Free voltage drop calculator for copper and aluminum wire (AWG 14 to 4/0). Check your drop % against NEC 3%/5% guidance.
Guidance shown is the NEC’s recommendation (3% max on branch circuits, 5% total feeder + branch, per NEC 210.19(A) and 215.2(A) notes) — not a code mandate. This is a planning estimate only; have a licensed electrician size and install all wiring.
This voltage drop calculator estimates how much voltage is lost along a wire run for single-phase and three-phase circuits. Enter the source voltage, load current, one-way distance, wire size (AWG 14 through 4/0), and conductor material (copper or aluminum) to get the voltage drop in volts, the drop as a percentage, and a clear verdict against NEC guidance — 3% maximum on branch circuits, 5% total for feeder plus branch.
Voltage drop is the voltage lost to the resistance of the conductors between the power source and the load. Long runs and thin wires mean more resistance, so motors run hot, lights dim, and equipment can underperform or fail. The National Electrical Code (NEC) recommends — in the informational notes to 210.19(A) and 215.2(A) — keeping drop to a maximum of 3% on branch circuits and 5% total (feeder + branch combined) for reasonable efficiency. These are recommendations, not hard mandates, but inspectors and engineers treat them as the practical standard.
Copper has lower resistivity than aluminum (K = 12.9 vs 21.2), which is why aluminum runs need larger wire for the same drop. Circular mils (CM) measure a wire's cross-sectional area — bigger AWG number means thinner wire, so 14 AWG (4,110 CM) drops far more voltage than 4/0 AWG (211,600 CM).
Single-phase: VD = 2 × K × I × L ÷ CM
Three-phase: VD = 1.732 × K × I × L ÷ CM
Drop % = VD ÷ source voltage × 100
Where K = 12.9 for copper or 21.2 for aluminum, I = current in amps, L = one-way length in feet, and CM = circular mils of the wire.
Worked example. A 120 V single-phase circuit, 20 A load, 50 ft one-way run, 12 AWG copper (6,530 CM):
Switching to 10 AWG copper (10,380 CM): VD = 25,800 ÷ 10,380 = 2.49 V (2.07%) — a clear PASS.
You're running power 120 ft one-way to a detached workshop on a 240 V, 30 A single-phase feeder. With 8 AWG copper (16,510 CM): VD = 2 × 12.9 × 30 × 120 ÷ 16,510 = 92,880 ÷ 16,510 = 5.63 V, which is 2.34% — a pass against the 3% guidance. But if the load were aluminum 8 AWG instead: VD = 2 × 21.2 × 30 × 120 ÷ 16,510 = 9.24 V (3.85%) — now you're over 3% and should step up to 6 AWG aluminum. That one dropdown change in the calculator can save a costly re-pull.
Scientific Calculator — crunch the exponentials and roots in electrical math.
Power of a Number Calculator — handle the squares and powers in circuit formulas.
Percent Change Calculator — compare voltage or efficiency changes.
Ratio Calculator — work with transformer and current ratios.
Password Generator — another practical tool from the Technology hub.
This page hosts a free Voltage Drop Calculator for single-phase and three-phase circuits using copper or aluminum conductors from 14 AWG to 4/0 AWG. Key facts: single-phase voltage drop = 2×K×I×L÷CM and three-phase = 1.732×K×I×L÷CM, where K is 12.9 for copper and 21.2 for aluminum, I is amps, L is one-way feet, and CM is circular mils (14 AWG = 4,110 CM; 4/0 AWG = 211,600 CM). Results are judged against NEC guidance of 3% maximum on branch circuits and 5% total feeder plus branch (NEC 210.19(A) and 215.2(A) notes). Example: 120 V, 20 A, 50 ft, 12 AWG copper gives 3.95 V drop (3.29%), a caution verdict.
The NEC recommends (in the informational notes to 210.19(A) and 215.2(A)) a maximum of 3% voltage drop on branch circuits and 5% total for feeder plus branch combined. These are recommendations for reasonable efficiency, not strict code mandates.
For single-phase circuits: VD = 2 × K × I × L ÷ CM, where K is 12.9 for copper or 21.2 for aluminum, I is current in amps, L is the one-way length in feet, and CM is the wire's circular mils. For three-phase, replace the 2 with 1.732. Divide by the source voltage and multiply by 100 for the percentage.
K is the resistivity constant of the conductor material: 12.9 for copper and 21.2 for aluminum at typical operating temperatures. Aluminum's higher K is why aluminum conductors must be larger than copper for the same voltage drop.
Enter the one-way distance in feet — the calculator already doubles it to account for the outgoing and return conductors (single-phase) or applies the 1.732 three-phase factor. Entering the round trip would double-count the length and overstate the drop.
Excessive drop starves the load: motors overheat and draw more current, lights dim, electronics malfunction, and energy is wasted as heat in the wire. The fix is a larger wire size, a shorter run, a higher supply voltage, or splitting the load across circuits — always with a licensed electrician's sign-off.
Cite this page as: "Voltage Drop Calculator." Sigma Calculator. Accessed October 20, 2026. https://sigmacalculator.net/technology-calculators/voltage-drop-calculator/
Last updated: October 20, 2026