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Wire Gauge & Ampacity Reference

Look up AWG wire properties and ampacity, size a gauge for a target current, and estimate voltage drop over a run.

Reference Calculator

Select by
Ampacity rating

Voltage drop over a run

Gauge
AWG 12
Ampacity
41 A chassis rating
Cross-section
3.31 mm²
Diameter
2.053 mm
Resistance
5.211 Ω/km copper @ 20 °C

Voltage drop

Voltage drop
1.04 V
Drop
8.7 %
Voltage at load
11 V
Conductor resistance
104 mΩ round-trip
Power lost in wire
10.4 W

Reference only

Ampacity depends on insulation, bundling, ambient temperature and duty. These figures are a design guide for copper — always defer to the applicable wiring code.

How Wire Gauge & Ampacity Reference works

American Wire Gauge is a geometric series, not an arbitrary table. Each step down in gauge number multiplies the diameter by about 1.123, which means six gauge steps double the diameter and roughly quadruple the cross-sectional area. Larger numbers mean thinner wire — a legacy of gauge originally counting drawing operations.

Resistance follows directly from geometry and material: it is proportional to length and inversely proportional to cross-sectional area, scaled by the material’s resistivity. Copper is about 1.68 × 10⁻⁸ Ω·m at 20 °C; aluminium is roughly 60% as conductive, which is why aluminium conductors need to be about two gauge sizes larger for the same resistance.

Ampacity — the current a conductor can carry continuously — is a thermal limit, not an electrical one. It depends on how fast heat escapes, so it varies with insulation temperature rating, ambient temperature, whether the conductor is in free air or bundled in a conduit, and how many current-carrying conductors share that space. This is why published ampacity tables always come with correction factors, and why a single figure per gauge is only ever a starting point.

For long runs, voltage drop usually governs the choice rather than heating. A conductor can be thermally comfortable while dropping enough voltage to leave the load out of specification. The common design target is to keep drop under 3% for a branch circuit and 5% overall — and because current flows out and back, the calculation must use twice the one-way distance.

Reference

  • Diameter (in) = 0.005 × 92^((36 − n) / 39) for AWG n
  • Area doubles every 3 gauges; diameter doubles every 6
  • R = ρ × L / A (copper ρ ≈ 1.68 × 10⁻⁸ Ω·m at 20 °C)
  • Voltage drop = I × R × 2 × one-way length
  • Power lost in the run = I² × R_total

How to use this reference

  1. Look up a gauge

    Select an AWG size to read its diameter, cross-sectional area, resistance per unit length, and typical ampacity.

  2. Or size for a current

    Enter your continuous current and let the tool suggest the smallest gauge that carries it thermally.

  3. Enter the run length

    Give the one-way distance — the calculation doubles it, since current returns along a second conductor.

  4. Check the voltage drop

    Compare the drop against your budget. On long runs this, not heating, usually decides the gauge.

Worked examples

The gauge-to-area rule

Given
AWG 10 versus AWG 16
Result
Diameter 2.59 mm versus 1.29 mm; area 5.26 mm² versus 1.31 mm²

Six gauges apart: exactly double the diameter and four times the area, as the geometric definition requires.

A 12 V run to a light

Given
10 A over 15 m one-way in AWG 14 copper
Result
About 2.5 V dropped — over 20% of the supply

The wire is thermally fine at 10 A. It is the voltage drop that makes this unworkable, which is typical of low-voltage DC runs.

Sizing to fix that drop

Given
The same 10 A and 15 m, targeting under 3%
Result
AWG 8 or larger

Three gauge steps up doubles the area and halves the drop each time. Low-voltage systems are almost always drop-limited.

When to use it

  • Choosing a conductor size for a DC power run in a vehicle, boat, or solar installation.
  • Checking whether an existing cable is adequate for a load you intend to add.
  • Estimating the voltage the load will actually see at the end of a long run.
  • Converting between AWG and the metric mm² sizes used outside North America.
  • Calculating power lost as heat in a distribution run.

Things to watch out for

  • Ampacity figures are estimates that depend on insulation rating, ambient temperature, bundling, and installation method. For anything mains-connected, follow your local electrical code — NEC, IEC, or national equivalent — not a general table.
  • Voltage drop, not heating, governs most long runs. Size for the drop and the thermal limit will usually take care of itself.
  • Copper resistance rises about 0.4% per °C. A conductor running hot has measurably more resistance than the 20 °C figure implies.
  • At mains frequencies the skin effect is negligible below roughly AWG 4/0, but it matters for large conductors and at high frequency, where the effective area shrinks.

Frequently asked questions

How does AWG numbering work?

It is a geometric series where larger numbers mean thinner wire. Each gauge step changes the diameter by a factor of about 1.123, so three steps double the cross-sectional area and six steps double the diameter. AWG 10 is exactly twice the diameter of AWG 16.

What gauge do I need for a given current?

Start from a published ampacity table for your insulation type and installation method, then check voltage drop over your actual run length. For anything longer than a few metres at low voltage, the drop almost always demands a larger conductor than heating alone would.

Why must I double the run length?

Because current flows out along one conductor and back along another. A 15 m run is 30 m of conductor in the circuit, and voltage drops in both directions. Halving the distance in the calculation is a very common and very consequential error.

How much voltage drop is acceptable?

The usual targets are 3% for a branch circuit and 5% total from source to load. Low-voltage DC systems are far less forgiving in relative terms: 2.5 V lost from a 12 V supply is over 20%, while the same loss from 230 V is trivial.

Can I use aluminium instead of copper?

Yes, and utilities do at scale for weight and cost. Aluminium is about 61% as conductive, so you need roughly two gauge sizes larger for equivalent resistance. It also creeps under pressure and oxidises, so terminations must be rated for aluminium.

How do AWG and mm² compare?

AWG 14 is about 2.08 mm², AWG 12 about 3.31 mm², and AWG 10 about 5.26 mm². The two systems do not line up on round numbers, so always convert by area rather than assuming a nearest equivalent is exact.

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