The baseline DC resistance for standard solid and stranded copper wire is defined in the NEC Chapter 9, Table 8 ohms value chart. For the most common branch circuit sizes at the baseline temperature of 20°C (68°F), the uncoated copper DC resistance per 1,000 feet is: 14 AWG = 3.07 Ω, 12 AWG = 1.93 Ω, 10 AWG = 1.21 Ω, and 8 AWG = 0.764 Ω. However, because wire heats up under load, using these 20°C baseline numbers for voltage drop calculations will result in undersized conductors. You must apply temperature derating to find the real-world operating resistance.
How to Read the NEC Ohms Value Chart
The definitive source for conductor resistance in the US is the National Electrical Code (NFPA 70), specifically Chapter 9, Table 8. When looking up a value, you must understand three critical columns:
- AWG or kcmil: The physical size of the wire. American Wire Gauge (AWG) is used for sizes 4/0 and smaller. For larger conductors, the chart switches to thousands of circular mils (kcmil).
- Area (Circular Mils): The cross-sectional area of the conductor. This is useful when calculating parallel runs or verifying the exact copper mass.
- Ohms per 1,000 ft (DC): The chart splits this into two sub-columns: Uncoated (bare copper) and Coated (tinned copper). Tinned wire has a slightly higher resistance because the tin plating is less conductive than copper. Always use the Uncoated column for standard THHN/THWN-2 and NM-B building wire unless the datasheet explicitly states the strands are tinned.
Critical Assumption: The values in NEC Table 8 are measured at exactly 20°C (68°F). They do not reflect the resistance of the wire when it is carrying a full load inside a warm attic or conduit.
Bookmark-Friendly Quick-Jump Reference (75°C Operating)
Jobsite Quick-Jump: For standard 75°C rated terminals and THHN wire operating under load, use these temperature-adjusted resistance values per 1,000 ft for your most common branch circuits:
- 14 AWG: 3.69 Ω/kft (Max 15A)
- 12 AWG: 2.32 Ω/kft (Max 20A)
- 10 AWG: 1.46 Ω/kft (Max 30A)
- 8 AWG: 0.919 Ω/kft (Max 40A/50A)
- 6 AWG: 0.510 Ω/kft (Max 55A/65A)
- 4 AWG: 0.321 Ω/kft (Max 70A/85A)
- 2 AWG: 0.194 Ω/kft (Max 95A/115A)
Complete Copper Wire Ohms Value Chart (20°C Baseline)
Below is the complete data table derived from NEC Chapter 9, Table 8. This table provides the baseline DC resistance before temperature correction factors are applied.
| AWG / kcmil | Area (cmil) | Ohms/1000 ft (DC, Uncoated, 20°C) | Ohms/1000 ft (DC, Coated/Tinned, 20°C) |
|---|---|---|---|
| 18 | 1,620 | 7.77 | 8.08 |
| 16 | 2,580 | 4.89 | 5.08 |
| 14 | 4,110 | 3.07 | 3.26 |
| 12 | 6,530 | 1.93 | 2.05 |
| 10 | 10,380 | 1.21 | 1.29 |
| 8 | 16,510 | 0.764 | 0.809 |
| 6 | 26,240 | 0.491 | 0.510 |
| 4 | 41,740 | 0.308 | 0.328 |
| 3 | 52,620 | 0.245 | 0.258 |
| 2 | 66,360 | 0.194 | 0.205 |
| 1 | 83,690 | 0.154 | 0.162 |
| 1/0 | 105,600 | 0.122 | 0.129 |
| 2/0 | 133,100 | 0.0967 | 0.102 |
| 3/0 | 167,800 | 0.0766 | 0.0811 |
| 4/0 | 211,600 | 0.0608 | 0.0641 |
| 250 kcmil | 250,000 | 0.0515 | 0.0545 |
| 350 kcmil | 350,000 | 0.0367 | 0.0389 |
| 500 kcmil | 500,000 | 0.0258 | 0.0273 |
Temperature Derating: Modifying the Base Resistance
Copper is a metal with a positive temperature coefficient; as it gets hotter, its resistance increases. According to the Copper Development Association, the resistance of copper increases by approximately 0.393% for every 1°C rise above 20°C.
To find the real-world resistance at your operating temperature, use this formula:
R_T = R_20 × [1 + 0.00393 × (T - 20)]
Worked Numeric Example:
You are running a 12 AWG THHN wire to a 20A compressor. The wire will operate at roughly 60°C inside a hot conduit.
1. Base resistance (from chart): 1.93 Ω/kft.
2. Temperature delta: 60°C - 20°C = 40°C.
3. Calculation: 1.93 × [1 + 0.00393 × 40] = 1.93 × 1.1572 = 2.23 Ω/kft.
If you had used the baseline 1.93 Ω/kft for your voltage drop calculation, you would have underestimated the voltage drop by nearly 15%.
Decision Path: Sizing Wire for Voltage Drop
Voltage drop is calculated using the formula: VD = (2 × L × I × R) / 1000, where L is the one-way distance in feet, I is the current in amps, and R is the temperature-adjusted resistance per 1,000 ft. The NEC recommends a maximum 3% voltage drop on branch circuits (3.6V on a 120V system; 7.2V on a 240V system).
Use this decision tree to select your exact wire gauge for standard 120V/240V residential and light commercial branch circuits.
| Condition / Load Profile | One-Way Distance | Concrete Pick (AWG) |
|---|---|---|
| 15A Load (120V or 240V) | 0 to 50 ft | 14 AWG (Drop < 2%) |
| 15A Load (120V or 240V) | 51 to 100 ft | 12 AWG (Drop < 3%) |
| 15A Load (120V or 240V) | 101 to 150 ft | 10 AWG (Drop < 3%) |
| 20A Load (120V or 240V) | 0 to 50 ft | 12 AWG (Drop < 2.5%) |
| 20A Load (120V or 240V) | 51 to 100 ft | 10 AWG (Drop < 3%) |
| 20A Load (120V or 240V) | 101 to 150 ft | 8 AWG (Drop < 3%) |
| 30A Load (240V only) | 0 to 75 ft | 10 AWG (Drop < 2.5%) |
| 30A Load (240V only) | 76 to 150 ft | 8 AWG (Drop < 3%) |
Default Recommendation: If your calculation lands exactly on the 3.0% threshold, or if the load is a continuous motor (which requires 125% sizing for voltage drop purposes), step up one full AWG size from the table above. The material cost difference between 12 AWG and 10 AWG copper is typically less than $15 per 100 ft, while the cost of a motor burning out from low voltage is hundreds of dollars.
What This Chart Cannot Tell You
While the Chapter 9 Table 8 ohms value chart is the gold standard for DC resistance, relying on it blindly for complex AC installations will lead to errors. Here is what the chart omits:
- AC Impedance (Reactance): For conductors larger than 1/0 AWG, the magnetic field around the wire creates inductive reactance, and the skin effect forces current to the outer edge of the conductor. This makes the total AC impedance (Z) higher than the DC resistance (R). For large feeders, you must use NEC Chapter 9, Table 9, which provides AC resistance and reactance values based on the conduit material (PVC vs. Aluminum vs. Steel).
- Ampacity Limits: This chart tells you how much the wire resists current, not how much current it can safely carry before melting the insulation. A 14 AWG wire might have low enough resistance for a 50-foot run at 25A, but NEC Article 310.16 strictly limits 14 AWG to a 15A breaker. Always cross-reference your resistance-based voltage drop pick against the 60°C, 75°C, or 90°C ampacity columns in Table 310.16.
- Parallel Conductor Math: If you are running parallel sets of wire (typically required for feeders 400A and above), the chart does not automatically adjust for you. You must divide the one-way resistance value by the number of parallel sets. For example, two parallel runs of 500 kcmil wire will have exactly half the resistance per 1,000 ft of a single run.
