Wire gauge:
why the amps alone never settle the size

Two tests, not one

Asking what size wire carries 15 amps sounds like it has one answer. It does not, because there are two separate failures to avoid. The first is overheating: too small a conductor warms up, damages its insulation and can start a fire. The second is excessive voltage drop: a conductor can be cool and safe yet lose so many volts over a long run that the load misbehaves.

Current rating, called ampacity, is set by the conductor size, the insulation type, the way the cable is installed and the ambient temperature. Voltage drop is set by the conductor size, the current, the length and the supply voltage. The smaller of the two sizes that pass is not good enough; you pick the one that passes both.

This is also why electricians ask for the cable length and the supply voltage before suggesting a size. A 15 A kettle lead in a kitchen, a 15 A feed to a garden pump 60 m away and a 15 A camera system on a 12 V battery all draw the same current. They need three quite different conductors.

What the gauge numbers mean

Metric cable is sold by cross-sectional area in square millimetres, such as 1.5, 2.5 or 4 mm². The American Wire Gauge uses a number that runs backwards: a larger number is a thinner wire. Each step of 6 gauge numbers doubles the diameter, and each step of 3 roughly doubles the area.

A 14 AWG wire has about 2.1 mm² of copper, close to the metric 2.5 mm² size, and 12 AWG at 3.3 mm² sits between 2.5 and 4. The sizes are similar but not identical, so use the table for your market, not a rounded conversion, when a rating matters.

Test one: current rating

Wiring codes publish tables of the current a given conductor may carry in each method of installation, such as in conduit, buried or clipped to a surface. The values depend on the code, the insulation and the number of cables bundled together, so there is no single table that applies everywhere.

• Continuous loads, which run for hours, are rated more strictly than intermittent ones. Some codes require the conductor to be rated at 125 per cent of a continuous load, which turns 15 A into about 18.8 A for sizing.
• Grouping several cables in one conduit reduces the allowed current for each.
• High ambient temperatures, for instance in a loft or near an engine, call for a correction factor.
• Insulation type changes the maximum conductor temperature and so the rating.

For these reasons, a rating from a general chart is a starting point. The code table for your installation method gives the figure that counts, and the protective device (fuse or breaker) must match the cable rather than the other way round.

Heat, not current alone, is the real limit. The table values assume the conductor can shed its heat to the surroundings. If you wrap the cable in insulation, coil it while loaded or enclose it in a hot space, the safe current falls well below the table value.

Test two: voltage drop over the run

Voltage drop for a two-wire circuit is 2 × I × ρ × L ÷ A, with copper's resistivity about 1.72 × 10⁻⁸ Ω·m. Rearranged, it gives the smallest area that keeps the loss within a chosen limit.

Run the same load on a 12 V battery system and the answer changes completely. A 3 per cent drop is only 0.36 V, so the same formula returns about 28.7 mm². The next standard size, 35 mm², is far thicker than any ampacity table would suggest. The current is identical; the supply voltage is what makes the run hard.

Solid, stranded and the choice of metal

The cross-section tells you how much metal is there, but not how the cable behaves when installed. Solid conductors are a single piece of metal and suit fixed wiring inside walls. Stranded conductors use many fine wires twisted together, which flex without breaking and are used in appliance leads, vehicles and anything that moves.

The metal matters too. Copper is the default for building wiring and electronics. Aluminium weighs less and costs less per ampere in large feeders, but its resistivity is about 1.6 times higher, so the cross-section for the same drop is correspondingly larger, and it needs terminals made for it to avoid loose connections. Copper-clad aluminium and tinned copper appear in marine and automotive cable, where corrosion resistance matters.

Finally, the insulation limits the temperature the conductor can run at, and therefore the current. PVC, cross-linked polyethylene and silicone each have a different maximum, and the right choice depends on the surroundings. A cable that is perfectly adequate in a cool conduit can be undersized when the same current runs through it beside a hot pipe.

Choosing and verifying the final size

1. 1Find the ampacity requirement from the code table for your installation method and add the continuous-load factor if one applies.
2. 2Compute the minimum area for the voltage drop limit, using the full round-trip length.
3. 3Pick the larger of the two sizes and move up to the next standard size if the result falls between them.
4. 4Check the protective device rating against the cable's current-carrying capacity.
5. 5Have the installation verified by a qualified electrician where it forms part of building wiring.