Wire gauge calculator: AWG, SWG and mm²

SWG and AWG

Two wire gauge systems are still in use. Standard Wire Gauge (SWG), also called Imperial or British Standard Wire Gauge, is used in the UK for enamelled copper wire and older equipment. American Wire Gauge (AWG) is used in North America and in most electronics datasheets. In both systems a bigger number means a thinner wire, but the sizes don't match: 14 SWG is 2.03 mm, while 14 AWG is 1.63 mm. Most UK suppliers now also give the diameter in millimetres, which is the least confusing way to buy wire. Mains and automotive cable is sold by cross-sectional area, such as 1.5 mm² or 2.5 mm².

Skin effect: why RF resistance is higher

At radio frequencies, current flows only in a thin layer near the surface of a conductor, the skin depth. In copper it's about 25 µm at 7 MHz and 6.6 µm at 100 MHz. A wire's RF resistance is therefore much higher than its DC resistance, and it depends on the surface area, not the cross-section. That's why magnetic loops use fat copper pipe, and why hollow tube works as well as solid rod for antenna elements. For a wire antenna at HF, the loss in normal wire is small compared with the radiation resistance, so 1.5 to 2 mm wire is plenty, but thin wire on a short or loaded antenna can waste real power.

Choosing antenna wire

For long wire antennas, strength matters more than conductivity. Hard-drawn copper stretches much less than soft copper, and copper-clad steel is stronger still: RF flows in the copper skin, so its RF resistance is close to copper's, even though its DC resistance (shown above) is higher. Insulated stranded wire is light and easy to handle for portable antennas, and the insulation lowers the resonant frequency slightly, so cut it a few percent shorter.

Current ratings

The current figure is a cautious rule of thumb of 3 A per square millimetre for wiring inside equipment. Safe current depends heavily on insulation, bundling, length and the voltage drop you can accept. For a 12 V supply lead, size the cable for voltage drop: a 100 W radio drawing 20 A through a 2 m lead of 2.5 mm² cable, 4 m of conductor there and back, loses about 0.55 V.

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