12 AWG wire resistance is the conductor’s opposition to current flow, expressed in ohms. For design and troubleshooting work, the useful result is usually not resistance per 1,000 ft alone. It is the calculated resistance of the actual conductor length—or the complete outgoing-and-return circuit path.
A 12 AWG wire resistance calculation starts with the resistance value for the selected conductor material, then adjusts that value for length and conductor temperature. Copper and aluminum do not produce the same resistance value, and resistance increases as conductor temperature rises.
The calculated resistance is commonly used for:
- Voltage-drop estimates on a 12 AWG branch circuit or small feeder
- Circuit performance reviews for long equipment runs
- Motor, control, and low-voltage load calculations where conductor impedance affects operation
- Comparing conductor options before changing raceway routing or circuit length
- Troubleshooting unexpected voltage loss under load
Use the AWG Wire Resistance Calculator to calculate the resistance for the installed or proposed conductor path.
Resistance Per 1,000 Feet
The calculator begins with the resistance of 12 AWG wire per 1,000 ft for the selected material. That reference value is then scaled to the conductor length entered in the calculator.
The basic one-conductor calculation is:
\(\displaystyle R_{\text{conductor}} = R_{\text{per 1000 ft}} \times \frac{L}{1000}\)
Where:
- \(R_{\text{conductor}}\) = calculated resistance of one conductor, in ohms
- \(R_{\text{per 1000 ft}}\) = resistance per 1,000 ft for the selected 12 AWG material and conductor temperature
- (L) = one-way conductor length in feet
A resistance result for one conductor is appropriate when reviewing a single phase conductor, neutral, grounding conductor, or another individual wire. It is not automatically the resistance seen by a load.
For a typical two-wire circuit, current travels out on one conductor and returns on another conductor. The circuit resistance must include both paths.
\(\displaystyle R_{\text{loop}} = 2 \times R_{\text{conductor}}\)
If the outgoing and return conductors are the same 12 AWG material, temperature, and length, the loop resistance is twice the one-way resistance.
Material and Temperature
Select 12 AWG in the calculator, then choose copper or aluminum and the applicable conductor temperature. The calculator uses those selections to determine the resistance basis before applying the entered length.
Copper and aluminum should not be treated as interchangeable in a resistance calculation. Even when the AWG size and route length are the same, the selected material changes the calculated result.
Conductor temperature also affects the result. A conductor carrying load in a warm location, an enclosed raceway, or a higher-temperature operating condition can have greater resistance than the same conductor at a lower temperature. Use the conductor temperature that matches the calculation purpose and the condition being evaluated.
Resistance temperature selection is separate from an insulation temperature rating, terminal rating, ampacity adjustment factor, or correction factor. Those selections belong to ampacity and code-compliance review, not to the arithmetic of conductor resistance.
Complete Circuit Path
For a 120 V branch circuit, a common voltage-drop path includes the ungrounded conductor and the grounded conductor. If both are 12 AWG and have the same one-way length, calculate the complete loop.
For a 240 V line-to-line load, the path commonly includes two ungrounded conductors. The same loop principle applies: include the resistance of every conductor carrying the load current along the complete circuit path.
Do not use only the one-way route length when estimating load-side voltage drop unless the calculator or formula already accounts for the return path.
A practical sequence is:
- Enter 12 AWG.
- Select copper or aluminum.
- Enter the one-way conductor length.
- Select the conductor temperature.
- Review the calculated one-conductor resistance.
- Include both conductors when the load circuit has an outgoing and return path.
- Apply the circuit resistance to a voltage-drop estimate.
Where circuit length, conductor size, or material changes along the route, calculate each section separately and add the resistance values. A circuit with a 12 AWG copper branch-circuit section and a different upstream feeder section does not have one uniform resistance value.
Calculation Example
Assume a circuit uses 12 AWG conductors with a one-way route length of (L) feet. The calculator provides a temperature-adjusted resistance value of \(R_{\text{per 1000 ft}}\) for the selected material.
The one-conductor resistance is:
\(\displaystyle R_{\text{conductor}} = R_{\text{per 1000 ft}} \times \frac{L}{1000}\)
For a two-wire circuit with equal conductors:
\(\displaystyle R_{\text{loop}} = 2 \times R_{\text{per 1000 ft}} \times \frac{L}{1000}\)
If the circuit current is (I), the resistive voltage-drop estimate is:
\(\displaystyle V_{\text{drop}} = I \times R_{\text{loop}}\)
The percentage voltage drop is then:
\(\displaystyle \%V_{\text{drop}} = \frac{V_{\text{drop}}}{V_{\text{system}}} \times 100\)
Enter the same conductor material, conductor temperature, length, and load conditions into the Voltage Drop Calculator to compare the resistance result with a voltage-drop estimate.
Resistance and Voltage Drop
Wire resistance is one input to voltage drop. A longer 12 AWG run produces more resistance, and higher circuit current produces more voltage drop across that resistance.
For a resistive approximation:
\(\displaystyle V_{\text{drop}} = I \times R\)
The resistance calculator provides the (R) portion of that relationship. The voltage-drop calculation adds load current and system voltage so the result can be reviewed as volts and percentage.
Voltage-drop review is especially useful when 12 AWG conductors serve:
- Long branch circuits
- Receptacle circuits with continuous or sustained loads
- Pumps, compressors, and other motor loads
- Outdoor equipment located far from the panelboard
- Equipment that has a minimum operating-voltage requirement
Motor circuits may require additional review because starting current, power factor, conductor impedance, and equipment characteristics can affect performance. A simple resistance calculation does not represent every AC circuit condition.
Ampacity and Circuit Review
Resistance does not establish 12 AWG ampacity, overcurrent protection, terminal limitations, conductor insulation requirements, or NEC compliance. Those decisions require a separate review of conductor material, insulation temperature rating, terminal rating, installation method, ambient temperature, adjustment factors, correction factors, current-carrying conductors, and the applicable code requirements enforced by the AHJ.
Use the Ampacity Calculator for a separate ampacity review. Keep that result separate from the resistance calculation:
| Calculation | Primary output | Typical use |
|---|---|---|
| 12 AWG wire resistance | Ohms | Circuit-path resistance and voltage-drop input |
| Voltage drop | Volts and percentage | Load performance and conductor-length review |
| Ampacity | Allowable current basis | Conductor and overcurrent-protection review |
A conductor can be adequate for ampacity yet still require a voltage-drop review because of a long route. Conversely, a low resistance result does not approve a conductor for the selected breaker size or installation condition.
Field Verification
Use actual route length rather than a straight-line building measurement. Include vertical risers, panelboard-to-raceway transitions, equipment connections, offsets, and the full installed routing distance. Raceway bends add physical route length, but bend layout and raceway fill must be evaluated separately from conductor resistance.
For existing circuits, measured operating voltage at the source and at the load can help verify whether calculated voltage drop aligns with field conditions. Loose terminations, damaged conductors, corroded connections, undersized conductors, and load changes can create voltage loss not represented by a nominal wire-resistance calculation.
The resistance calculator estimates conductor resistance from 12 AWG size, material, length, and temperature. It does not evaluate breaker sizing, ampacity, raceway fill, termination quality, fault-current performance, or code compliance.