Voltage Drop Calculator
Computes the voltage drop across a wire run given current, one-way distance, wire gauge, and conductor material. Essential for ensuring devices receive adequate voltage at the end of long cable runs.
About this calculator
Voltage drop occurs because every conductor has resistance, and current flowing through that resistance dissipates energy as heat while reducing the voltage available at the load. The formula used here is: V_drop = 2 × I × d × R_unit / 1000, where I is current in amperes, d is the one-way distance in feet, and R_unit is the resistance per 1,000 feet for the chosen gauge and material. The factor of 2 accounts for both the outgoing and return conductors in a single-phase circuit. Resistance values differ by material: copper is a better conductor than aluminum, so aluminum wires of the same gauge have higher resistance (roughly 1.64× that of copper). The NEC recommends limiting voltage drop to 3% for branch circuits and 5% total (feeder plus branch), because excessive drop causes equipment to run hot, motors to lose torque, and lighting to dim.
How to use
Suppose you are wiring a 20 A circuit with 12 AWG copper wire over a one-way distance of 75 feet. Enter current = 20 A, distance = 75 ft, wire_gauge = 12, material = copper. The resistance constant for 12 AWG copper is 1.93 Ω per 1,000 ft. V_drop = 2 × 20 × 75 × 1.93 / 1000 = 2 × 20 × 75 × 0.00193 = 5.79 V. On a 120 V circuit that is 5.79 / 120 = 4.8% drop — above the NEC 3% guideline. You should upsize to 10 AWG (constant 1.21): V_drop = 2 × 20 × 75 × 1.21 / 1000 = 3.63 V, or 3.0% — right at the limit.
Frequently asked questions
What is the maximum allowable voltage drop for a residential branch circuit?
The NEC recommends, but does not mandate, a maximum voltage drop of 3% on any single branch circuit and no more than 5% combined across the feeder and branch circuit. These are guidelines found in NEC 210.19(A) and 215.2(A) informational notes, not hard code requirements, but many inspectors and engineers treat them as design targets. Exceeding 3% causes noticeable dimming of lights, reduced motor starting torque, and premature wear on appliances sensitive to under-voltage conditions.
Why does wire material matter for voltage drop calculations?
Copper and aluminum have different electrical resistivities. Copper's resistivity is approximately 1.72 × 10⁻⁸ Ω·m, while aluminum's is about 2.82 × 10⁻⁸ Ω·m — roughly 64% higher. This means an aluminum conductor of the same gauge will produce about 1.64 times more voltage drop than copper for the same current and distance. Aluminum wiring is less expensive and lighter, making it popular for large feeder conductors, but you must upsize aluminum conductors (typically by two AWG sizes) to match the ampacity and voltage-drop performance of copper.
How can I reduce voltage drop without changing wire gauge?
The most practical alternatives are to shorten the run by relocating the panel or adding a sub-panel closer to the load, reduce the current draw by splitting loads across multiple circuits, or increase the supply voltage (e.g., using 240 V instead of 120 V halves the current for the same wattage, cutting drop by half). You can also use parallel conductors — running two cables side by side effectively doubles the conductor cross-section and halves resistance. For critical loads like motors or sensitive electronics, a dedicated circuit with oversized conductors is always the safest long-term solution.