Voltage Drop Calculator

Calculate voltage drop across any cable run in seconds. Enter voltage, current, wire length, and conductor size to get the drop in volts and percent, plus the maximum safe cable length. Free, metric-based, works for copper and aluminum, single-phase and three-phase.

What is Voltage Drop Calculator?

Voltage drop is the reduction in electrical potential that happens whenever current flows through the resistance of a wire. It is unavoidable — every conductor, no matter how good, has some resistance — but it becomes a real problem when a cable run is long, the conductor is undersized, or the load current is high. Left unchecked, excessive voltage drop causes motors to run hot and lose torque, lights to dim or flicker, and electronic equipment to behave unpredictably or fail early.

This calculator uses the standard electrician's voltage-drop formula based on conductor resistivity, cable length, cross-sectional area, and load current. It supports both single-phase (out-and-back) and three-phase (√3) wiring, and both copper and aluminum conductors, so it works for household wiring, industrial feeders, solar installations, and long outdoor or underground cable runs anywhere in the world — the formula is purely physical and does not depend on any single country's electrical code.

Formula

Single-phase: Vdrop = (2 × ρ × L × I) / A

Three-phase: Vdrop = (√3 × ρ × L × I) / A

Drop % = (Vdrop / Vsupply) × 100

Voltage at load = Vsupply − Vdrop

Where: ρ = conductor resistivity (Ω·mm²/m), L = one-way cable length (m), I = load current (A), A = conductor cross-sectional area (mm²). Copper ≈ 0.0172 Ω·mm²/m, Aluminum ≈ 0.0283 Ω·mm²/m at 20°C.

How to Calculate

Choose the system type: single-phase for most household and small commercial circuits, or three-phase for larger motors and industrial feeders.
Select the conductor material — copper or aluminum — since aluminum has significantly higher resistance for the same size.
Enter the supply voltage in volts and the load current in amps that the circuit will actually carry.
Enter the one-way cable length in meters (measure the physical run from source to load, not there-and-back).
Enter the conductor's cross-sectional area in mm² (check the cable's printed size or a wire gauge chart).
Read the voltage drop, drop percentage, voltage delivered to the load, and the maximum cable length that keeps you under the 3% guideline.

Example

A single-phase 230V circuit supplies a 16A load through a 2.5 mm² copper cable run 25 meters one-way. Voltage drop = (2 × 0.0172 × 25 × 16) / 2.5 ≈ 5.5 V, which is about 2.4% of 230V — comfortably inside the 3% guideline, and the load actually receives roughly 224.5V.

Key Benefits

Instantly compares single-phase and three-phase wiring without redoing the math by hand
Shows both the raw voltage drop and the percentage against your supply voltage side by side
Calculates the maximum cable length that stays within the standard 3% guideline
Works with any currency and any country's supply voltage — the physics doesn't change
Copper vs. aluminum comparison helps you decide if upsizing the conductor or switching material is more cost-effective

Common Mistakes to Avoid

Entering the round-trip cable length instead of the one-way distance — the formula already accounts for the return path.
Mixing up AWG and mm² conductor sizes, which produces a voltage drop that is off by a large margin.
Ignoring three-phase loads and always using the single-phase (×2) formula, which overstates the real drop by roughly 15%.
Forgetting that aluminum conductors need a larger cross-sectional area than copper to carry the same current with the same drop.

Pro Tips

As a rule of thumb, keep voltage drop under 3% for branch circuits and under 5% for the total feeder-plus-branch run.
If a run is borderline, it is usually cheaper to go up one standard conductor size than to redesign the circuit path.
For long DC solar or battery runs, voltage drop matters even more — recalculate using your actual system voltage, not 230V or 120V.
Keep a note of the maximum recommended length shown here when planning future extensions to the same circuit.

Key Terms Explained

Voltage drop: The loss of electrical potential across a conductor caused by its resistance, measured in volts.
Resistivity: A material property describing how strongly a conductor resists current flow, measured in Ω·mm²/m.
Cross-sectional area: The thickness of the conductor's copper or aluminum core, measured in mm², not counting insulation.
Three-phase factor (√3): The multiplier used instead of 2 for balanced three-phase circuits, reflecting the 120° phase relationship between conductors.

Common Use Cases

Sizing cable runs for household sub-panels, workshops, and outbuildings
Checking solar panel and battery wiring for excessive line loss over long DC or AC runs
Verifying motor feeder cables in industrial and commercial three-phase installations
Planning extension cords or temporary power runs for construction sites and events
Diagnosing dimming lights or underperforming appliances at the end of a long circuit

Frequently Asked Questions

What is voltage drop and why does it matter?

Voltage drop is the loss of electrical potential that occurs as current travels through the resistance of a cable. Every conductor has some resistance, so the voltage that reaches a motor, appliance, or light fixture is always slightly lower than the voltage at the source. If the drop is too large, equipment runs hotter, motors lose torque, lights dim, and sensitive electronics can malfunction or fail prematurely.

What is an acceptable voltage drop percentage?

Most electrical codes (including IEC and NEC guidance) recommend keeping voltage drop under 3% for a branch circuit and under 5% for the combined feeder and branch circuit. Staying within these limits keeps equipment operating efficiently and avoids nuisance tripping, flickering lights, and premature motor wear.

How do I reduce voltage drop on a long cable run?

You can reduce voltage drop by increasing the conductor's cross-sectional area (a thicker wire has less resistance), shortening the cable run where possible, switching from aluminum to copper (copper has roughly 61% of aluminum's resistivity), or raising the supply voltage and using a step-down transformer near the load.

Why does three-phase use a different formula than single-phase?

A single-phase circuit needs a complete out-and-back loop, so the current effectively travels through twice the conductor length, which is why the formula uses a factor of 2. A balanced three-phase circuit shares the load across three conductors 120° apart, and the vector sum of that arrangement works out to a factor of √3 (about 1.73) instead of 2 — noticeably less drop for the same cable and load.

Does this calculator work for both metric and imperial wire sizes?

The calculator uses cross-sectional area in square millimeters (mm²), the standard used across most of the world (IEC 60228). If your cable is specified in AWG (common in North America), convert the AWG size to mm² first — for example, 12 AWG is approximately 3.31 mm² and 10 AWG is approximately 5.26 mm².

Does temperature affect the result?

Yes. This calculator uses standard resistivity values for copper and aluminum at 20°C. Conductor resistance rises with temperature, so a cable that is heavily loaded or installed in a hot environment will have a slightly higher real-world voltage drop than the value shown here. For critical installations, apply your local code's temperature correction factors.