How to calculate voltage drop

Voltage drop is the voltage a conductor "loses" to its own resistance between the source and the load. Get it wrong and lights dim, motors struggle to start, and you quietly waste energy as heat in the wire. This guide walks through the formula, where the numbers come from, the NEC's 3% and 5% targets, copper versus aluminum, and two fully worked examples you can repeat on paper.

The formula

The everyday voltage-drop formula for a single conductor run is:

Voltage drop = K × length (ft) × current (A) × (R ÷ 1000)

Each piece has a job. K is a phase constant: use 2 for a single-phase circuit and √3 (about 1.732) for three-phase. The factor of 2 on single-phase exists because current travels out on one wire and returns on another, so the electricity actually covers twice the one-way distance. Length is the one-way run in feet — the formula handles the round trip for you, so you measure from the panel to the load once. Current is the load in amps. R is the conductor's resistance in ohms per 1000 feet, which is why we divide by 1000: it converts "per 1000 ft" resistance into the resistance of your actual run length.

Once you have the volts dropped, two more lines finish the picture:

Percent drop is the number electricians actually argue about, because the guidance is written as a percentage rather than a fixed number of volts.

Where the resistance numbers come from

The resistance value R is not something you guess — it is published. This calculator uses the DC resistance figures from NEC Chapter 9, Table 8 (Conductor Properties), given in ohms per 1000 feet for uncoated copper and aluminum at roughly 75 °C. For example, copper 12 AWG is listed at 1.93 ohms per 1000 ft, copper 10 AWG at 1.21, and 6 AWG at 0.491. Aluminum runs higher: 12 AWG aluminum is 3.18 ohms per 1000 ft. Using a published table is what makes two people's calculations match. Table 8 is a DC reference; real AC circuits with significant reactance (long runs in steel conduit, large conductors) are better checked against the AC impedance values in NEC Table 9, which is one reason this tool is an estimate rather than a code-compliance verdict.

The NEC 3% and 5% guidance

The National Electrical Code does not mandate a maximum voltage drop for most general wiring. Instead, informational notes (formerly "fine print notes") recommend keeping a branch circuit at or below 3%, and the feeder plus branch circuit combined at or below 5%. Because these are recommendations, an inspector usually cannot fail an installation on voltage drop alone — but ignoring them is poor practice. Excessive drop means lights that dim when the microwave runs, motors that draw extra current and run hot, electronics that brown out, and energy lost as heat for the life of the circuit. Most designers simply treat 3% as the target and only spend up to 5% when a long feeder makes 3% impractical.

Copper vs aluminum

Aluminum is lighter and cheaper per amp, which is why utilities and many large feeders use it. The trade-off is resistance: for the same size, aluminum's resistance is noticeably higher than copper's, so it drops more voltage over the same run. In practice, matching a copper circuit's drop with aluminum usually means going up one or two wire sizes. Aluminum also needs antioxidant compound and connectors rated for it (look for AL or CU-AL markings), and it is rarely used below 12 AWG — which is why this calculator's aluminum list starts at 12 AWG while copper includes 14 AWG. When you compare options, compare the installed result (drop, ampacity, and termination cost), not just the price per foot.

Worked example 1: a 120 V branch circuit

Suppose you run copper 12 AWG, 100 feet one-way, feeding a 20 A single-phase load at 120 V. Plug in the numbers: K = 2 (single-phase), length = 100, current = 20, and R = 1.93 ohms per 1000 ft.

Voltage drop = 2 × 100 × 20 × (1.93 ÷ 1000) = 7.72 V. Percent drop = 7.72 ÷ 120 × 100 = 6.43%. The voltage at the load is 120 − 7.72 = 112.28 V. That is well past the 3% target and even past 5%, so 12 AWG is too small for this distance. Stepping up to copper 8 AWG (R = 0.764) gives 2 × 100 × 20 × 0.000764 = 3.06 V, or about 2.5% — back inside the 3% guidance. This is exactly the kind of "the breaker size says 12 AWG but the distance says bigger" situation voltage-drop sizing is meant to catch.

Worked example 2: a 480 V three-phase feeder

Now take copper 4 AWG (R = 0.308), 200 feet one-way, carrying 60 A on a 480 V three-phase feeder. Here K = √3 ≈ 1.732.

Voltage drop = 1.732 × 200 × 60 × (0.308 ÷ 1000) ≈ 6.40 V. Percent drop = 6.40 ÷ 480 × 100 ≈ 1.33%, and the voltage at the load is about 473.6 V. Comfortably inside 3%. Two things stand out compared with example 1: the higher 480 V system voltage makes the same number of dropped volts a much smaller percentage, and the √3 factor (instead of 2) further reduces three-phase drop. Higher voltage is the single most effective lever for long runs — it is why feeders and EV or shop circuits are often run at 240 V instead of 120 V.

Why it matters

Voltage drop is easy to ignore because nothing fails immediately — it just costs you slowly. Undersized conductors waste energy as heat every hour they carry load, shorten the life of motors and ballasts, and produce the flicker and brown-outs that get blamed on "bad wiring." Sizing for drop up front is cheap insurance. Remember, though, that voltage drop is only one of several checks: the conductor must also satisfy ampacity, temperature derating, conduit fill, and overcurrent protection. Always use the larger of the voltage-drop size and the ampacity size, and confirm the final design against the current NEC and your local code.

FAQ

What is the voltage drop formula?

Voltage drop = K × length (ft) × current (A) × (R ÷ 1000). K is 2 for single-phase and √3 (about 1.732) for three-phase, and R is the conductor's DC resistance in ohms per 1000 ft from NEC Chapter 9 Table 8. Divide the drop by source voltage and multiply by 100 for percent drop.

Why does length use a factor of 2 for single-phase?

Current flows out on one conductor and back on another, so a single-phase run is effectively twice the one-way length. The factor of 2 accounts for the round trip. Three-phase uses √3 instead because of how the phase voltages combine.

How much voltage drop is too much?

The NEC informational notes suggest 3% or less on a branch circuit and 5% or less for feeder plus branch combined. They are recommendations, not enforceable limits, but excessive drop wastes energy, dims lights, and can keep motors and electronics from starting or running properly.

Try the numbers yourself in the voltage drop calculator, or jump to a common run like wire size for 50 amps at 100 feet.

Based on NEC Ch.9 Table 8 (DC resistance, ~75 °C, uncoated conductors).

Educational estimate only, based on NEC Chapter 9 Table 8 DC resistance. Not a substitute for a licensed electrician or your local code (AHJ). It does not fully model ampacity, temperature derating, power factor, or continuous-load factors. Verify against the current NEC and local code before wiring. Based on NEC Ch.9 Table 8.