Wire Gauge Calculator

Circuit type:

Choose the circuit model before sizing the voltage drop.

Load current:

Enter the current that the conductor must carry.

One-way run length:

Use the one-way cable run, not the round-trip total.

System voltage:

Use nominal line voltage for the load, such as 12, 24, 48, 120, or 240 volts.

Maximum voltage drop:

Set the largest acceptable drop from source to load.

Conductor material:

Choose the conductor material for resistance and ampacity checks.

Ampacity temperature column:

Select the conductor temperature rating to use for this planning check.

{{ error }}

Load sizing margin:

Use 100% for the entered load, or raise it when a design margin applies.

Comparison gauge:

Adds a selected-gauge line to the snapshot and JSON without changing the recommendation.

Decimal places:

Use 0-6 places for displayed numbers and exports.

places

Metric	Value	Sizing note	Copy
{{ row.metric }}	{{ row.value }}	{{ row.note }}

Gauge	Ampacity	Voltage drop	Drop percent	Resistance	Status	Copy
{{ row.gauge }}	{{ row.ampacity }}	{{ row.voltageDrop }}	{{ row.dropPercent }}	{{ row.resistance }}	{{ row.status }}

Export to PDF Fullscreen

Embed:

Customize

Include current inputs

Size

Advanced

Width

Height

Aspect ratio

Max height

Collapsible embed

Allow fullscreen

Referrer policy

Sandbox tokens

Wire gauge affects two things that show up quickly in real circuits: how much current a conductor can carry and how much voltage is lost before power reaches the load. A long low-voltage run can need a much larger conductor than the amp reading alone suggests, while a short higher-voltage run may be limited mostly by ampacity.

American Wire Gauge, or AWG, uses smaller numbers for larger conductors. Moving from 12 AWG to 10 AWG increases conductor area and lowers resistance, so the same current loses less voltage over the same distance. The tradeoff is practical: larger wire costs more, takes more room, and may be harder to terminate, but undersized wire can leave a device with weak voltage or an unsafe current margin.

Load current

Amps set the heating and voltage-drop load.

One-way run

The source-to-load distance is expanded by the circuit path.

Voltage limit

The allowed drop sets the maximum loss from source to load.

Gauge choice

A candidate must pass both drop and ampacity checks.

Wire sizing is a two-gate planning check: voltage drop can force a larger conductor even when ampacity already passes.

Voltage drop is especially important for 12 V, 24 V, and 48 V systems because a small number of lost volts can become a large percentage of the supply. The same 0.5 V drop is modest on a 120 V circuit and serious on a 12 V circuit. Three-phase circuits also use a different path multiplier, so the circuit model must match the way the load is supplied.

A gauge result is a planning estimate, not installation approval. Final conductor selection still depends on conductor type, insulation, terminal ratings, overcurrent protection, ambient temperature, raceway fill, bundled conductors, and the electrical rules that apply where the circuit will be installed.

Technical Details:

AWG size controls the conductor diameter and cross-sectional area used for resistance. The calculation treats each candidate as a solid round conductor for geometry and resistance. Stranded wire with the same AWG has the same nominal conductor area, but its outside diameter can be larger because of strand construction and insulation.

Voltage drop follows Ohm's law. The conductor resistance is found from material resistivity, conductor area, temperature, and path length. The voltage lost is current multiplied by path resistance, then divided by nominal system voltage to get a percentage. Ampacity is checked separately against the selected material and temperature column.

Formula Core

The core calculation starts with AWG geometry, then converts that geometry into resistance and voltage drop.

\begin{array}{lcl} AWG diameter in mm & = & 0.127 \times 92^{(36 - n) / 39} \\ Area & = & π \times {(d / 2)}^{2} \\ Resistance per meter & = & ρ (T) / A \\ Voltage drop percent & = & 100 \times I \times R / V \end{array}

Wire gauge formula variables
Symbol	Meaning in this calculator	Important boundary
`n`	AWG index. 18 through 1 are ordinary numbered gauges; 1/0, 2/0, 3/0, and 4/0 use 0, -1, -2, and -3.	The candidate list stops at 4/0.
`I`	Entered load current in amperes.	Must be greater than zero.
`V`	Nominal system voltage used for the percentage drop.	Must be greater than zero.
`R`	Path resistance after material, area, temperature, one-way length, and circuit multiplier are applied.	Uses a two-conductor path for DC and single-phase, and sqrt(3) for three-phase.
`ρ(T)`	Material resistivity adjusted from 20 C to the selected temperature column.	Copper and aluminum use different base resistivity and temperature coefficient values.

Temperature affects the resistance estimate because metallic conductors have higher resistance when warmer. The selected 60 C, 75 C, or 90 C column is used both for the reference ampacity lookup and the resistance temperature adjustment. That makes the result useful for planning, but it does not replace the rule that final ampacity must match the terminal rating and installation conditions.

Wire gauge result gates and boundary rules
Gate or rule	How it is checked	How to read it
Voltage drop	Candidate drop percent must be less than or equal to the maximum voltage drop setting.	A conductor can fail here even when the ampacity column is high enough.
Ampacity	Reference ampacity must be greater than or equal to design current.	Design current is the entered load multiplied by the load sizing margin.
Load sizing margin	Changes only the ampacity design-current check.	Voltage drop still uses the entered load current.
Aluminum small gauges	18, 16, and 14 AWG aluminum candidates have voltage-drop rows but no reference ampacity in this tool.	They cannot become the recommended gauge because the ampacity gate is missing.
Out of range	No listed candidate through 4/0 passes both gates.	The summary reports that the run needs a conductor larger than the listed range or a changed design.

The sizing math runs in the browser after the page is loaded. No separate wire-sizing request is sent for the calculation, and the JSON view is built from the visible result state.

Everyday Use & Decision Guide:

Start with the actual load current, the one-way source-to-load run, and the nominal load voltage. Do not double the length yourself for DC, two-wire AC, or single-phase AC. The circuit type applies the path multiplier, and entering a round-trip length would double-count the run.

For many first-pass checks, a 3% drop limit is a reasonable planning target. Sensitive low-voltage electronics may deserve a tighter limit, while a rough feasibility check may use a wider limit to see what changes. If the summary says Drop-limited, the run length or voltage-drop setting is driving the recommendation more than the ampacity table.

Use the material and temperature column deliberately. Copper and aluminum do not have the same resistance or reference ampacity. The 90 C column can make a conductor look better on paper, but final installation may still be limited by lower-temperature terminals. For ordinary building wiring, the terminal rating and local code review matter as much as the calculator result.

Wire Size Check is the quick audit view for the recommended gauge, design current, circuit path, voltage drop, ampacity, geometry, resistance basis, and planning caution.
Gauge Candidate Ledger is the best place to see why smaller conductors failed and how much margin larger conductors have.
Voltage Drop Map shows the drop percent by gauge with the selected limit marked, making the drop boundary easier to spot.
JSON preserves the inputs, recommendation, selected comparison gauge, validation errors, and chart rows for later review.

The comparison gauge is useful when you already have a conductor size in mind. It does not change the recommendation. It adds a selected-gauge line so you can see whether that known size passes, fails voltage drop, fails ampacity, or lacks a reference ampacity.

Step-by-Step Guide:

Use the controls in this order so the recommendation and ledger describe the same circuit.

Choose Circuit type first. Pick DC or two-wire AC, Single-phase AC, or Three-phase AC so the path multiplier matches the circuit.
Enter Load current, One-way run length, and System voltage. A result appears only when current, length, and voltage are greater than zero.
Set Maximum voltage drop between 0.1% and 20%, then choose Conductor material and the Ampacity temperature column.
Open Advanced if the ampacity check needs a load sizing margin, if you want to inspect a known comparison gauge, or if the displayed decimal places need adjustment.
Read the summary first. Recommended wire gauge means the smallest listed candidate passed both gates; No listed AWG meets this run means the listed range through 4/0 did not pass.
Open Wire Size Check for the sizing note and caution, then use Gauge Candidate Ledger or Voltage Drop Map to see the boundary between failing and passing gauges.
If a validation message appears, fix the exact field named in the alert, such as a zero current, zero length, zero voltage, or voltage-drop limit outside the accepted range.

Interpreting Results:

The recommended gauge is the smallest listed conductor that passes both the reference ampacity check and the selected voltage-drop limit. Smaller conductors may still have enough ampacity, but fail because their resistance loses too much voltage over the run.

Read the voltage-drop percent before treating the gauge as settled. A recommendation just under the limit is more sensitive to measurement error, actual conductor temperature, and load changes than one with broad headroom. The Drop-limited and Ampacity-limited badges identify which gate is closest to the boundary.

How to interpret wire gauge calculator outputs
Output cue	What it means	What to verify next
Recommended wire gauge	The first listed AWG candidate passed both ampacity and voltage-drop checks.	Confirm conductor type, terminal temperature, and overcurrent protection.
Voltage drop	The predicted voltage loss and percentage at the entered load current.	Check that one-way length and nominal voltage are correct.
Reference ampacity	The lookup value for the selected material and temperature column.	Apply required derating for ambient temperature, bundled conductors, conduit fill, and local rules.
Comparison gauge	The selected known gauge is reported without changing the recommendation.	Use its status line to see whether the known size actually passes.
Larger than 4/0	No candidate in the built-in list passed both gates.	Reduce current, shorten the run, raise system voltage, use parallel conductors where allowed, or get a formal design.

A passing result does not certify a circuit. It means the entered planning model passed the two checks this calculator performs. Installation details outside those checks can still require a larger conductor or a different wiring method.

Worked Examples:

Short 12 V DC run

A 10 A load on a 15 ft one-way copper run at 12 V, with a 3% drop limit and the 75 C column, recommends AWG 8. The result shows about 0.23 V of drop, or 1.91%, with 50 A reference ampacity. In the ledger, AWG 10 is close but still fails the selected drop limit at about 3.04%.

Low-voltage run with a margin

A 20 A load over 30 ft one-way at 12 V, copper, 75 C, and a 125% load sizing margin produces a Design current of 25 A. The recommendation becomes AWG 3, with about 0.29 V of drop, or 2.40%. AWG 4 has enough ampacity but misses the 3% voltage-drop limit by a small amount, so the larger conductor is selected.

Three-phase aluminum feeder check

A 40 A three-phase load over 45 m one-way at 400 V, aluminum, 75 C, and a 3% drop limit recommends AWG 6. The ledger reports about 8.07 V of drop, or 2.02%, with 50 A reference ampacity. AWG 8 has 40 A ampacity but fails the voltage-drop gate at about 3.21%.

Run outside the listed range

An 80 A load over 100 ft one-way at 12 V with copper and a 3% drop limit returns Larger than 4/0. Even 4/0 stays near 7.95% drop in this model. That result points to a design change rather than a small gauge adjustment: shorten the run, reduce current, increase voltage, or use a formal conductor plan.

FAQ:

Should I enter one-way length or round-trip length?

Enter one-way length. The calculator applies the out-and-back path for DC, two-wire AC, and single-phase AC, and it applies the three-phase path multiplier when that circuit type is selected.

Why did a larger wire become necessary when ampacity already passed?

Voltage drop and ampacity are separate checks. A conductor can carry the current according to the reference ampacity column but still lose too much voltage over a long or low-voltage run.

What does load sizing margin change?

It changes the design current used for the ampacity pass/fail check. The voltage-drop calculation still uses the entered load current, so a 125% margin does not inflate the voltage-drop volts.

Why do some aluminum gauges show no reference ampacity?

The built-in aluminum reference table starts at 12 AWG. Smaller aluminum candidates may still appear in the voltage-drop ledger, but they cannot pass the recommendation gate without a reference ampacity.

Does a passing result mean the conductor is code-compliant?

No. The result checks voltage drop and a reference ampacity column. Final selection still needs conductor insulation, terminal temperature, ambient correction, bundling, raceway conditions, overcurrent protection, and applicable code review.

Why did I get an input error?

Current, one-way length, and system voltage must be greater than zero. Maximum voltage drop must be between 0.1% and 20%, and the material and circuit type must be supported choices.

Glossary:

AWG: American Wire Gauge, a stepped gauge system where smaller numbers mean larger conductors.
Ampacity: The current a conductor can carry under stated conditions of use.
Voltage drop: The voltage lost across conductor resistance while current flows to the load.
One-way run: The source-to-load cable length before the circuit path multiplier is applied.
Design current: The entered load current after the load sizing margin is applied for ampacity checking.
Path resistance: The total resistance of the conductor path used in the voltage-drop calculation.

References:

American Wire Gauge reference for the AWG diameter relation and gauge notation.
NIST Circular 31 copper wire tables for copper and aluminum resistivity context.
Copper Development Association wire installation guide for voltage-drop planning context.