Free Wire Size Calculator

Selecting the Right Wire Gauge: A Complete Electrical Cable Sizing Tool

Choosing the correct wire thickness is critical for any electrical project—whether you're running power to a garden pond pump, wiring a tiny house, or connecting a shed. Undersized cables cause voltage drops, overheating, and potential safety hazards. This versatile wire gauge calculator handles both DC and AC systems, including single‑phase and three‑phase setups, 12 V low‑voltage lighting, and more. It replaces bulky wire size charts and gives you the recommended AWG (American Wire Gauge) or cross‑sectional area in mm².

How Wire Size Is Determined

The size of a wire refers to its cross‑sectional area. A larger area reduces resistance, allowing more current to flow with less voltage loss—similar to water moving through a wide pipe. The basic relationship comes from Ohm’s law and Pouillet’s law:

$$V = I R \qquad R = \frac{\rho L}{A}$$

where:
$V$ = voltage drop (V),
$I$ = peak current (A),
$R$ = wire resistance (Ω),
$\rho$ = electrical resistivity of the conductor (Ω·m),
$L$ = one‑way cable length (m),
$A$ = cross‑sectional area (m²).

Combining the two gives the required area:

$$A = \frac{\rho \, I \, L}{V}$$

Because square meters are impractical for wiring, the result is multiplied by $1\,000\,000$ to express it in mm². The calculator then matches this area to the nearest AWG size (always rounding up to the next thicker gauge for safety).

Adjusting for AC Three‑Phase Systems

For three‑phase circuits, three cables carry the load instead of one. No return conductor exists, so the factor of 2 disappears, and a $\sqrt{3}$ term accounts for the relationship between line and phase values:

$$A = \frac{\sqrt{3} \;\rho \;I \;L}{V}$$

Whether you need a DC wire size calculator, an AC single‑phase tool, or a three‑phase wire sizing helper, this calculator adapts automatically.

Temperature Effects on Conductor Resistivity

Resistivity changes with temperature. Copper at 20 °C has $\rho_1 = 1.68 \times 10^{-8}\; \Omega·\text{m}$ and a temperature coefficient $\alpha = 0.00404$. To find resistivity at another temperature $t_2$:

$$\rho_2 = \rho_1 \left[ 1 + \alpha (t_2 - t_1) \right]$$

For example, copper at 50 °C reaches about $2.0 \times 10^{-8}\; \Omega·\text{m}$. The calculator lets you input the maximum expected operating temperature so the wire size accounts for real‑world conditions.

Step‑by‑Step Usage Guide

1. Select the electrical system – DC, AC single‑phase, or AC three‑phase.
2. Enter the source voltage (e.g., 12 V, 120 V, 230 V).
3. Set the allowable voltage drop – typically 3 % for efficient operation; 5 % is the upper limit.
4. Choose the conductor material – copper or aluminum.
5. Input the peak current – use the highest expected load.
6. Specify the one‑way cable run (distance from source to farthest point).
7. Adjust the operating temperature if it differs from the default 20 °C.

The tool outputs the required cross‑sectional area, the corresponding AWG gauge, and the wire diameter. It also displays the unit in circular mils (cmil) or kilo circular mils (kcmil) for convenience (1 cmil ≈ $5.067 \times 10^{-4}\,\text{mm}^2$).

Practical Example

Suppose you have an AC single‑phase system at 120 V, 3 % allowable drop (3.6 V), copper conductor, 25 A peak current, a 100 m cable run, and a maximum ambient temperature of 50 °C. Using the resistivity calculated for copper at 50 °C, the required cross‑sectional area is:

$$A = \frac{\rho \, I \, L}{V}$$

With $\rho \approx 2.0 \times 10^{-8}\,\Omega·\text{m}$, $A \approx 13.1\,\text{mm}^2$. The calculator picks the next standard AWG size—typically 6 AWG—and also gives the value in kcmil. This result ensures the cable stays within the voltage drop limit and operates safely at the expected temperature.

Important: All results are informational guides. Always consult a qualified electrician and follow local codes before any electrical installation.