Voltage Drop Calculator
Calculate voltage drop across a DC cable run. Enter wire length, cross-section (AWG or mm²), current and material — get voltage drop, power loss and percentage instantly.
Last updated: May 2026
Enter wire length, gauge and current to calculate voltage drop.
Vdrop = 2 × ρ × L × I ÷ A (round-trip)
Why voltage drop matters
Every wire has resistance. Current flowing through that resistance causes a voltage drop — the load receives less voltage than the supply provides. In a 12 V LED strip installation with thin wire over 10 m, that drop can be 1–2 V, causing dimming at the far end. In automotive wiring, excessive drop can prevent a starter motor from cranking. The IEC guideline for fixed building installations is a maximum 3% drop from source to load.
The formula uses the round-trip length (2 × one-way): Vdrop = 2 × ρ × L × I ÷ A, where ρ is resistivity in Ω·mm²/m, L is one-way length in metres, I is current in amps and A is cross-section in mm².
Common applications
- LED strip lighting over long runs (12 V or 24 V)
- Solar panel cable sizing (low voltage, high current)
- Automotive accessories and trailer wiring
- Low-voltage outdoor lighting and garden circuits
- PCB trace resistance estimation (convert mm² to trace width)
Frequently Asked Questions
Why does the calculator multiply length by 2?
Current flows from the supply, through the positive wire to the load, and back through the negative (return) wire to the supply. Both wires have resistance and both contribute to the total voltage drop. If you are running 10 m of cable from a 12 V battery to a load, the total wire length in the circuit is 20 m. Using the one-way length and forgetting to double it gives half the actual drop — a common error in LED strip and solar installations.
What is an acceptable voltage drop percentage?
IEC 60364 recommends a maximum 3% drop for fixed installations (lighting and power circuits). For sensitive electronics, 1–2% is a better target. Automotive: Society of Automotive Engineers (SAE) allows up to 10% for non-critical circuits, 3% for critical circuits. For 12 V LED strips, a 5% drop (0.6 V) is usually acceptable at the far end — more causes visible dimming. Higher-voltage systems (24 V, 48 V) tolerate more absolute drop while keeping the percentage within limits.
How do I reduce voltage drop without changing the wire gauge?
Three options: (1) Reduce run length — split one long run into two shorter runs fed from both ends. (2) Increase supply voltage — a 24 V system carries the same wattage at half the current, dropping voltage drop to one quarter for the same wire. (3) Add a booster or buck converter at the far end to compensate. Increasing wire gauge is usually the most practical and lowest-cost option for fixed installations.
Does voltage drop apply to AC mains wiring?
Yes, the same principle applies, but the calculation uses RMS current and single-phase AC has the same resistive drop formula. Three-phase circuits use a different factor (√3) and the neutral carries only imbalance current. For AC mains (230 V), a 3% drop is only 6.9 V — usually acceptable. The bigger concern in AC mains is earth fault loop impedance for fuse/breaker protection, not purely resistive drop.