	0.52	649.1 mΩ	6.491 V	54.09%	no
22	0.33	1.032 Ω	10.32 V	86.01%	no
24	0.2	1.641 Ω	16.41 V	136.77%	no

Formula

R_max = (V_supply × drop%) / I    then find the gauge that beats it

Worked example

Carrying 10 A over 10 m at 12 V within 3% needs AWG 7 or thicker — giving 318.5 mV of drop.

V_allowed = V_supply × drop%
12 V × 3%
360 mV
R_max = V_allowed / I
360 mV / 10 A
36 mΩ
Across 20 m of conductor — the run doubled, because current returns.
find the gauge with R/m ≤ R_max / L
1.8 mΩ/m needed
AWG 7 at 1.593 mΩ/m

Frequently asked questions

What wire gauge do I need for a long 12 V run?

Thicker than you expect. At 12 V a 3% drop is only 0.36 V, so a 10 A load over 10 m needs about AWG 10 in copper. The same current at 240 V would allow 7.2 V of drop and need far less copper for the same percentage.

Why does the calculation double my cable length?

Because current flows out and back. A 10 m run means 20 m of conductor in the loop, and the drop happens along all of it. Forgetting this is the most common voltage-drop mistake and makes the answer half what it should be.

Is 3% voltage drop a rule or a guideline?

A widely used design guideline, not a law of physics. Some codes recommend 3% on a branch circuit and 5% total. What matters is whether your load still works at the reduced voltage — motors and LED drivers care more than resistive heaters.

Should I size for voltage drop or for current rating?

Whichever demands more copper. Short runs are usually limited by ampacity — how much heat the wire can shed. Long runs are limited by voltage drop, often needing much thicker wire than the current alone would suggest. This tool checks both.

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