Wire resistance calculator
Free wire resistance calculator for copper and aluminum by AWG or kcmil: resistance and conductance by length and temperature, with K values shown.
Project inputs
Updates as inputs change. Unit changes preserve the same physical values.
Method & assumptions
DC resistance from the conductor's area and the resistivity of annealed copper or 1350 aluminum at 20 °C, corrected for temperature. AC resistance is slightly higher for large conductors (skin effect), and reactance is not included; NEC Chapter 9 Table 8 and Table 9 and manufacturer data give listed values.
Results do not check every aspect of an electrical installation. The calculators follow US (NEC) practice only; a regional preset changes defaults such as voltage and units, and no result certifies NEC compliance.
Sources and validation →Worked example
Conductor size: 12 AWG · Material: Copper · Length: 100 ft · Path: One conductor (the length entered) · Conductor temperature: 75 °C · Construction: Concentric stranded (about 2% longer)
Resistance: 0.197 Ω · Conductance: 5.076 S · Resistance per 1000 ft: 1.97 Ω/1000 ft · K at this temperature: 12.865 Ω·cmil/ft
This example uses the stated inputs; results for the entered values appear above.How wire resistance is calculated
Wire resistance is the opposition a conductor offers to current, set by its material, cross-sectional area, length and temperature. For conductors sized in AWG or kcmil it is calculated as R = K × L ÷ cmil, where K is the material's resistivity in ohm-circular mils per foot, L the length in feet and cmil the conductor's area in circular mils. Conductance is the reciprocal, G = 1 ÷ R, in siemens.
At 20 °C, K is 10.37 for annealed copper and 17.00 for 1350 aluminum. Resistance rises with temperature by the factor 1 + α × (T − 20), with α = 0.00393 per °C for copper and 0.00403 for aluminum, and concentric stranding adds about 2% because the strands are slightly longer than the cable. At 75 °C, stranded, K comes to about 12.9 for copper and 21.2 for aluminum, the values commonly used in voltage drop calculations.
Worked example
A 120 V branch circuit runs 100 ft in 12 AWG stranded copper, so the current travels 200 ft out and back. At 75 °C, K is 12.86 and the circuit resistance is 12.86 × 200 ÷ 6,530 = 0.394 Ω, a conductance of 2.54 S.
At a load of 16 A, the voltage drop is I × R = 16 × 0.394 = 6.30 V, or 5.3% of 120 V. The voltage drop calculator applies the same resistance to single-phase and three-phase circuits.
Table
| Size | Copper 20 °C | Copper 75 °C | Aluminum 20 °C | Aluminum 75 °C |
|---|---|---|---|---|
| 14 AWG | 2.5738 | 3.1302 | 4.2195 | 5.1547 |
| 12 AWG | 1.6200 | 1.9701 | 2.6557 | 3.2444 |
| 10 AWG | 1.0191 | 1.2394 | 1.6707 | 2.0410 |
| 8 AWG | 0.6407 | 0.7792 | 1.0504 | 1.2832 |
| 6 AWG | 0.4031 | 0.4903 | 0.6609 | 0.8074 |
| 4 AWG | 0.2534 | 0.3082 | 0.4155 | 0.5076 |
| 2 AWG | 0.1594 | 0.1939 | 0.2613 | 0.3193 |
| 1/0 AWG | 0.1002 | 0.1218 | 0.1642 | 0.2006 |
| 2/0 AWG | 0.0795 | 0.0967 | 0.1303 | 0.1592 |
| 4/0 AWG | 0.0500 | 0.0608 | 0.0820 | 0.1001 |
| 250 kcmil | 0.0423 | 0.0515 | 0.0694 | 0.0847 |
| 350 kcmil | 0.0302 | 0.0368 | 0.0495 | 0.0605 |
| 500 kcmil | 0.0212 | 0.0257 | 0.0347 | 0.0424 |
Computed with the calculator above from resistivity, the temperature coefficient and a 2% stranding increment. NEC Chapter 9 Table 8 lists values for specific stranding classes, which may differ slightly.
Limitations
The calculator gives DC resistance. For AC, large conductors have a slightly higher effective resistance because of skin and proximity effects, and reactance adds to the impedance; NEC Chapter 9 Table 9 and manufacturer data give AC values. Coated copper, compact stranding and conductor alloys also change the result slightly.
The conductor temperature has to be estimated: a loaded conductor runs warmer than the air around it, up to its insulation rating. For voltage drop, 75 °C is a common assumption; for fault current, a cold conductor gives the conservative (higher) current. Listed resistances in NEC Chapter 9 Table 8 may differ slightly from these calculated values.
Frequently asked questions
What is the resistance of 12 AWG copper wire?
About 1.62 Ω per 1,000 ft at 20 °C and 1.97 Ω per 1,000 ft at 75 °C for stranded conductors, by the calculation above.
What is conductance?
Conductance is the reciprocal of resistance, G = 1 ÷ R, measured in siemens (S). A conductor with a resistance of 0.5 Ω has a conductance of 2 S; the larger the conductance, the more easily current flows.
Why does wire resistance increase with temperature?
In metals, a higher temperature makes the atoms vibrate more, which scatters the electrons carrying the current. For copper the resistance rises by about 0.39% for each degree Celsius above 20 °C.
How does aluminum compare with copper?
Aluminum has about 1.6 times the resistivity of copper, so an aluminum conductor needs a larger area for the same resistance, typically about two AWG sizes larger. For the same conductance it is considerably lighter.
How is wire resistance used to calculate voltage drop?
Voltage drop equals current times the resistance of the conductors the current flows through: the round trip for a single-phase circuit, or the line conductor times √3 for a balanced three-phase circuit. The voltage drop calculator applies this to a circuit.
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