A copper wire resistance chart provides the baseline DC resistance per unit length—typically measured in Ohms per 1,000 feet (Ω/kft) or Ohms per kilometer—for standard American Wire Gauge (AWG) sizes. If you are sizing conductors for a solar array, calculating voltage drop for a long feeder run, or designing a high-current DC bus, you need the exact baseline resistance before you can apply temperature corrections or bundling derations. The direct answer for standard 12 AWG solid copper wire is 1.588 Ω/kft at 25°C (77°F), while 12 AWG stranded is 1.980 Ω/kft.

Below is the master reference table, followed by the exact formulas and rules for adjusting these baseline numbers to match your specific jobsite or breadboard conditions.

The Master Copper Wire Resistance Chart (25°C Baseline)

How to read this table: This chart lists the DC resistance for uncoated copper conductors at a standard ambient temperature of 25°C (77°F). The Area (cmil) column shows the cross-sectional area in circular mils, which dictates the physical current-carrying capacity. The Solid and Stranded columns show resistance per 1,000 feet. Stranded wire always has a slightly higher resistance than solid wire of the same AWG because the spiral lay (twist) of the individual strands makes the actual electrical path longer than the physical length of the cable. Use the bookmark-friendly IDs on the most common DIY and trade sizes to jump back quickly.

Table 1: Uncoated Copper Wire DC Resistance at 25°C (77°F). Source: Adapted from Southwire Conductor Data and NEC Chapter 9, Table 8 baseline metrics.
AWG Size Area (cmil) Solid (Ω / 1,000 ft) Stranded (Ω / 1,000 ft)
18 1,624 6.385 7.951
16 2,583 4.016 4.992
14 4,107 2.525 3.140
12 6,530 1.588 1.980
10 10,380 0.9989 1.240
8 16,510 0.6282 0.7776
6 26,240 0.3951 0.4913
4 41,740 0.2485 0.3086
2 66,360 0.1563 0.1941
1/0 105,600 0.0983 0.1225
4/0 211,600 0.0490 0.0608
Pro Tip for Quick Math: To find the resistance of a specific length of wire, multiply the Ω/kft value by your total circuit length (out and back) and divide by 1,000. For example, a 50-foot one-way run of 10 AWG stranded wire (100 feet total loop) has a baseline resistance of: (1.240 × 100) / 1000 = 0.124 Ω.

Which Column Applies to Your Installation?

Choosing the wrong column is the most common error when calculating voltage drop for branch circuits and feeders. Your selection depends on the physical construction of the wire and the temperature rating of the insulation.

Solid vs. Stranded Conductors

If you are pulling NM-B (Romex) through wall cavities for standard 15A or 20A residential receptacles, you are using solid conductors. Use the Solid column. If you are pulling THHN/THWN through EMT conduit, wiring a solar combiner box with PV wire, or building a battery bank with flexible welding cable, you are using stranded conductors. Always use the Stranded column for these applications. As the AWG size increases (lower numbers), manufacturers stop producing solid wire entirely because it becomes too stiff to bend; notice that 8 AWG and larger in standard building wire is almost exclusively stranded.

Temperature Columns (25°C vs. 75°C vs. 90°C)

The table above provides the baseline at 25°C (77°F). However, the National Electrical Code (NEC) Chapter 9, Table 8 officially lists DC resistance at 75°C (167°F) because that is the standard operating temperature for most modern terminations. If your wire will be operating under load in a hot attic or inside a conduit on a sun-baked roof, the 25°C baseline will dangerously understate your actual resistance.

How Derating and Temperature Modify Base Resistance

Copper is a highly temperature-sensitive metal. As the conductor heats up from ambient temperature or from I²R (current squared times resistance) heating, its resistance increases. This creates a feedback loop: higher resistance generates more heat, which further increases resistance.

To adjust the 25°C baseline values from the chart above to your actual operating temperature, use the standard temperature coefficient formula for copper:

RT = R25 × [1 + 0.00393 × (T - 25)]

Where RT is the new resistance, R25 is the chart baseline, and T is the operating temperature in °C.

Worked Example: 6 AWG THHN in a Hot Attic

Imagine you are running a 6 AWG stranded feeder to a subpanel. The baseline resistance is 0.4913 Ω/kft at 25°C. However, the wire is routed through an attic that reaches 50°C (122°F) in the summer, and the wire itself heats up to 75°C under load.

  • Step 1: Calculate the temperature delta: 75°C - 25°C = 50°C.
  • Step 2: Multiply by the copper coefficient: 50 × 0.00393 = 0.1965.
  • Step 3: Add 1: 1 + 0.1965 = 1.1965.
  • Step 4: Multiply by baseline: 0.4913 × 1.1965 = 0.5878 Ω/kft.

By ignoring temperature derating, you would have underestimated the resistance of that 6 AWG wire by nearly 20%. In a long run, this miscalculation is exactly what causes a 240V welder to trip its breaker due to severe voltage drop.

Bundling Derating Note: While NEC Article 310.15(C)(1) provides bundling derating factors (e.g., multiplying ampacity by 80% for 4-6 current-carrying conductors in a raceway), this adjusts the allowable current (ampacity) to prevent insulation meltdown. It does not change the physical resistance of the copper. However, bundling traps heat, raising the operating temperature (T in the formula above), which indirectly increases your operating resistance.

What This Resistance Chart Cannot Tell You

A DC resistance chart is a foundational tool, but it is not a complete engineering model. If you are designing high-frequency circuits, sizing service entrance conductors, or passing an electrical inspection, you must account for the following physical realities that a simple lookup table ignores.

1. AC Impedance and the Skin Effect

This chart lists DC resistance. In alternating current (AC) circuits, especially at 60Hz and higher, current tends to migrate toward the outer surface of the conductor—a phenomenon known as the skin effect. For standard 60Hz residential wiring (14 AWG to 4/0 AWG), the skin effect is negligible, and DC resistance is perfectly adequate for voltage drop calculations. However, in high-frequency applications (like inverters outputting high-frequency PWM, or RF antenna feeds), the effective cross-sectional area shrinks, and the AC impedance (Z) will be significantly higher than the DC resistance (R) listed here.

2. Termination Temperature Limits (NEC 110.14(C))

The chart tells you the resistance of the wire, but it does not dictate the maximum temperature you can safely use for sizing. Under NEC 110.14(C), even if you use 90°C THHN wire, you must size the circuit based on the lowest temperature rating of any connected termination. Since most standard residential breakers and receptacles are rated for 75°C, you must use the 75°C ampacity column for your final sizing, regardless of the wire's 90°C insulation. The resistance chart cannot protect you from melting a plastic breaker lug by overloading a wire that is technically capable of handling the current.

3. Actual Voltage Drop Under Load

Resistance is only half of the voltage drop equation. To find the actual voltage drop, you must apply Ohm's Law (V = I × R). A 10 AWG wire might have a low baseline resistance, but if you pull 30A through a 200-foot loop to an RV pedestal, the voltage drop will be:

  • Total loop length: 200 ft (0.2 kft)
  • Resistance at 75°C (approx): 1.24 × 1.1965 = 1.48 Ω/kft
  • Total Resistance: 1.48 × 0.2 = 0.296 Ω
  • Voltage Drop: 30A × 0.296 Ω = 8.88 Volts

On a 120V circuit, an 8.88V drop is 7.4%—well past the NEC's recommended 3% maximum for branch circuits. The chart gives you the 'R', but you must supply the 'I' and the distance to find out if your design will actually work on the bench or in the field.