Voltage Drop Calculator
Pick a wire gauge and cable length, see the voltage drop in volts and percent. AWG and mm², with temperature derating and round-trip handling for camper, boat, RV, and off-grid DC wiring.
Voltage Drop Calculator
= 240W
= 16.4 ft (one way, round-trip calculated automatically)
Advanced settings
Voltage Drop
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VoltPlan designs full electrical systems for campers, boats, and off-grid setups with automatic wire sizing, fuse placement, and solar configurations.
Design Your System12V voltage drop chart: maximum cable length per wire gauge
Longest one-way run (positive plus negative conductor counted) that keeps the drop under 3% (0.36V) on a 12V system, copper at 20°C. Halve the current or double the voltage and the length doubles.
| Wire gauge | 5 A | 10 A | 20 A | 30 A | 50 A | 100 A |
|---|---|---|---|---|---|---|
| 1 mm² / 16 AWG | 2.0 m | 1.0 m | — | — | — | — |
| 1.5 mm² / 14 AWG | 3.0 m | 1.5 m | — | — | — | — |
| 2.5 mm² / 13 AWG | 5.0 m | 2.5 m | 1.3 m | — | — | — |
| 4 mm² / 12 AWG | 8.1 m | 4.0 m | 2.0 m | — | — | — |
| 6 mm² / 10 AWG | 12 m | 6.0 m | 3.0 m | 2.0 m | — | — |
| 10 mm² / 8 AWG | 20 m | 10 m | 5.1 m | 3.4 m | 2.0 m | — |
| 16 mm² / 6 AWG | 32 m | 16 m | 8.0 m | 5.4 m | 3.2 m | — |
| 25 mm² / 4 AWG | 50 m | 25 m | 13 m | 8.4 m | 5.0 m | — |
| 35 mm² / 2 AWG | 71 m | 35 m | 18 m | 12 m | 7.1 m | 3.5 m |
| 50 mm² / 1/0 AWG | 101 m | 50 m | 25 m | 17 m | 10 m | 5.0 m |
— the gauge cannot safely carry that current, regardless of length: pick a larger wire first.
Voltage drop formula
V_drop = I × (R_km ÷ 1000) × L × 2 L_max = 0.36V ÷ (I × R_km ÷ 1000 × 2)
I is the current in amps, R_km the conductor resistance in Ω per km (6 mm² copper: 2.98 Ω/km), L the one-way length in metres and the ×2 the return conductor. At 24V the allowed 3% drop is 0.72V, so every length in the chart doubles.
Frequently Asked Questions
How do I calculate voltage drop?
V_drop = current × resistance × length × round-trip factor. The calculator does this for you when you pick the wire gauge and length; the chart above turns the same formula around into a maximum cable length.
What is acceptable voltage drop for 12V?
3% maximum (0.36V) is the industry standard. Critical electronics under 1%. Lighting can tolerate up to 5%.
Why is 12V so demanding on wire size?
3% of 12V is only 0.36V. Combined with much higher current for the same wattage, 12V wiring needs to be heavier than 120V or 230V household runs.
Does temperature affect drop?
Yes. Copper resistance rises about 0.4% per °C above 20°C. Set the temperature in advanced settings to match the install location.
What is round-trip?
Current flows out and back, so the drop happens in both conductors. Multiply the one-way length by 2 — enabled by default.
How do I bring drop back into spec?
Use a larger wire gauge, shorten the cable run, increase the system voltage (12 → 24 → 48V), or split the load across multiple feeds.
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Once you have the wire gauge, size the matching fuse with the 125% rule and placement guidance.