Free Wire Size Calculator Online

Wire Size Calculator: Find a Recommended AWG Gauge for Amperage and Voltage Drop

A wire size calculator takes a circuit's current, the length of the run, the source voltage, and the conductor material, then works out the smallest standard wire gauge that can carry that load safely and without an excessive voltage drop over the distance. This tool checks two separate conditions at once — how much current the wire can carry (ampacity) and how much voltage the wire loses to resistance over its length (voltage drop) — and returns the smallest American Wire Gauge (AWG) size that clears both. Everything runs as JavaScript in the page itself, so a result appears the instant you change a field, with no upload and no server round trip involved.

Picking a wire gauge by guesswork is a common source of two very different failures: a wire too thin for the current can overheat inside a wall, and a wire that's technically rated for the amperage but stretched over a long run can still lose enough voltage that a motor stalls, a light dims, or an appliance underperforms. This calculator exists to catch both problems in a single pass rather than checking ampacity and voltage drop separately and hoping the two answers line up.

How to Use the Wire Size Calculator

1. Choose Single Phase or Three Phase for the circuit — this changes the voltage-drop formula's multiplying factor, described below.
2. Choose the conductor material: Copper or Aluminum. Aluminum conductors have higher resistance per foot, so the calculator swaps in a different resistance value and a different ampacity ceiling for each gauge.
3. Enter the Current the circuit will carry, in amps.
4. Enter the One-Way Length of the run, in feet — measured from the source to the load in one direction, not the round trip.
5. Enter the Source Voltage, such as 120 or 240.
6. Optionally adjust Max Drop %, the largest voltage-drop percentage you're willing to accept. It defaults to 3%, a figure commonly cited as a reasonable target for branch circuits.
7. The page immediately shows the recommended AWG size, along with that gauge's ampacity rating, the calculated voltage drop in volts, and the drop as a percentage of the source voltage.

If no standard gauge between 14 AWG and 4/0 satisfies both checks for the numbers you entered, the tool reports that directly instead of quietly picking an undersized wire — at that point the fix is usually to shorten the run, raise the source voltage, split the load across two circuits, or loosen the maximum drop percentage.

Why Wire Gauge Selection Matters

• Planning a circuit before pulling wire. Knowing roughly which gauge you'll need before a supply run lets you budget material cost and confirm the conduit or raceway you're using can physically fit the conductor.
• Long outdoor or detached-structure runs. A wire feeding a detached garage, a shed, a well pump, or an RV pad might be well within its ampacity rating and still drop several volts simply because the run is long — this is exactly the scenario voltage-drop checking is meant to catch.
• Comparing copper against aluminum. Aluminum conductors are lighter and often cheaper per foot for larger gauges, but because they carry roughly 1.6 times the resistance of copper at the same gauge, a run that's fine in copper may need to jump up a size (or two) in aluminum to hit the same voltage-drop target.
• Sanity-checking a size someone else specified. If a contractor, an online forum, or an old set of plans lists a gauge, plugging the same current, length, and voltage into the calculator gives you a quick second opinion on whether that number still holds for your setup.
• Understanding why a longer extension cord or feeder underperforms. A tool, pump, or heater running noticeably weaker at the end of a long cord is frequently a voltage-drop symptom rather than a fault with the device itself.

How the Recommendation Is Calculated

The calculator works through the standard AWG sizes from smallest to largest — 14, 12, 10, 8, 6, 4, 3, 2, 1, 1/0, 2/0, 3/0, then 4/0 — and stops at the first one that passes both tests below.

The first test is ampacity: does this gauge's current rating meet or exceed the current you entered? Ampacity here comes from a 60°C column styled after NEC Table 310.16, assuming no more than three current-carrying conductors bundled together and a 30°C ambient temperature — a commonly cited baseline that lines up with familiar residential breaker sizing, such as 15A for 14 AWG and 20A for 12 AWG.

The second test is voltage drop, using the same formula as this site's voltage-drop calculator: for single-phase circuits, Vdrop = 2 × L × I × R / 1000, where L is the one-way length in feet, I is the current in amps, and R is the conductor's resistance in ohms per 1000 feet at that gauge. The factor of 2 accounts for current traveling out to the load and back again on the return conductor. Three-phase circuits use Vdrop = √3 × L × I × R / 1000 instead, without the round-trip doubling, which is the standard adjustment for a balanced three-phase load. The resulting drop is then expressed as a percentage of the source voltage and compared against the Max Drop % target.

The table below lists the exact ampacity and resistance figures the calculator draws from for each gauge.

AWG	Copper ampacity (60°C)	Aluminum ampacity (60°C)	Copper resistance (Ω/1000 ft)
14	15A	not standard for branch circuits	2.525
12	20A	15A	1.588
10	30A	25A	0.999
8	40A	30A	0.628
6	55A	40A	0.395
4	70A	55A	0.249
3	85A	65A	0.197
2	95A	75A	0.156
1	110A	85A	0.124
1/0	125A	100A	0.0983
2/0	145A	115A	0.0779
3/0	165A	130A	0.0618
4/0	195A	150A	0.049

Aluminum's resistance figure is never a separate hand-typed table — it's always the copper value in the table above multiplied by 1.6, a widely used approximation for how much more resistive aluminum is at a given gauge compared to copper. Note that 14 AWG aluminum has no entry at all, since aluminum conductors that thin aren't a standard or permitted choice for branch-circuit wiring, so the calculator skips straight past that row when material is set to aluminum.

Worked Example: A 20-Amp, 100-Foot, 120-Volt Run

Take a 20A load, 100 feet one-way, on a 120V single-phase circuit, copper conductor, with the default 3% maximum drop. Checking 12 AWG first: its ampacity is 20A, which just barely meets the 20A load, so it passes the first test. But its voltage drop is 2 × 100 × 20 × 1.588 / 1000 = 6.35 volts, which is 6.35 / 120 = 5.29% of the source voltage — well over the 3% target, so 12 AWG fails the second test despite passing the first.

Moving up to 10 AWG: ampacity is 30A (fine), but the drop is 2 × 100 × 20 × 0.999 / 1000 = 3.996 volts, or 3.33% — still above 3%. Moving up again to 8 AWG: ampacity is 40A (fine), and the drop is 2 × 100 × 20 × 0.628 / 1000 = 2.512 volts, which rounds to 2.51 volts, or 2.51 / 120 = 2.09% — under the 3% target. So the calculator recommends 8 AWG for this run, even though 12 AWG is technically rated to carry 20 amps on its own. This is the entire point of checking both conditions together: ampacity alone would have suggested a wire two sizes smaller than what actually keeps the voltage drop in a reasonable range over that distance.

This Tool Produces a Planning Estimate — It Does Not Replace an Electrician

Every number this calculator returns is an estimate meant for early planning and for learning how gauge, ampacity, and voltage drop interact — it is not a substitute for a real electrical design and it should never be treated as the final word for a live installation. Undersized wire is a fire hazard: a conductor forced to carry more current than it can safely dissipate as heat can overheat inside a wall, a conduit, or an enclosure, potentially damaging insulation and creating an ignition risk long before a breaker ever trips. Real wire sizing must always follow the electrical code that applies where the work is being done — the National Electrical Code (NEC) in the United States, or your local equivalent — and any circuit you actually intend to install should be reviewed and installed by a licensed electrician who can account for ambient temperature, conduit fill, number of conductors, terminal ratings, and every other local requirement this simplified calculator does not model. Treat the output here as a starting conversation, not a finished plan.

Assumptions Worth Knowing Before You Rely on a Result

The ampacity figures assume a 60°C temperature rating, no more than three current-carrying conductors bundled in the same raceway, and a 30°C ambient — change any of those conditions (a hotter attic, a bundle of six conductors, a higher-temperature insulation rating) and the real permitted ampacity for a given gauge can be different from what's shown here. The resistance table is measured at 20°C for solid copper wire; stranded conductors and different operating temperatures shift resistance slightly. The 3% default drop target is a commonly cited planning guideline rather than a fixed legal limit, and some circuits — long feeders in particular — are sometimes designed to a combined service-plus-feeder target instead. None of these caveats make the tool less useful for sketching out a plan; they simply mark the line between "good starting estimate" and "code-compliant final design."

Why did the calculator recommend a bigger wire than the breaker size would suggest?

Because ampacity and voltage drop are two independent limits, and whichever one is stricter for your specific length and voltage wins. A 20A breaker only tells you the wire must be rated for at least 20A — it says nothing about how far that circuit runs. Once the run gets long enough, voltage drop becomes the binding constraint and pushes the recommendation up past what ampacity alone would require.

Does the one-way length include the return path?

No — enter only the distance from the source to the load in one direction. The single-phase formula already multiplies that length by 2 internally to account for the conductor running out and the neutral or return conductor running back; entering a round-trip length would double-count that factor and inflate the drop estimate.

Should I use single-phase or three-phase for a typical home circuit?

Residential branch circuits in North America are almost always single-phase, so that's the correct default for household outlets, lighting, and most appliance circuits. Three-phase applies to certain commercial, industrial, or larger mechanical loads where the service itself is three-phase.

Why is the aluminum ampacity lower than copper at the same gauge?

Aluminum has a lower conductivity than copper, so a given cross-sectional area of aluminum can't dissipate heat from the same current as safely as the equivalent copper size — the ampacity tables reflect that by rating each aluminum gauge lower than its copper counterpart, which is also why aluminum installations commonly step up one or two gauges compared to an equivalent copper circuit.

Related Electrical Tools

If you already know the wire gauge and just need the resulting voltage drop and end voltage for a run, the voltage drop calculator applies the same formula directly against a gauge you specify. For working out any single unknown among voltage, current, resistance, and power on a circuit, the Ohm's law calculator covers the underlying relationships this tool builds on. Once a circuit is sized and running, the electricity cost calculator turns a device's wattage and daily runtime into an estimated monthly or yearly cost.