How DC Voltage Drop Works
Voltage drop occurs because all physical electrical wires have electrical resistance. When current (Amperes) flows through a cable, this resistance converts some electrical energy into heat, causing the voltage delivered at the appliance terminal to be lower than the battery source.
In low-voltage DC systems (such as 12V and 24V electrical setups in campervans, 4WDs, caravans, and boats), voltage drop is far more critical than in household 240V AC mains. A 1.2V drop on a 240V circuit is only 0.5%, but a 1.2V drop on a 12V battery drops your terminal voltage to 10.8V — enough to trigger the low-voltage cutoff error on your fridge, inverter, or diesel heater.
The DC Voltage Drop Formula
Our calculator uses the standard electrical engineering formula for two-conductor DC circuits:
- Length: One-way distance in meters (the factor of 2 accounts for both the positive feed and negative return conductors).
- Current: Circuit load in Amperes.
- Resistivity (ρ): Resistivity of oxygen-free copper conductor at operating temperature (~0.018 Ω·mm²/m at 30°C).
- Area: Cable core cross-sectional area in mm².
3% vs. 10% Voltage Drop: Which Should You Choose?
| Application | Target Drop | Why It Matters |
|---|---|---|
| Critical Circuits: Inverters, 12V Fridges, DC-DC Chargers, Solar MPPT, Marine Navigation | ≤ 3% | Ensures battery chargers sense battery voltage accurately, prevents inverters and compressor fridges from shutting down prematurely under peak load. |
| Non-Critical Circuits: General LED lighting, water pumps, USB sockets, exhaust fans | ≤ 10% | LED lights and momentary water pumps can tolerate a slight voltage dip without malfunction, allowing lighter, cheaper cable runs. |
Standard Metric Cable Sizes vs. AWG / B&S
| Metric Area (mm²) | AWG / B&S Equivalent | Continuous Safe Ampacity (in air, 105°C) |
|---|---|---|
| 1.5 mm² | 16 AWG / 16 B&S | 20 A |
| 2.5 mm² | 14 AWG / 14 B&S | 30 A |
| 4.0 mm² | 12 AWG / 12 B&S | 40 A |
| 6.0 mm² | 10 AWG / 10 B&S | 50 A |
| 10.0 mm² | 8 AWG / 8 B&S | 70 A |
| 16.0 mm² | 6 AWG / 6 B&S | 95 A |
| 25.0 mm² | 4 AWG / 4 B&S | 130 A |
| 35.0 mm² | 2 AWG / 2 B&S | 160 A |
| 50.0 mm² | 1/0 AWG / 0 B&S | 210 A |
| 70.0 mm² | 2/0 AWG / 00 B&S | 265 A |