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.
Steps:
- 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.
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.
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
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
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.
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.
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.
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.