How To Calculate The Size Of A Cable

Thicker cables offer less resistance to electric flow. They offer more electrons to carry a charge and a greater number of paths through which the electrons can travel. As a result, given the same voltage, a thicker cable carries more current. Choose a cable's exact thickness to meet a target level of resistance. The other relevant factors are the length of the cable, which external needs usually dictate and the resistivity of the cable's material.

Step 1

Divide the voltage running through the cable by your target current. If, for instance, 120 volts will act on the cable, and you want 30 amps to run through it: 120 / 30 = 4. This is your target resistance, measured in ohms.

Step 2

Multiply the cable's length by its material's resistivity. Copper, for instance, has a resistivity of 1.724 x 10^-8 ohm meters at room temperature. If your cable must measure 30,000 meters in length:

30,000 x 1.724 x 10^-8 = 0.0005172 ohm sq. m.

Step 3

Divide the answer by your target resistance:

0.0005172 / 4 = 0.0001293.

This is the cable's necessary cross-sectional area.

Step 4

Divide the cable's area by pi: (0.0001293) / 3.142 = 4.1152 x 10^-5.

Step 5

Find the square root of this answer: (4.1152 x 10^-5) ^ 0.5 = 0.006415. This is the cable's radius, measured in meters.

Step 6

Multiply your answer by 39.37 to convert it to inches: 0.006415 x 39.37 = 0.2526.

Step 7

Multiply the answer by 2: 0.2526 x 2 = 0.5052 inches. This is the cable's necessary thickness. It approximately corresponds with the standard 16-gauge cable.