3% Voltage Drop Rule
On critical DC circuits the loss on the cable is expected not to exceed 3% of the supply voltage. In a 12 V system that means 0.36 V, in a 24 V system 0.72 V. On non-critical circuits such as lighting, a tolerance of up to 10% is accepted; remote control and driver supplies count as critical.
The rule is not a comfort preference but a design margin. An electronic board usually shuts itself down or limits its output below a certain voltage; every volt lost on the cable eats into that margin. Also, a measurement is not taken while the battery is not already full: a nominally 12 V system can fall to 11.5 V under load, and the cable loss adds on top of that.
- Critical circuits: remote control receiver, driver power input, valve coils, sensor supply — target 3%.
- Non-critical ones: cabin lighting, warning lamps — up to 10% is acceptable.
You can reverse the calculation to find the cross section: A = 2·ρ·L·I/ΔU. To carry a load drawing 20 A at 12 V over 5 m with 0.36 V loss, A = 2·0.0175·5·20/0.36 ≈ 9.7 mm², so in practice 10 mm² is needed. If the same load were supplied at 24 V, the current would halve and the permitted loss would double, and the required cross section would become noticeably smaller.
The context in which this term is used: Installation, redundancy and field continuity