AWG Wire Resistance Calculator

Related tools: Wire Size Calculator, Voltage Drop Calculator, and Wire Derating Calculator.

An AWG wire resistance calculator estimates the resistance of a conductor path from its AWG size, conductor material, operating temperature, one-way length, and the number of current-carrying conductors included in the path.

The primary result is Circuit resistance in ohms. That value is used in voltage-drop calculations, branch-circuit and feeder conductor comparisons, and preliminary conductor-sizing work. Higher circuit resistance produces more voltage drop at a given load current and can affect equipment performance, especially on long runs serving motors, HVAC equipment, pumps, EV charging equipment, and other continuous or high-current loads.

The calculator also reports the estimated AWG diameter, circular-mil area, resistance at 20 C, temperature-adjusted resistance, and the temperature factor applied to the selected conductor material.

Resistance Path and Voltage Drop

Conductor resistance is the opposition to current flow through the metal conductor. For a circuit with a known load current, resistance is commonly converted into an estimated resistive voltage drop:

\(\displaystyle V_D = I \times R_{\text{circuit}}\)

Where:

• V_D = voltage drop in volts
• I = circuit current in amperes
• R_{\text{circuit}} = total circuit resistance in ohms

For a 120 V branch circuit, a long conductor run can lose several volts under load. For a 480 V feeder, the same resistance may represent a smaller percentage voltage drop, but the actual design review must still consider load type, motor starting behavior, conductor reactance where applicable, and the installation conditions.

The resistance calculation does not establish ampacity. Ampacity depends on conductor insulation temperature rating, terminal rating, ambient temperature, bundling, raceway conditions, adjustment factor requirements, correction factor requirements, and applicable code rules. Resistance supports voltage-drop and circuit-performance analysis; it is not a substitute for ampacity selection.

Calculator Inputs

Input	Electrical use
AWG size (AWG)	Selects the conductor diameter and estimated circular-mil area. Lower AWG numbers represent larger conductors and generally lower resistance.
Conductor material	Selects the material constant used for resistance estimation. Copper and aluminum conductors have different resistivity values.
Conductor temperature (C)	Adjusts resistance from the 20 C reference condition to the entered conductor temperature. Resistance rises as conductor temperature rises.
One-way length (ft)	Sets the physical conductor length from source to load. The circuit path may include more than one conductor over that distance.
Current-carrying conductors	Multiplies the one-way resistance by the number of conductors included in the current path. A typical single-phase line-to-neutral or line-to-line circuit commonly uses two conductors in the resistance path.

For a basic two-wire circuit, enter the actual source-to-load distance in One-way length (ft) and enter 2 for Current-carrying conductors. The calculation then includes the outgoing and return conductor resistance.

Do not use the conductor count as a substitute for determining current-carrying conductors for ampacity adjustment. Here, the field represents the conductors included in the electrical resistance path.

AWG Diameter and Circular Mils

The calculator estimates conductor geometry from the standard AWG progression. AWG diameter is shown in mils, where one mil equals 0.001 inch. The calculated circular-mil area is derived from the conductor diameter:

\(\displaystyle \text{Circular mil area} = d^2\)

Where d is conductor diameter in mils.

For the 12 AWG example, the calculator produces:

• AWG diameter: 80.8081 mil
• Circular mil area: 6529.9468 cmil

Circular-mil area is useful because conductor resistance varies inversely with conductor cross-sectional area. Larger conductors have more conductive material available for current flow, which reduces resistance and voltage drop.

A conductor’s listed AWG size does not independently define its finished installed performance. Stranding, conductor construction, insulation system, terminations, operating temperature, and manufacturer data can affect the final engineering or installation decision.

Resistance at 20 C

The calculator first estimates Resistance at 20 C in ohms per 1,000 feet using the selected conductor material and the estimated AWG circular-mil area.

The general resistance relationship is:

\(\displaystyle R_{20} = \frac{K \times 1000}{CM}\)

Where:

• R_{20} = resistance at 20 C in ohms per 1,000 ft
• K = material resistance constant
• CM = conductor area in circular mils

The selected Conductor material supplies the material constant. Copper normally has lower resistance than aluminum at the same conductor size, so an aluminum conductor usually requires a larger cross-sectional area when resistance or voltage-drop performance is the controlling design condition.

The calculator displays the result as Resistance at 20 C, providing a reference value before the entered conductor temperature is applied.

Temperature-Adjusted Resistance

Conductors do not retain the same resistance at all temperatures. As conductor temperature increases, metallic conductor resistance increases. The calculator applies a temperature coefficient to the 20 C resistance and displays:

• Temperature factor
• Adjusted resistance

The adjusted relationship is:

\(\displaystyle R_T = R_{20} \times F_T\)

Where:

• R_T = adjusted resistance at the entered conductor temperature
• R_{20} = resistance at 20 C
• F_T = temperature factor

The displayed Temperature factor shows the multiplier applied to the 20 C resistance. At 20 C, the factor is 1 x, so Adjusted resistance equals Resistance at 20 C.

For voltage-drop review, temperature adjustment is relevant because a conductor carrying load current may operate above the 20 C reference condition. The entered temperature should be a defensible basis for the design or operating scenario; it is not automatically the insulation temperature rating or terminal rating.

Circuit Resistance Calculation

The calculator converts adjusted resistance per 1,000 feet into the resistance of the complete entered conductor path:

\(\displaystyle R_{\text{circuit}} = R_T \times \frac{L \times N}{1000}\)

Where:

• R_{\text{circuit}} = Circuit resistance in ohms
• R_T = Adjusted resistance in ohms per 1,000 ft
• L = One-way length in feet
• N = Current-carrying conductors included in the resistance path

The result represents conductor resistance only. It does not add resistance from splices, lugs, breakers, disconnects, bus connections, terminations, contact resistance, or connected equipment.

Calculation Example

Using the displayed example values:

Field	Entered value
AWG size (AWG)	12 AWG
Conductor material	Copper
Conductor temperature (C)	20
One-way length (ft)	100
Current-carrying conductors	2

The calculator produces:

Result	Value
AWG diameter	80.8081 mil
Circular mil area	6529.9468 cmil
Resistance at 20 C	1.5882 ohm/1000 ft
Temperature factor	1 x
Adjusted resistance	1.5882 ohm/1000 ft
Circuit resistance	0.3176 ohm

The circuit-resistance calculation is:

\(\displaystyle 0.3176 = 1.5882 \times \frac{100 \times 2}{1000}\)

If the circuit carries 15 A, the conductor-only resistive voltage drop is:

\(\displaystyle V_D = 15 \times 0.3176 = 4.764 \text{ V}\)

For a nominal 120 V circuit, that is approximately:

\(\displaystyle \frac{4.764}{120} \times 100 = 3.97\%\)

That percentage is a voltage-drop calculation based on conductor resistance. It does not determine whether 12 AWG is code-compliant for the circuit. Conductor sizing must separately address overcurrent protection, required ampacity, insulation temperature rating, terminal limitations, equipment requirements, and the installation rules enforced by the AHJ.

Field Verification

Use the calculated resistance to compare conductor sizes or materials before finalizing a branch circuit or feeder layout. A lower-resistance conductor may reduce voltage drop, but it can also change raceway fill, pulling difficulty, bending space, lug compatibility, cost, and termination requirements.

Verify the final installation against the actual conductor and system conditions, including:

• Manufacturer conductor data and conductor construction.
• Actual conductor temperature under the expected load condition.
• The complete circuit path, including phase, neutral, or return conductors as applicable.
• Circuit configuration, including AC impedance and reactance where those factors are material.
• Ampacity after applicable ambient-temperature correction and current-carrying-conductor adjustment factors.
• Terminal ratings, insulation temperature rating, overcurrent protection, and equipment listing requirements.
• Raceway fill, conductor pulling conditions, bending layout, and available termination space.
• The locally adopted electrical code and AHJ requirements.

The calculator provides a transparent resistance estimate from AWG geometry, material, temperature, length, and conductor count. Final conductor selection requires installation-specific electrical and code review.