How to Calculate the Electrical Wire Size

The method of determining wire gauge sizes follows naturally from the way in which manufacturers make wire. A metal rod is heated and drawn through a hole (drawing die) that’s smaller than the original wire. The gauge of the wire is determined by the number of draws needed to produce wire of the desired size. For example, the original rod is 0 gauge, wire from the first draw is 1 gauge, wire from the second draw is 2 gauge and so on.

Examine the variables in calculating the wire size. The primary variables in each method are the size of the rod used to start the drawing process and the reduction ratio of each draw.

Determine the size of gauges larger than 0 gauge. These are calculated by applying the same reduction ratio. For example, if gauge 1 is 89 percent of gauge 0, gauge 0 will be 89 percent of gauge 00, which will be 89 percent of gauge 000 and so on.

Use the American wire gauge (AWG) system adopted in 1857 to calculate wire size. This method defined a range of wire sizes with two objectives; the ratio of the largest size with the smallest size would be an integer and the resulting gauges would generally correspond to proprietary gauges in use at the time. In this case, the AWG standard defines 0000 gauge wire as 0.46 inches and 36 gauge as 0.005 inches.

Derive the formula for calculating the AWG wire sizes. This will be given by the general formula Dn = (D0)r ^((N-(d + n))/N), where Dn is the diameter of n gauge wire, r is the ratio between the largest and smallest wire, N is the number of draws needed to obtain the smallest wire from the l, and d is the offset of the largest gauge from 0.

Substitute values into the general formula given in Step 4. For the formula Dn = (D0)r ^((N-(d + n))/N), the AWG standard uses D0 = 0.005, r = 0.46/0.005 = 92, N = 39 and d = -3. This provides the following formula for the AWG standard: Dn = 0.005 in. x 92^((36-n)/39).