The exact resistance of copper wire depends on its AWG size and operating temperature. At the standard 75°C (167°F) baseline used in the NEC, 12 AWG uncoated copper wire has a resistance of 1.98 ohms per 1,000 feet, while 10 AWG sits at 1.24 ohms per 1,000 feet. If you are calculating voltage drop for a standard 120V residential branch circuit, these base values are your starting point.
The Copper Wire Ohm Resistance Chart (NEC Chapter 9, Table 8)
This chart lists the direct current (DC) and alternating current (AC) resistance of uncoated copper wire. How to read this table: The 'Area (cmil)' column represents the cross-sectional area in circular mils. The 'Ohms/kft' column is your baseline multiplier for voltage drop calculations, representing the resistance of 1,000 feet of wire at 75°C. To find the resistance of a specific run, multiply the Ohms/kft value by your total wire length (out and back) in thousands of feet.
| AWG Size | Area (cmil) | DC Ohms/kft @ 75°C | AC Ohms/kft @ 75°C |
|---|---|---|---|
| 14 | 4,110 | 3.14 | 3.14 |
| 12 | 6,530 | 1.98 | 1.98 |
| 10 | 10,380 | 1.24 | 1.24 |
| 8 | 16,510 | 0.778 | 0.778 |
| 6 | 26,240 | 0.491 | 0.491 |
| 4 | 41,740 | 0.308 | 0.308 |
| 3 | 52,620 | 0.245 | 0.245 |
| 2 | 66,360 | 0.194 | 0.194 |
| 1 | 83,690 | 0.154 | 0.154 |
Which Column Applies to Your Installation?
For 95% of residential and light commercial DIY projects, the DC Ohms/kft and AC Ohms/kft columns are identical. At standard 60Hz power, the 'skin effect' (where AC current migrates to the outer edge of the conductor) is negligible in wire sizes smaller than 1/0 AWG. If you are wiring a standard 120V/240V branch circuit using THHN or NM-B, use the DC column for your math.
Uncoated vs. Coated: This table assumes bare, uncoated copper. If you are using tinned copper wire (common in marine environments or specific solar PV cables to prevent oxidation), the resistance increases by approximately 10% to 15%. For tinned wire, multiply the table values by 1.10 to maintain accurate voltage drop calculations.
How Temperature Derating Modifies the Base Value
The NEC Table 8 baseline is 75°C. However, copper resistance scales linearly with temperature. If your wire is running through a cold basement or a scorching attic, the actual resistance will differ from the chart. According to Engineering Toolbox material standards, copper has a temperature coefficient of resistance ($\alpha$) of roughly 0.00393 per °C.
The Derating Formula:
$R_{actual} = R_{table} \times [1 + 0.00393 \times (T_{actual} - 75)]$
Worked Example: You are running 10 AWG copper through an unconditioned attic that reaches 50°C (122°F) in the summer.
1. Base resistance (10 AWG) = 1.24 $\Omega$/kft.
2. Temperature delta = 50°C - 75°C = -25°C.
3. Multiplier = 1 + (0.00393 $\times$ -25) = 1 - 0.09825 = 0.90175.
4. Actual resistance = 1.24 $\times$ 0.90175 = 1.118 $\Omega$/kft.
In this cold environment, your voltage drop will actually be lower than the chart predicts.
Decision Path: Picking the Right Wire for a 3% Voltage Drop
The NEC recommends a maximum 3% voltage drop on branch circuits (3.6V on a 120V nominal system). Use this decision tree to select your exact wire size for a standard 15A continuous load on a 120V circuit. Note: Total wire length is double the one-way distance to account for the neutral return path.
| One-Way Distance | Total Wire Length | Calculated Drop (14 AWG) | Calculated Drop (12 AWG) | Concrete Pick (AWG) |
|---|---|---|---|---|
| 25 ft | 50 ft | 2.35V (Pass) | 1.48V (Pass) | Use 14 AWG |
| 50 ft | 100 ft | 4.71V (Fail) | 2.97V (Pass) | Use 12 AWG |
| 75 ft | 150 ft | 7.06V (Fail) | 4.45V (Fail) | Use 10 AWG (Drop: 2.79V) |
| 100 ft | 200 ft | 9.42V (Fail) | 5.94V (Fail) | Use 8 AWG (Drop: 2.33V) |
The Rule: If your calculated drop exceeds 3.6V, move up one AWG size in the master chart and recalculate. Never rely on the breaker's ampacity rating alone to dictate wire size for long runs; a 14 AWG wire will safely carry 15A without tripping a breaker, but at 100 feet, your appliance will only see 110V, causing motors to overheat and draw excess current.
Quick-Jump Reference: Most Queried AWG Sizes
Bookmark this section for quick bench-side math. These values assume a 75°C operating temperature and uncoated copper.
- 14 AWG (3.14 $\Omega$/kft): Standard for 15A lighting circuits. Max recommended one-way run for 15A at 3% drop: 38 feet.
- 12 AWG (1.98 $\Omega$/kft): Standard for 20A receptacle circuits. Max recommended one-way run for 15A at 3% drop: 60 feet.
- 10 AWG (1.24 $\Omega$/kft): Common for 30A dryer/RV outlets and long 20A runs. Max recommended one-way run for 20A at 3% drop: 60 feet.
- 8 AWG (0.778 $\Omega$/kft): Used for 40A ranges and long 30A runs. Max recommended one-way run for 30A at 3% drop: 77 feet.
- 6 AWG (0.491 $\Omega$/kft): Standard for 50A subpanel feeders and EV chargers. Max recommended one-way run for 40A at 3% drop: 91 feet.
When in doubt, measure the actual voltage at the furthest receptacle under a loaded condition using a true-RMS multimeter. If the reading falls below 114V on a 120V nominal system, your wire resistance is too high for the applied load, regardless of what the chart dictates.
