There are many sources of wire table information for round magnet wire (see references below). Most refer to the American Wire Gauge System, where gauge numbers increase in sequence as wire diameter decreases exponentially. Wire diameter can be calculated directly without using a lookup table with a simple exponential formula fit to the wire diameters at two gauges. 4/0 gauge which is 0.460" diameter and 36 gauge which is 0.005" diameter can be used to obtain excellent agreement with published tables. Note that 4/0 just means "four zeros" and sometimes appears as 0000. Numerically, 4/0 is represented as -3 when calculating wire diameter.
I've based my table data on the resistivity of IACS copper at 20°C, which is 1.7241 × 10-6 Ohm-cm. I've used 2.828 × 10-6 Ohm-cm for aluminum. To calculate weight for different wire gauges, I've used a density of 8.89 g/cm3 for copper and 2.701 g/cm3 for aluminum. These numbers can vary depending on the purity and treatment of the metal. They will also vary dramatically with temperature, especially resistivity. These numbers are only good at 20°C, which is typical room temperature. Obviously, in many applications the wire will be carrying current and can be much hotter than room temperature. I may add a feature in the future to allow you to enter a different temperature when obtaining wire table data. In case you are wondering why I included weight in this table, there are two reasons. First, spooled wire is often sold by weight instead of length. Second, the weight of a finished coil can be critical in many applications. Think "voice coil", as in loud speakers and microphones.
The diameters given here are for bare round wire, but most wire in real applications has a layer of insulation on it. For hookup wire and telephone cable you can easily strip off the insulation to get a good measurement of diameter. For magnet wire, getting a good reading is more of a challenge. You can scrape or sand off the insulation, but you will likely remove some copper as well. I've been burning the insulation off of magnet wire with hot solder, and then removing as much of the solder as I can before measuring diameter. Magnet wire insulation seems to run from 0.0005" to 0.001" thickness, so in many cases you can estimate a wire's gauge number with the insulation on. Obviously, these tables won't help if you have square or stranded wire. A good way to get started looking at different wire gauges is to buy the assortment carried by Radio Shack under their catalog number #278-1345 which contains spools of 22 ga., 26 ga., and 30 ga. enameled magnet wire. To measure diameter you'll need a good micrometer like the Mitutoyo in the picture above, or a dial caliper that reads in 0.001" increments.
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