Wire Gauge Calculator

A 12 V cable run: why length changes the answer

Suppose a 12 V supply feeds a 10 A load through 3 metres of copper cable. With an equal return path, current travels through 6 metres of conductor. At a 3% drop limit, the cable may lose at most 0.36 V.

At a conductor temperature of 20 °C, the model requires about 2.874 mm² of metal. AWG 13 has about 2.624 mm², so it misses that target. AWG 12 has about 3.309 mm² and meets it. Its calculated loop resistance is about 0.03126 Ω, giving a drop of about 0.3126 V, or 2.605%. The load receives about 11.687 V and the two conductors dissipate about 3.126 W together.

Double the one-way run to 6 metres and the required area doubles. The answer moves to AWG 9, about 6.634 mm². If that size is unavailable, use an available larger conductor after checking the installation requirements. A lower AWG number means a larger cross-section.

The 12v wire gauge calculator preset is an example, not a vehicle or marine installation specification. A chassis return, connectors, battery sag, or unequal return cable adds effects outside this equal-conductor model. The 24 V preset uses the same physics with a larger allowed loss in volts for the same percentage.

What the wire gauge size result tells you

This wire gauge length calculator chooses from every whole AWG size between 40 and 4/0. It sizes for voltage drop only. A short run can therefore produce a wire that is too thin to carry the load safely even though the calculated drop is small. Check ampacity, overcurrent protection, insulation, terminals, bundling and the applicable installation rules separately before selecting a cable.

The allowed-drop percentage is your design input. The default 3% is an example target, not a claim about a universal legal limit. If the required cross-section exceeds 4/0, the tool reports that limit rather than silently returning an undersized cable.

For a DC load specified in watts, first determine the actual cable current. For an ideal constant-power load, current is power divided by voltage at the load; supply voltage alone may understate current when cable loss is significant. Starting current and equipment requirements can also control cable selection.

AWG conversion: diameter and area measure different things

Choose Convert AWG to use the wire gauge conversion calculator. AWG 36 is defined here by a 0.127 mm bare diameter, giving an area of about 0.012668 mm². The scale is geometric: the diameter ratio between adjacent sizes is the 39th root of 92. NBS Handbook 100, section 2.2 explains this relationship.

For the larger sizes, enter 0 for 1/0, −1 for 2/0, −2 for 3/0, and −3 for 4/0. Fractional AWG numbers are rejected. The displayed diameter is an equivalent solid bare conductor diameter. A stranded bundle and its insulation have larger outside dimensions; measuring the jacket does not give the metal cross-section. SWG is a different gauge system and is not supported here.

How resistance and voltage drop are calculated

For gauge number $n$, bare diameter $d$ in millimetres and area $A$ in square millimetres are:

For one-way length $L$ in metres, current $I$ in amperes, and material resistivity $\rho$ in Ω·mm²/m:

Required area follows by rearranging the drop equation: $A_{required}=2\rho LI/\Delta V_{allowed}$. The tool selects the smallest supported conductor whose area is at least that value. All calculations use full precision; displayed dimensions and electrical values use six significant digits.

Resistance rises with conductor temperature. The linear model uses $\rho_T=\rho_{20}[1+\alpha(T-20)]$, with copper values 0.017241 Ω·mm²/m and 0.00393/°C, and electrical-grade aluminum values 0.028264 Ω·mm²/m and 0.00403/°C. These reference values come from NBS Handbook 100 and NBS Handbook 109, table 1. Real cable construction and alloy can change resistance; use manufacturer data for the final design.

Enter the expected conductor temperature, not simply the surrounding air temperature. This model accepts −20 to 100 °C to estimate resistance; it does not predict self-heating or approve an insulation temperature rating. AC reactance, power factor, skin effect, and three-phase circuits are outside the DC model.