Conductor formula tool
Wire Resistance Calculator
Estimate one-way and round-trip conductor resistance from material, size, and length without turning the result into wire sizing.
Calculate Wire Resistance
Enter conductor material, size, and one-way length to estimate one-way and round-trip resistance near 20 C.
Result
Resistance
0.1588 ohm
One-way conductor, 12 AWG, near 20 C.
Total resistance
0.3175 ohm
Result notes
Keep the entered values, assumptions, and result together when adding this calculation to job notes or submittal records. Final installation choices should align with the applicable code edition, equipment listing, manufacturer instructions, local amendments, and AHJ requirements.
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Related charts
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Use this voltage drop chart: 3% equals 3.6 V at 120 V, 7.2 V at 240 V, and 14.4 V at 480 V; 5% equals 6 V, 12 V, and 24 V.
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Use a conductor material comparison chart to document copper, aluminum, and copper-clad aluminum assumptions after wire-size and ampacity calculator results.
Related guides
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Voltage Drop Formulas & Calculation Reference
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Formula and field context
Estimate one-way and round-trip conductor resistance from material, size, and length without turning the result into wire sizing.
Formula context
Wire Size Chart
A wire size chart is useful only when it keeps the field sequence visible: calculated load first, usable ampacity second, terminal temperature and derating third, then voltage drop and equipment instructions. Use the calculator result as the starting point and verify the adopted NEC edition, manufacturer data, and AHJ requirements before treating the conductor as ready for the job.
Formula
Conductor planning screen = calculated load plus duty factors, then compare usable ampacity, terminal limit, voltage-drop target, and equipment instructions.Variables to keep with the result
- Calculated load is the amperes from the load schedule, nameplate, or calculator result.
- Usable ampacity is the conductor value after material, insulation, terminal temperature, ambient correction, and conductor-count adjustment are considered.
- Voltage drop is a performance screen that can require a larger conductor even when ampacity is acceptable.
Formula and variables
Conductor resistance can be estimated from R = rho x L / A. R is resistance in ohms, rho is material resistivity, L is conductor length, and A is conductor cross-sectional area. This worksheet uses common copper and aluminum resistivity near 20 C, converts feet to meters, converts mm2 to square meters, and returns one-way and round-trip resistance for documentation.
U.S. field context and example
Wire resistance supports voltage-drop notes, test-record comparisons, temporary lead planning, low-voltage control work, and troubleshooting where measured resistance needs a reasonableness check. For example, a 100 ft one-way run of 12 AWG copper is about 0.159 ohm near 20 C, or about 0.318 ohm round trip. The round-trip value matters when current leaves and returns through two conductors, such as a simple two-wire DC or single-phase circuit path.
Assumptions and limits
The result is an electrical resistance estimate, not a conductor selection. Stranding, actual conductor area, temperature, alloy, splices, terminations, and measurement lead resistance can change the field value. Use the voltage-drop calculator when load current and acceptable drop must be reviewed. Use the wire-size and ampacity calculators when the conductor must be selected for an installation.
Common mistakes
Common mistakes include using one-way resistance when the circuit path is round trip, mixing feet and meters, treating aluminum like copper, and ignoring temperature. When a resistance value will be compared to a meter reading, document whether the measured path includes one conductor, two conductors, equipment bonding paths, terminations, or connected loads.