Voltage Drop Calculator - Wire Sizing Tool

Formula

Single-phase: Vd = 2 × I × (R/1000) × L | Three-phase: Vd = √3 × I × (R/1000) × L

I = current (A), R = resistance per 1,000 ft from NEC Table 8 (Ω), L = one-way distance (ft).

Worked Examples

Example 1: 20A, 12 AWG, 100 ft

Problem:120V single-phase, 20A load, 12 AWG copper, 100 ft run

Solution:Vd = 2 × 20 × (1.98/1000) × 100

Result:7.92 V drop (6.6%) — oversized; use 10 AWG

Example 2: 20A, 10 AWG, 100 ft

Problem:120V single-phase, 20A load, 10 AWG copper, 100 ft run

Solution:Vd = 2 × 20 × (1.24/1000) × 100

Result:4.96 V drop (4.1%) — acceptable

Frequently Asked Questions

What is an acceptable voltage drop?

The National Electrical Code addresses this in informational notes rather than mandatory rules: no more than 3 percent on a branch circuit, and no more than 5 percent combined across feeder and branch circuit, measured at the farthest outlet. Because these are recommendations for reasonable efficiency they are not enforceable by an inspector, but most engineers treat them as design limits. Sensitive or critical loads are usually held under 2 percent.

Why does AWG matter?

American Wire Gauge runs backwards: a lower number means a thicker conductor and therefore less resistance per foot. NEC Chapter 9 Table 8 lists stranded copper at 1.98 ohms per 1,000 ft for 12 AWG but only 0.491 ohms per 1,000 ft for 6 AWG, four times less. Each three-gauge step roughly halves the resistance, so going from 12 AWG to 9 AWG would cut the drop in half for the same run.

How do I reduce voltage drop?

There are four levers, in rough order of practicality. Increase the conductor size, which is the usual fix and the reason long runs are upsized a gauge or two. Shorten the run by relocating the panel or subpanel closer to the load. Reduce the current by splitting the load across two circuits. Or raise the supply voltage, since a 240 V circuit carries the same power at half the current and so suffers a quarter of the percentage drop.

Does AC vs DC matter?

For most practical wire-sizing calculations, DC and AC resistance are treated the same. AC introduces reactance in large conductors, but this is negligible for typical branch circuits.

What is the factor of 2 in the formula?

Current has to complete a circuit, so it flows out along the hot conductor and back along the neutral. The copper it passes through is therefore twice the one-way distance you measure from panel to load, and the single-phase formula doubles the length to account for it. Three-phase circuits use the square root of 3, about 1.732, instead, because the return current is shared across the phases rather than carried by a dedicated conductor.

How do I convert voltage drop into a percentage?

Divide the calculated drop by the nominal supply voltage and multiply by 100. A 5.2 V drop on a 120 V branch circuit is 5.2 / 120 x 100 = 4.3 percent, which exceeds the 3 percent recommendation and calls for a larger conductor. The same 5.2 V drop on a 240 V circuit is only 2.2 percent, which is why higher-voltage distribution tolerates far longer runs for the same conductor size.

Are these copper or aluminum values?

Voltage Drop Calculator - Wire Sizing Tool uses copper conductor resistance values from NEC Chapter 9 Table 8. Aluminum has about 1.6× higher resistance — multiply the result by 1.6 for aluminum wire.

Background & Theory

Every conductor is also a resistor. When current flows down a branch circuit, part of the source voltage is consumed inside the copper instead of reaching the load, and that consumed part is voltage drop. The arithmetic is simply Ohm's law applied to the wire itself. The two circuit multipliers used here exist because the current path is never one-way: a single-phase circuit sends current out on one conductor and back on another, so a 100 foot run is really 200 feet of copper, which is the factor of 2. A balanced three-phase circuit returns current through the other phase conductors rather than a neutral, and the line-to-line geometry yields a factor of sqrt(3), about 1.732. The resistance values behind each AWG option come from NEC Chapter 9 Table 8 for uncoated copper at 75 degrees C, the rating most branch-circuit insulation carries. Two consequences follow. Copper resistance climbs roughly 0.4 percent per degree C, so a conductor running hot inside a crowded conduit drops more than the table predicts. Aluminum carries about 1.6 times the resistance of copper at the same gauge, so scale the answer up for aluminum runs. The missing voltage does not vanish, it becomes heat at a rate of current squared times resistance, which is why an undersized long run both starves the load and warms the wall cavity. Motors suffer worst, because low terminal voltage raises current draw, increases slip and shortens winding life.

History

Georg Simon Ohm published the relationship that underpins this calculation in 1827, in a monograph on the galvanic circuit that much of the German academic establishment initially dismissed before the Royal Society vindicated him with the Copley Medal in 1841. Standardised wire sizing came next. The American Wire Gauge scale, still known as Brown and Sharpe gauge after the Providence toolmakers who fixed it in 1857, defined diameters as a geometric progression, which is why every three-step drop in gauge number roughly halves the resistance per foot and why the table in this calculator falls in such regular ratios. Voltage drop became an economic problem during the war of the currents in the 1880s. Edison's low-voltage direct-current distribution could not push usable power much beyond a mile from the generating station without unacceptable loss, whereas the Westinghouse alternating-current system used transformers to raise voltage, cut current for the same delivered power, and shrink drop across long lines. Codified guidance followed the technology. The National Electrical Code, first published in 1897 and maintained today by the National Fire Protection Association as NFPA 70, carries the familiar targets in informational notes: about 3 percent drop on a branch circuit and 5 percent combined with the feeder. Those figures are recommendations for efficient operation rather than enforceable rules, though many jurisdictions and design specifications adopt them as hard limits.