Cable Voltage Drop

Formula

Description

The total voltage drop in a cable includes both the outgoing and return conductors, hence the factor of 2. This round-trip drop is the voltage difference between what leaves the source and what arrives at the load. Excessive voltage drop causes devices to malfunction, reduces motor torque, dims LED lighting, and wastes energy as heat in the cable. Industry standards typically require less than 3% voltage drop for branch circuits and less than 5% for combined feeder and branch circuits. For 12V DC systems, even a 0.5V drop represents a 4% loss.

Variables

• Vdrop — Total round-trip voltage drop (V)
• I — Load current (A)
• R — Resistance per meter of one conductor (Ω/m)
• L — One-way cable length (m)

Practical Notes

Resistance per meter for common copper wire: 14 AWG = 0.0083 Ω/m, 16 AWG = 0.0132 Ω/m, 18 AWG = 0.021 Ω/m, 22 AWG = 0.053 Ω/m. For long LED strip runs, inject power at both ends or at intervals. For solar panel wiring, voltage drop directly reduces energy harvest. Use thicker wire or higher voltage (with DC-DC conversion at the load) for long runs.