Calculators · EV and electrical

12 V and 24 V wire size, voltage drop and fuse calculator

In low-voltage vehicle and boat wiring, a cable that is too thin loses a noticeable share of the voltage along its length, so lights dim and motors run slowly. The loss depends on current, cable length and cross-section. These calculators give the voltage drop of a cable, the minimum cross-section for an acceptable drop, and a fuse rating to protect the circuit. The cable must also be rated for the current it carries.

Voltage drop in a cable

Calculates the voltage lost in the supply and return conductors of a copper cable. Around 3% is the usual limit for important circuits and 10% for less critical ones.

Results
Voltage drop—VVoltage lost in the cable.
Voltage drop—%As a share of system voltage.
Voltage at the load—VWhat the device actually receives.

Formula
R = ρ × 2L / A   (ρ = 0.0172 Ω·mm²/m for copper)
V_drop = I × R

Minimum cable size for an allowed drop

Works out the smallest conductor cross-section that keeps voltage drop within your limit, then picks the next standard size up. Longer runs and higher currents need thicker cable.

Results
Minimum cross-section—mm²Calculated minimum.
Standard size to use—mm²Next standard metric cable size up.

Formula
A = ρ × 2L × I / (V × max% / 100)

Fuse rating

A fuse should be rated a little above the normal current so it does not blow in use — commonly 125% — and never above what the cable can safely carry. This picks the next standard blade or mega fuse size.

Results
Calculated rating—ACurrent multiplied by the fuse factor.
Standard fuse—ANext standard fuse size — check the cable is rated above it.

Formula
fuse ≥ current × factor

Worked example: 20 A lights 5 m from the battery

  1. In 2.5 mm² cable the round trip of 10 m has 0.0172 × 10 / 2.5 = 0.069 Ω of resistance.
  2. At 20 A that drops 1.38 V — 11.5% of 12 V, far too much for lights.
  3. For a 3% limit the cable needs 9.6 mm², so use 10 mm².
  4. Protect the circuit with a 25 A fuse (20 A × 125%).

Metric cable sizes and nearest AWG

Cross-sectionNearest AWGResistance per metre
1.5 mm²16 AWG11.5 mΩ
2.5 mm²14 AWG6.9 mΩ
4 mm²12 AWG4.3 mΩ
6 mm²10 AWG2.9 mΩ
10 mm²8 AWG1.7 mΩ
16 mm²6 AWG1.1 mΩ
25 mm²4 AWG0.69 mΩ

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About these results

Results are theoretical estimates from standard engineering formulas. Real-world figures depend on conditions these formulas do not capture. Always follow the manufacturer's specifications for maintenance, loading and safety decisions.

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