What is voltage drop?
Every conductor has some resistance, so as current flows through a cable a small portion of the source voltage is “lost” along the way. That loss is called voltage drop, and it’s what determines whether your load receives usable voltage or something noticeably lower than the nameplate value.
How to use this calculator
Enter the load current in amperes, the one-way length of the run, the system voltage, choose your wire size (in mm² or AWG), and the calculator returns the voltage drop in volts and as a percentage, plus the voltage that actually reaches the load. Unlike a wire-size calculator that picks a gauge for you, this tool answers the opposite question: “for this specific wire, how much voltage will I lose?”
Acceptable voltage drop limits
Common guidance across most electrical codes:
- Under 3% — recommended for branch circuits carrying lighting or sensitive electronics. Green zone.
- 3–5% — acceptable only when it represents the total drop from source to final load (feeder + branch combined). Yellow zone.
- Over 5% — excessive. Lights flicker, motors run hot, VFDs and control gear can misbehave. Use a larger wire.
The formula
For a single-phase or DC circuit:
Vdrop = 2 × I × L × (ρ / A)
where I is current in amperes, L is one-way length in meters, ρ (rho) is the resistivity of the conductor material (about 0.0225 Ω·mm²/m for copper at operating temperature, 0.036 for aluminium), and A is the cross-sectional area in mm². The factor of 2 accounts for the current going out and returning. For three-phase, that factor becomes √3 (about 1.732).
Why voltage drop causes real-world problems
Motors are the classic case: an induction motor started with 10% low voltage draws more starting current, runs hotter, and shortens its own lifespan. Incandescent lighting dims and LED drivers can flicker. On DC systems, servos and cameras reset. PLCs and control gear usually tolerate ±10% but their attached sensors and actuators may not. On long solar or off-grid DC runs, voltage drop is often the single dominant sizing constraint.
Important limitations
This calculator uses conductor DC resistance at typical operating temperature. It does not include reactance (the inductive component of AC impedance, which matters at higher currents and larger cable sizes), temperature correction beyond the ~75°C assumption, power factor, or code-specific derating. For final installations, always verify with the applicable code (NEC Chapter 9 Table 8/9 in the US, IEC 60364 or BS 7671 or LST EN 60364 in Europe) and have installations checked by a licensed electrician.