🔌 Voltage Drop Calculator

Voltage drop is the reduction in electrical potential that occurs as current flows through a conductor. Every wire has some resistance, and as current passes through it, a portion of the voltage is lost as heat. The National Electrical Code (NEC) recommends keeping voltage drop below 3% for branch circuits and 5% for the total feeder plus branch run. Exceeding these limits can cause motors to overheat, lights to dim, and electronics to malfunction.

The fundamental formula for DC or single-phase AC resistive voltage drop is V_drop = 2 × I × ρ × L / A, where the factor of 2 accounts for both the supply and return conductors, I is the current (A), ρ is the resistivity of the conductor material, L is the one-way wire length (m), and A is the cross-sectional area (m²). For three-phase systems, the formula uses √3 instead of 2. Copper has a resistivity of about 1.68 × 10⁻⁸ Ω·m at 20°C, while aluminum is higher at 2.65 × 10⁻⁸ Ω·m, meaning aluminum wires of the same size will drop more voltage.

This calculator uses the standard voltage drop equations for single-phase and three-phase circuits. For single-phase, it computes V_drop = 2 × I × R, where R = ρ × L / A. For three-phase, it uses V_drop = √3 × I × R. The percentage drop is V_drop / V_source × 100.

The derived metric here is the voltage drop percentage, which tells you if your installation is within code limits. The NEC recommends a maximum 3% drop for branch circuits and 5% for the combined feeder and branch circuit. The IEC 60364 standard similarly suggests limiting drop to 4-5% for lighting and 5-7% for other loads.

1. Select the conductor material: copper or aluminum.
2. Enter the current flowing through the wire in amperes (A).
3. Enter the one-way length of the wire in metres (m).
4. Enter the cross-sectional area in mm² (or use AWG reference).
5. Select single-phase or three-phase, then click Calculate.

What is an acceptable voltage drop?

The NEC recommends a maximum of 3% voltage drop for branch circuits and 5% total (feeder + branch). For example, on a 120 V circuit, a 3% drop means losing no more than 3.6 V. Drops exceeding 5% can cause equipment malfunction and energy waste.

Why does the formula multiply by 2 for single-phase?

Current must travel from the source to the load and then return. Both the supply wire and the return wire have resistance, so the total path length is twice the one-way distance. Three-phase systems use √3 (about 1.732) instead because of the phase displacement between conductors.

How does temperature affect voltage drop?

Copper resistance increases approximately 0.393% per °C above 20°C. A wire carrying current in a 60°C attic has about 16% more resistance than at room temperature, which proportionally increases the voltage drop. Always consider the actual operating temperature.

Should I use copper or aluminum wire?

Copper has about 60% lower resistance per same cross-section, so it drops less voltage. Aluminum is lighter and cheaper for large gauge utility runs, but it requires a larger cross-section to match copper’s performance. For residential wiring, copper is standard.

Does voltage drop differ for AC vs DC?

For purely resistive AC loads, the DC resistance formula applies directly. For AC circuits with inductive loads (motors, ballasts), the cable’s reactance adds to the voltage drop. This calculator computes the resistive component; for AC inductive circuits, the actual drop may be 10-20% higher.

What AWG size do I need for a 15 A circuit at 30 m?

For a 15 A, 120 V single-phase circuit at 30 m, AWG 12 copper (3.31 mm²) yields about 3.8% drop, which exceeds the 3% branch-circuit guideline. Stepping up to AWG 10 (5.26 mm²) brings the drop down to about 2.4%.

Use AWG tables to cross-reference wire gauge with cross-sectional area: AWG 12 ≈ 3.31 mm², AWG 10 ≈ 5.26 mm², AWG 8 ≈ 8.37 mm². Wire resistance increases with temperature: copper's resistance rises roughly 0.393% per °C above 20°C, so in hot attics or conduits, expect higher drops than calculated. For AC inductive loads (motors, transformers), the impedance includes reactance, which increases the effective voltage drop beyond the pure resistance value. When running long cable runs (over 30 m / 100 ft), upsizing the wire one or two gauges is typically more economical than accepting the energy loss from voltage drop over the cable's lifetime.