Wiring / conductor sizing

Wire gauge and voltage drop

Convert an AWG size into real conductor geometry, then check the load current against ampacity and compute the round-trip voltage drop over the run. Copper and aluminium are both supported.

Load checkWithin limitCopper loss147.46 WResistivity1.724e−8

Conductor area

5.261 mm²

2.588 mm diameter

Voltage drop

4.915 V

2.14% of 230 V

Loop resistance

163.842 mΩ

Over 25.0 m each way

Ampacity

30 A

Load is within the limit

AWG conductor table

Diameter, cross-section and resistance for each common size, with the ampacity used here.

Copper

AWG conductor table
AWG	Diameter	Area	Ω per km	Ampacity
14 AWG	1.628 mm	2.081 mm²	8.285	15 A
12 AWG	2.053 mm	3.309 mm²	5.210	20 A
10 AWG	2.588 mm	5.261 mm²	3.277	30 A
8 AWG	3.264 mm	8.366 mm²	2.061	40 A
6 AWG	4.115 mm	13.302 mm²	1.296	55 A
4 AWG	5.189 mm	21.151 mm²	0.815	70 A
3 AWG	5.827 mm	26.670 mm²	0.646	85 A
2 AWG	6.544 mm	33.631 mm²	0.513	95 A
1 AWG	7.348 mm	42.408 mm²	0.407	110 A
0 AWG	8.251 mm	53.475 mm²	0.322	125 A

Ampacity figures are the 60 °C copper column, derated 22% for aluminium. Installation method and ambient temperature change the real limit.

Voltage drop by run length

Round-trip drop at the selected current and conductor size.

Drop across the AWG range

Each step of three AWG numbers roughly halves the cross-section and doubles the drop.

Canonical equations

The exact relationships implemented by this engine.

d = 0.127·92^((36−n)/39) · n = AWG number

A = πd²/4 · circular mils ÷ 1.273e6 gives mm² too

r = ρ/A · r is resistance per metre; ρ is temperature dependent

V_d = I·R_loop = 2·I·r·L · the factor 2 covers both conductors

Show model notation and computationOpen the symbolic form, the variable definitions, and the step-by-step derivation when you need the detail.

A clear path from inputs to results.

Wire sizing is two separate checks that often disagree: will the conductor carry the current without overheating, and will the voltage still be usable at the far end? This engine computes the geometry from the AWG definition, then runs both checks on the same conductor.

Step-by-step methodWorked exampleAssumptions explained

How to use

Select the AWG size. Lower numbers are thicker.

Choose copper or aluminium.

Enter the load current.

Enter the one-way run length. The drop calculation doubles it for the return conductor.

Enter the supply voltage to get the drop as a percentage.

Check both badges: ampacity for safety, voltage drop for performance.

The formula

d = 0.127 \cdot 92^{(36-n)/39}\ \text{mm}, \qquad A = \frac{\pi d^2}{4}, \qquad R = \frac{\rho L}{A}, \qquad V_d = 2IRL

Reference: ASTM B258 (AWG diameters); NEC Article 310.16 (ampacity tables); IEC 60228 for metric equivalents.

Worked example

10 AWG copper, 30 A, 25 m run at 230 V (default inputs)

Inputs

10 AWG copper.
Load 30 A.
Run length 25 m each way.
Supply 230 V.

Calculation path

1
Diameter: 0.127 × 92^(26/39) = 2.588 mm; area = 5.261 mm².
2
Resistance: 1.724×10⁻⁸ / 5.261×10⁻⁶ = 3.277 Ω per km.
3
Loop resistance over 25 m each way: 0.0819 Ω.
4
Voltage drop: 2 × 30 × 0.0819 = 4.915 V, or 2.14% of 230 V.
5
Ampacity at 60 °C is 30 A, so the load is exactly at the limit.

Result

5.26 mm² · 3.28 Ω/km · 4.92 V drop (2.14%) · 30 A ampacity — within the limit but with no margin.

Interpreting the result

Both checks pass here, but only just. The 2.14% drop is inside the 3% branch-circuit guideline, and the current exactly equals the ampacity column. Real installations need margin: derating for ambient temperature, conduit fill and bundling can cut ampacity by 20% or more, which would make 10 AWG inadequate at 30 A.

Assumptions & limitations

Resistivity is at 20 °C. Copper rises about 0.4% per kelvin, so a conductor at 70 °C has roughly 20% more resistance and more drop.

Ampacity uses the 60 °C copper column, derated 22% for aluminium. Insulation rating and installation method change this materially.

The load is resistive with unity power factor. Reactive loads need the impedance rather than the resistance.

The run is a simple two-wire circuit. Three-phase runs drop the factor of 2 and use √3 instead.

No derating for ambient temperature, conduit fill, or adjacent conductors is applied.

Frequently asked questions

What does AWG actually measure?

Cross-sectional area, encoded logarithmically: each three-gauge step halves the area, and each six-gauge step halves the diameter. The formula reproduces the historical wire-drawing process.

Which limit governs, ampacity or voltage drop?

Ampacity is a safety limit and must never be exceeded. Voltage drop is a performance limit. On short runs ampacity governs; on long runs voltage drop usually does.

Does this handle three-phase?

No — it models a single-phase two-wire circuit. For three-phase, the drop is √3 × I × R × L and the current per conductor is lower for the same power.

Why is aluminium derated?

It has about 61% of copper’s conductivity, so the same cross-section carries less current for the same temperature rise. Aluminium also creeps under terminal pressure, which is why it needs different connectors.

How much voltage drop is acceptable?

Common guidance is 3% for a branch circuit and 5% total from the service entrance. Equipment tolerances and motor starting currents can demand tighter limits.

Why does my calculated drop differ from a cable manufacturer’s table?

Manufacturers quote impedance at operating temperature and include reactance, which matters for large conductors and high currents. This engine uses DC resistance at 20 °C, so it understates drop on big cables.