Voltage drop calculator
Check if your cable is large enough for the run. Results are shown against the 3% and 10% thresholds from BS EN ISO 13297.
Related tools
Worked example
You're wiring a 12V bilge pump (8A) with a 4 m one-way cable run using 2.5 mm² cable:
- Total cable length: 4 m × 2 = 8 m (out and return)
- Resistance of 2.5 mm²: 7.41 mΩ/m × 8 m = 59.3 mΩ
- Voltage drop: 8A × 0.0593Ω = 0.47V (3.9% on 12V)
- Result: marginal — upgrade to 4 mm² cable to bring drop below 3%
Try those values above to verify.
How voltage drop is calculated
The calculator uses the actual resistance of the cable (mΩ/m for copper) and doubles the run length to account for both positive and return conductors.
Voltage drop = Current (A) × Total cable resistance (Ω)
ABYC E-11 and BS EN ISO 13297 recommend: 3% maximum for critical circuits and 10% maximum for non-critical loads.
A 3% drop on a 12V system is only 0.36V — far less than it sounds, which is why proper cable sizing matters.
Cable resistance reference
| Cable CSA | Resistance (mΩ/m) | Typical use |
|---|---|---|
| 1.5 mm² | 12.1 | Cabin lighting, small instruments |
| 2.5 mm² | 7.41 | Navigation lights, pumps up to 10A |
| 4 mm² | 4.61 | Larger pumps, windlass controls |
| 6 mm² | 3.08 | Inverters, bow thrusters (short runs) |
| 10 mm² | 1.83 | Battery to distribution panel |
| 16 mm² | 1.15 | High-current runs, engine start |
| 25 mm² | 0.727 | Bow thruster, windlass main feed |
Frequently asked questions
What is the maximum voltage drop allowed on a boat?
BS EN ISO 13297 and ABYC E-11 both specify a maximum of 3% for critical circuits (navigation lights, bilge pumps, VHF radio) and 10% for non-critical loads. On a 12V system, 3% is just 0.36V.
Can I use automotive cable for marine wiring?
Not recommended. Marine wiring uses tinned copper conductors, which resist corrosion in the damp, salt-air environment of a boat. Standard automotive cable uses bare copper, which corrodes rapidly and can fail without warning.
Why does cable length matter so much?
Resistance increases linearly with length. A 10 m run has twice the voltage drop of a 5 m run at the same current. Routing cables efficiently — and running them as close to the battery as safely possible — reduces volt drop and heat.
What happens if voltage drop is too high?
Equipment runs below its design voltage: pumps run slower, lights dim, electronics may behave erratically. The cable also runs hotter, increasing fire risk. Always size cables to keep drop within limits.