Vehicle wiring must carry the load current with an acceptable voltage drop and survive vibration, heat, moisture and chemical exposure. Most circuits use the metal body or engine block as the return conductor, which saves weight but makes every ground connection part of the circuit.
Voltage drop in a 3 m run
Problem. A 2.5 mm² copper conductor runs 3 m to a 15 A load. Use ρ = 1.72 × 10⁻⁸ Ω·m and ignore the return path for this step. Estimate the one-way drop.
Solution. A = 2.5 × 10⁻⁶ m². R = (1.72 × 10⁻⁸ × 3) ÷ (2.5 × 10⁻⁶) = 0.0206 Ω. V_drop = 15 × 0.0206 = 0.31 V one way. The complete circuit also includes the return path and connections, so the total drop is higher.
Voltage-drop diagnosis
To find a high-resistance connection, measure the voltage across it while the circuit is operating at normal load. A connection that should be near zero volts but shows a significant drop is the fault. This test evaluates the circuit under real current, which a resistance measurement with the circuit off may not reveal.
- Size conductors for current, voltage drop and environmental conditions.
- The body and engine block are part of the return path.
- Corrosion at a ground point raises resistance and affects several systems.
- Voltage-drop testing under load localises the fault.
- Wire colour and connector conventions vary by manufacturer.
- R = ρL/A and V_drop = I × R describe conductor behaviour.
- The body is a return conductor, not an ideal zero-volt node.
- Voltage-drop testing must be done under load.
- Connector and ground corrosion cause widespread symptoms.
Worked example
A 12 V circuit draws 10 A through a ground connection showing 0.8 V. The connection resistance is R = V ÷ I = 0.8 ÷ 10 = 0.08 Ω, and it dissipates P = 0.8 × 10 = 8 W as heat at the joint.