🔌 Wire Size Calculator

The Right Wire Size Is Two Questions, Not One

Choosing a wire gauge feels like it should be one lookup, but it is really two. The wire has to carry the current without overheating, and it has to deliver that current over the distance without the voltage sagging too far. The correct size is whichever of those two answers is larger.

The first question is ampacity — the current a conductor can carry continuously without overheating, fixed by the NEC for each size, material and temperature rating. The second is voltage drop — the voltage lost to the wire's resistance over the run, which grows with distance and current. A gauge can be perfectly safe for the amps and still drop too much voltage over a long pull.

This Wire Size Calculator answers both at once. Enter the load current, voltage, one-way distance and material; it finds the smallest AWG that meets the NEC ampacity table and the smallest that stays under your voltage-drop limit, then recommends whichever is larger — the size that satisfies both rules.

How the Wire Size Calculator Works

Fill in the circuit, click once. Here is what each field does and how the recommendation is reached.

How Wire Sizing Works: Ampacity and Voltage Drop

Two independent checks decide the gauge. The calculator runs both and takes the larger result.

Check 1 — ampacity. Every wire size has an ampacity: the current it can carry continuously without its insulation overheating. The NEC fixes these values in Table 310.16 for each material and temperature rating. The calculator scans from the smallest gauge up and picks the first one whose ampacity meets your load current. That is the safety floor.

Check 2 — voltage drop. Drop is Ohm's law over the run: 2 × current × resistance-per-foot × one-way distance for DC and single-phase (the 2 covers the return conductor), or √3 × the rest for three-phase. The calculator finds the smallest gauge whose drop stays under your limit, expressed as a percentage of the supply voltage.

Taking the larger. The two checks rarely agree. Short runs are limited by ampacity; long runs by voltage drop. The recommended size is whichever is bigger, because the wire must clear both hurdles. The result panel labels which one was binding so you can see why.

A worked example. A 50-amp load at 240 V, copper, 100 feet one-way. Ampacity needs 8 AWG (rated 50 A at 75°C). Voltage drop on 8 AWG is 2 × 50 × 0.628 × (100 ÷ 1000) = 6.28 V, or 2.6% of 240 V — under the 3% limit. Both checks land on 8 AWG, so 8 AWG is the answer.

Parallel conductor sets change the drop term. Running two identical sets side by side halves the effective resistance, which halves the drop — useful on large feeders. Enable Advanced mode to set the number of sets; the calculator divides the resistance accordingly before checking the limit.

Wire Size by Amps & Ampacity Charts

The quick lookup most people want first — wire size by amperage — plus the ampacity and resistance values the calculator works from.

Wire Size by Amperage (Copper & Aluminum, 75°C)

These are NEC Table 310.16 sizes at the 75°C column, with the small-conductor limits of NEC 240.4(D) applied (14 AWG max 15 A, 12 AWG max 20 A, 10 AWG max 30 A copper) — the size needed to carry the current on a short run. Long runs may need a larger gauge for voltage drop, which is what the calculator above checks. Note that dwelling service and feeder conductors may use the reduced sizes allowed by NEC 310.12 (for example, 4 AWG copper or 2 AWG aluminum for a 100-amp dwelling service).

Conductor Resistance (Ω per 1000 ft)

Reading the resistance table: a smaller AWG number is a thicker wire with lower resistance, so it drops less voltage. Aluminum runs about 1.6× the resistance of copper for the same size — which is why aluminum usually lands one to two sizes larger than copper for the same job.

Voltage Drop Limits

Where Voltage Drop Takes Over from Ampacity

Up to the listed distance, ampacity sets the size. Beyond it, the 3% voltage-drop limit forces a larger gauge. Higher voltage stretches that crossover much farther out — and low-voltage DC reaches it almost immediately, which is why solar and automotive runs need heavy wire.

What Determines the Wire Size You Need

Eight variables decide the gauge. The first three set the ampacity floor; the rest swing the voltage-drop result.

Copper vs Aluminum & Phase Types

Two choices shape the result before you touch a number: the conductor metal, and the type of electrical system.

How to Size Wire for a Run: A Phase-by-Phase Roadmap

Picking a wire size is two questions answered in order — can it carry the current, and can it keep the drop in check. Here is the path.

The Cost of the Wrong Wire Size

Undersize and you pay in heat and hardware; oversize and you pay in copper. The right size is the cheapest path over the life of the circuit.

An undersized wire that still passes ampacity can lose real voltage over distance — energy turned into heat instead of useful work, billed year after year for nothing but a warm cable. Worse, a wire that is genuinely too small for its load runs hot, ages its insulation early, and trips breakers. Sizing it right the first time avoids both.

The opposite mistake costs too. Copper is expensive, and jumping two sizes "to be safe" on every run adds up fast across a job. The goal is not the biggest wire — it is the smallest wire that clears both the ampacity and voltage-drop limits, which is exactly what the calculator returns.

Real Wire-Sizing Examples

Three circuits, three different binding constraints — each worked through with verified math so you can check your own.

Wire-Sizing Best Practices

Good wire sizing is a handful of habits applied before the cable is pulled. These are the ones that matter most.

The first habit is checking drop on every run that is even moderately long. Short circuits rarely have a problem, so they get a pass — but a run of fifty feet or more, especially at 120 volts, deserves a quick calculation. Catching a marginal circuit on paper is free; catching it after the wall is closed is not.

The second is treating ampacity and voltage drop as two separate hurdles, both of which the wire must clear. A gauge can be perfectly safe for the current and still drop too much voltage over distance. When the two rules disagree, the larger wire wins — that is the size that satisfies both.

Six Habits for Sizing Wire Right

When This Calculator Is the Wrong Tool

The ampacity and drop math here is exact, but a real installation has factors a quick calculation does not capture. Here is where the output needs an expert's eye.

1. It uses resistance only, not full impedance. On AC circuits, conductors also have reactance, and in steel conduit that reactance is significant. This calculator works from DC resistance, which is accurate for most planning but slightly optimistic for large AC conductors in metallic conduit.

2. It does not apply derating. Ampacity here comes straight from NEC Table 310.16. It does not reduce for more than three current-carrying conductors in a raceway, high ambient temperature, or conduit fill — all of which can require a larger conductor. On bundled or hot installs, apply the derating factors.

3. Resistance values are at a reference temperature. Conductor resistance rises as the wire heats under load. The table values assume a standard temperature, so a hot circuit will drop slightly more voltage than calculated.

4. It assumes a balanced, steady load. The math is for a constant current draw. Motors drawing heavy startup current, or unbalanced three-phase loads, behave differently and need a more detailed analysis.

Where to go instead: for code-critical work, a licensed electrician and the full NEC are the authority — including derating, Chapter 9 Table 9 for AC impedance in conduit, and the dwelling-service allowances of 310.12. This calculator is a fast, accurate first pass for sizing wire by ampacity and voltage drop, not a substitute for a code-compliant design.

Electrical Terms You'll See On This Page

Quick reference for the electrical terms used throughout this wire size calculator.

Voltage Drop

The voltage lost in a conductor as current flows through its resistance, between the source and the load.

Resistance

A conductor's opposition to current flow, measured in ohms. It rises with length and falls with wire size.

AWG

American Wire Gauge — the standard sizing system for wire. A smaller AWG number means a thicker wire.

Ampacity

The maximum current a conductor can carry continuously without overheating, fixed by NEC Table 310.16 for each size, material and temperature rating. It sets the minimum wire size for a given load.

Termination Rating

The temperature rating of the terminals a wire lands on — 60, 75 or 90°C. Conductor sizing uses the column matching the lowest-rated component, usually 75°C.

Ohm's Law

The relationship V = I × R — voltage equals current times resistance — the basis of the drop formula.

One-Way Distance

The length of the run measured in a single direction, from the source to the load.

Phase Factor

The multiplier in the drop formula — 2 for DC and single-phase, 1.732 for three-phase.

Conductor

The metal wire that carries current — usually copper or aluminum.

NEC

The National Electrical Code, the US standard for electrical installation, including voltage drop guidance.

Parallel Sets

Two or more identical conductor runs wired side by side to share current and lower effective resistance.

Branch Circuit

The final wiring run between the last overcurrent device and the load it serves.

Feeder

A circuit between the service equipment and a branch-circuit panel further down the line.

Load Current

The current, in amps, that the connected equipment draws from the circuit.

Voltage at Load

The voltage actually reaching the equipment — the source voltage minus the voltage drop.

Frequently Asked Questions

The most common questions about sizing electrical wire.

The Electrical Cluster & More

Wire sizing is one part of an electrical job. Start with its sibling calculator and the guides behind the numbers, then the rest of the build.