Wire Gauge Resistance Calculator Formula

Understand the math behind the wire gauge resistance calculator. Each variable explained with a worked example.

Use the Wire Gauge Resistance Calculator

Formulas Used

Wire Diameter

wire_diameter = diameter_mils

One-Way Resistance

wire_resistance = resistivity * length_ft / area_cmil

Round-Trip Resistance

round_trip = 2 * resistivity * length_ft / area_cmil

Variables

VariableDescriptionDefault
awg_numberAWG Gauge Number12
length_ftWire Length (one way)(ft)100
resistivityResistivity(ohm-cmil/ft)10.37
diameter_milsDerived value= 1000 * pow(92, (36 - awg_number) / 39) / 200calculated
area_cmilDerived value= pow(diameter_mils, 2)calculated

How It Works

Wire Gauge Resistance

AWG Diameter

d(mils) = 1000 x 92^((36-AWG)/39) / 200

Resistance

R = rho x L / A

Copper: rho = 10.37 ohm-cmil/ft at 20 C.

Worked Example

100 ft of 12 AWG copper.

awg_number = 12length_ft = 100resistivity = 10.37
  1. 01Diameter = 80.81 mils
  2. 02Area = 6530 cmil
  3. 03R = 10.37 x 100 / 6530 = 0.1588 ohms
  4. 04Round-trip = 0.3177 ohms

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