Wire Resistance Calculator
Estimate DC resistance from American Wire Gauge, material resistivity, conductor path length, and operating temperature. Then trace current into voltage drop, load-end voltage, and I²R heating without confusing resistance math with code ampacity.
Describe the conductor path
A 100.0 ft current path of nominal 12 AWG copper at 75.0°F is estimated at 0.16125 Ω. At 20.00 A, the path drops 3.225 V and dissipates about 64.50 W. This is a resistance screen, not an ampacity or code-compliance result.
Resistance, AWG geometry, and temperature
For a uniform conductor, resistance equals material resistivity multiplied by length and divided by metallic cross-sectional area. American Wire Gauge defines nominal solid-wire diameter by a geometric progression. Gauge 36 is 0.005 inch, and a change of 39 gauge steps corresponds to a diameter ratio of 92. The calculator converts diameter to circular area before applying the resistance equation.
area = π × diameter² ÷ 4
ρT = ρ20 × [1 + α × (T°C − 20°C)]
R = ρT × total path length ÷ area
voltage drop = current × R; heat loss = current² × R
The temperature relationship is linearized around the entered 20°C reference. NIST copper tables report a 20°C coefficient near 0.00393 per degree Celsius for 100% conductivity copper. Actual alloy, temper, strand construction, plating, cold work, purity, and temperature range change resistance. Use manufacturer data or a measurement when precision matters.
One-way distance versus current path
A 50 ft load location usually does not mean only 50 ft of conductor resistance. A two-wire DC or single-phase circuit sends current outward and returns it, giving 100 conductor-feet in the simple model. This calculator multiplies one-way length by the number of current-path conductors.
Do not automatically use two for every system. Balanced three-phase line-to-line voltage drop uses a connection-specific relationship rather than three times one-way DC resistance. Neutral current depends on loading and harmonics. Parallel conductors divide current and resistance only when length, material, size, terminations, and current sharing are appropriate.
Connections, splices, switches, fuses, contacts, bus bars, and source impedance add resistance beyond the wire itself. Measure under controlled load if end-to-end performance is critical.
Material and construction matter
The copper preset uses the NIST reference resistivity shown in the Copper Wire Tables. The aluminum preset is a planning value, not a substitute for the conductor manufacturer’s published DC resistance. Custom mode leaves entered resistivity and coefficient under user control.
AWG geometry describes nominal metallic diameter for a solid-equivalent conductor. Stranding changes overall outside diameter and lay length; compact, compressed, flexible, plated, and specialty constructions may differ. Insulation thickness does not add conducting area.
AC resistance can exceed DC resistance because of skin effect, proximity effect, magnetic conduit, induced currents, and conductor arrangement. The difference is usually small for low-frequency small conductors but can be material for large conductors, high frequency, or unfavorable geometry.
Worked 12 AWG copper run
Enter 12 AWG, a 50 ft one-way run, two current-path conductors, 75°F metal temperature, 20 A, and 120 V. The AWG relationship gives a nominal bare diameter of about 0.08081 inch, or 2.053 mm. Cross-sectional area is about 3.309 mm² or 6,530 circular mils.
Seventy-five degrees Fahrenheit is approximately 23.89°C. Applying the entered copper coefficient to 1.7241 × 10⁻⁸ Ω·m at 20°C gives about 1.75045 × 10⁻⁸ Ω·m. The 100 ft total path is 30.48 m, producing approximately 0.16125 Ω, or 1.6125 Ω per 1,000 ft at that temperature.
At 20 A, voltage drop is about 3.225 V, which is 2.687% of 120 V. The idealized load-end voltage is 116.775 V. Wire heat is I²R, about 64.50 W spread along both conductors. That does not prove the conductor is thermally acceptable because insulation, bundling, ambient, termination, overcurrent protection, and code rules are separate.
Factors this resistance model does not include
| Factor | Why it changes the project | What to verify |
|---|---|---|
| Conductor tolerance | Actual diameter, conductivity, and strand lay change DC resistance. | Manufacturer resistance data and applicable product standard. |
| Termination temperature | Allowable ampacity and equipment use can be limited by terminal ratings. | Equipment labeling, conductor insulation, adopted NEC, and AHJ requirements. |
| Ambient and bundling | Heat dissipation changes when conductors share raceway, cable, insulation, or hot spaces. | Applicable adjustment and correction rules, installation method, and measured conditions. |
| Load behavior | Starting, cycling, harmonics, and inrush differ from a steady current. | Duty cycle, RMS current, peak behavior, equipment voltage tolerance, and startup data. |
| AC effects | Skin, proximity, magnetic material, and configuration can raise impedance. | Frequency-specific manufacturer impedance or engineering calculation. |
| Fault performance | Fault current and clearing time require the complete source-loop impedance. | Available fault current, protective-device curves, conductor withstand, bonding, and study. |
Resistance is not ampacity
A wire can have a numerically acceptable voltage drop and still be too small under the adopted electrical code, equipment instructions, or thermal conditions. Conversely, an ampacity-suitable conductor may need to be larger for voltage performance, motor starting, efficiency, or project criteria. This calculator does not select conductors, raceways, terminals, breakers, fuses, or grounding and bonding.
For U.S. premises wiring, use the NFPA 70 edition adopted by the jurisdiction, local amendments, listing and labeling, manufacturer instructions, and qualified design. Verify conductor material and compatibility at terminations. Aluminum conductors require listed equipment and correct preparation, torque, and installation practice. Never improvise energized measurements on hazardous circuits.
How to validate a field resistance estimate
First identify the actual conductor from markings, drawings, purchase records, or manufacturer data. Record gauge, material, stranding, length, parallel count, connectors, and route. Estimate metal temperature under the relevant load rather than assuming room temperature. Compare the calculated resistance with published ohms-per-thousand-feet values at a known temperature.
For a low-resistance measurement, ordinary two-lead meter resistance can be dominated by probe and contact resistance. Use a suitable four-wire Kelvin method, calibrated instrument, stable current, clean contacts, and temperature correction when accuracy requires it. De-energize and isolate the conductor according to applicable safe-work procedures before resistance testing.
A loaded voltage-drop test includes connections and source impedance, which may be desirable for troubleshooting but is not the same quantity as wire-only resistance. Measure source and load voltages simultaneously or under stable load, use appropriately rated instruments, and include current. Investigate hot connections, damaged strands, unexpected splices, and unequal parallel-path current when measured loss exceeds the documented model.
From a planning number to a documented design
Keep a calculation record with the conductor description, reference resistance source, assumed metal temperature, one-way route length, number of current-carrying paths, load current, source voltage, and rounding. State whether the result is wire-only or includes connections. If the load varies, preserve normal, startup, and worst credible current cases rather than reporting one unlabeled number.
Compare the screen with the equipment’s allowable input-voltage range at both steady load and startup. A small percentage at nominal current can become a larger transient drop during motor starting or converter inrush. If resizing is considered, recheck protection, fault current, raceway fill, pulling limits, terminals, parallel-conductor rules, physical routing, cost, and embodied material. The best conductor decision balances safety, performance, efficiency, installation, and maintainability across the whole system.
Wire resistance FAQs
Why does the default use two conductors?
A simple DC or single-phase load has an outgoing and return path. Multiplying 50 ft one way by two models 100 conductor-feet. Other systems require their connection-specific method.
Does lower AWG mean larger wire?
Yes. In the AWG system, a smaller gauge number has a larger nominal diameter. The aught sizes continue below zero: 0 is 1/0, −1 represents 2/0, and so on in this input.
Why does resistance rise with temperature?
For copper and aluminum over ordinary operating ranges, lattice vibration increases resistivity as temperature rises. The calculator uses a linear coefficient, while precise work uses material-specific data over the needed range.
Can this choose a safe wire size?
No. Resistance and voltage drop are only part of conductor design. Ampacity, insulation, terminals, ambient, bundling, protection, load type, installation method, code, and fault performance also control.
Does stranding change the result?
It can. The AWG equation gives nominal solid-wire geometry. Strand count, individual strand diameter, lay, compaction, plating, and manufacturing tolerance affect finished conductor resistance and outside diameter.
Is this valid for high-frequency current?
It calculates DC resistance with a temperature adjustment. High frequency and large AC conductors may need skin-effect, proximity-effect, magnetic-enclosure, and arrangement analysis using frequency-specific impedance data.
When multiple conductors form parallel current paths, combine their branch values with the parallel circuit resistance calculator.
References
These U.S. metrology, electrical-science, and workplace-safety sources support the conductor geometry, copper data, circuit relationships, and safety boundaries.