Voltage Drop
Voltage drop is the number of volts lost before reaching the load end because of the conductor's own resistance. In a DC circuit the outgoing and return conductors are counted together: ΔU = 2·L·I·ρ/A. The loss is directly proportional to current and distance and inversely proportional to cross-section; the lost energy turns into heat.
In the formula L is the one-way distance (m), I the current (A), A the cross-section (mm²), and ρ the resistivity of copper, approximately 0.0175 Ω·mm²/m. Let us make it concrete with an example: if you take a load drawing 20 A over 5 m with 4 mm², ΔU = 2·5·20·0.0175/4 ≈ 0.88 V. If you use 10 mm² on the same run, the loss falls to 0.35 V.
- The effect is large at low voltage: 0.88 V is insignificant at 230 V but is 7% of the supply at 12 V.
- The lost voltage does not vanish: it turns into heat on the cable and heats the connection ends.
- It is measured under load: voltage measured with no load always looks fine.
This is usually the source of the field complaint "the device works at the battery, but it is weak when fitted in place". If the voltage at the driver or remote control input is a volt below the voltage at the battery terminals while the motor runs, the cross-section must be increased.
The context in which this term is used: Installation, redundancy and field continuity