How to Determine Wire Size for Amperage per Foot

Measure the distance the circuit must traverse. This distance is the entire round trip of the circuit, not only the distance from the current source.

Add together the electrical loads that will be placed upon the circuit. The term "electrical load" refers to any device that draws current from the circuit at any voltage. For example, a 15-amp heater and a 10-amp pump require 25 amps of electrical current. Amperage is the unit of measure for electrical current and is equal to 1 Coulomb per second.

Divide the total amperage by 0.80. The resulting figure is the amperage rating the wire must meet. Wire amperage ratings list a value that shows the maximum instantaneous amperage allowed. This maximum value can only be sustained for a short time. The continuous amperage rating of the wire is 80 percent of this value. For example, a standard wall outlet is most commonly installed on a 15-amp circuit that may sustain a constant 12 amp load: 12 Amps / 0.8 = 15 Amps.

Consult a wire amperage chart to determine the minimum required diameter for the circuit length determined above. If the amperage rating of your application places your required wire very near the demarcation point between two wire sizes, it is always safer to go with the wire that has a larger cross-sectional area. Commonly, wires are measured by "gauge," with lower numbers signifying larger cross-sectional area.