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 Common wire sizes.

Common copper wire sizes and resistance

 AWG Max I * Inch Dia mm Dia per 1000' ** per km ** 22 0.918 0.025 0.644 16.46 54 20 1.46 0.032 0.812 10.35 33.95 18 2.32 0.04 1.024 6.51 21.36 16 3.69 0.05 1.291 4.09 13.42 14 5.87 0.064 1.628 2.57 8.43 12 9.33 0.08 2.053 1.62 5.31 10 14.8 0.101 2.588 1.02 3.34 8 23.6 0.128 3.264 0.64 2.1 6 37.5 0.162 4.115 0.4 1.31 4 59.6 0.204 5.189 0.25 0.83 2 94.8 0.257 6.544 0.16 0.52 1 119 0.289 7.348 0.12 0.39 * max safe current in a tightly wound coil ** at 25 degrees C Source: The Radio Amatures Handbook 1978

Resistance of other materials compared to copper
where copper = 1

 Aluminium : 1.6 Brass : 3.7 - 4.9 Gold : 1.4 Silver : 0.94 Iron : 5.68 Lead : 12.8 Steel : 7.6 - 12.7 Tin : 6.7 Zinc : 3.4 Nickel : 5.1 Source: The Radio Amatures Handbook 1978

Voltage drop on long cables.

The length of our cable from the supply ( windmill ) to the load ( batteries ) can have a big impact on the power generated. All wire has a resistance, and any resistance is a loss, so we need to reduce this loss to a minium. But just how much is lost?

Say our windmill is supplying 20 amps of power, and we have a 12 volt battery under charge. We have a power cable from our windmill to our battery of 100 meters, and the cable size is 6 AWG.

Using the cable cross reference here, we know that 6 AWG wire ( 4.1mm or 0.16inches diameter ) has a resistance of 1.3 ohms per 1km, or 0.13 ohm for our 100m length. Now there are two wires + and -, so we double that to get 0.26 ohms.

So if the windmill is supplying 20 amps, using ohms law ( here ) we calcualte 20 amps over our 0.26 ohm cable equals 5.2 volts lost in the cable. To work out how many watts is lost in the cable, we use watts = volts * amps, so 5.2 volts * 20 amps = 104 watts. As our battery is 12 volts, its using 12 volts * 20 amps = 240 watts. So our windmill is making 104watts + 240watts = 344 watts, 104 watts of which is wasted in the cable.

If we increase our wire size to 4 AWG, our total cable resistance drops to 0.16 ohms (0.83 ohms per 100m, times 2). This works out to 3.2 volts lost in the cable, or 64 watts wasted.