The DC resistance of standard annealed, uncoated copper wire at 75°C (167°F) ranges from 6.385 ohms per 1,000 ft for 18 AWG down to 0.0490 ohms per 1,000 ft for 4/0 AWG. These baseline values, sourced directly from NEC (NFPA 70) Chapter 9, Table 8, are the foundation for calculating voltage drop and power loss in any DC or single-phase AC branch circuit.
Before you size a wire based purely on breaker ampacity, you must verify that the resistance over your specific run length won't cause a voltage drop exceeding 3% for branch circuits or 5% for the entire feeder-to-branch path. Below is the complete reference data, followed by a concrete decision path to select your exact gauge.
How to Read This AWG Wire Resistance Table
This table uses the 75°C (167°F) DC resistance column for uncoated copper. This is the correct column to use for modern THHN/THWN-2 conductors terminated on standard 75°C-rated breakers, lugs, and receptacles. If you are using NM-B (Romex) cable, the insulation is rated for 90°C, but NEC 330.104 restricts the ampacity to the 60°C column; however, the physical resistance of the copper itself is still best calculated using the 75°C baseline unless the wire is sitting in a freezing environment.
Complete AWG Copper Wire Resistance Data (NEC Chapter 9, Table 8)
| AWG Size | Area (Circular Mils) | DC Resistance at 75°C (Ω / 1,000 ft) | DC Resistance at 75°C (Ω / km) |
|---|---|---|---|
| 18 | 1,620 | 6.385 | 20.94 |
| 16 | 2,580 | 4.016 | 13.17 |
| 14 | 4,110 | 3.070 | 10.07 |
| 12 | 6,530 | 1.980 | 6.496 |
| 10 | 10,380 | 1.240 | 4.068 |
| 8 | 16,510 | 0.778 | 2.552 |
| 6 | 26,240 | 0.490 | 1.607 |
| 4 | 41,740 | 0.308 | 1.010 |
| 3 | 52,620 | 0.245 | 0.803 |
| 2 | 66,360 | 0.194 | 0.636 |
| 1 | 83,690 | 0.154 | 0.505 |
| 1/0 | 105,600 | 0.122 | 0.400 |
| 2/0 | 133,100 | 0.0967 | 0.317 |
| 3/0 | 167,800 | 0.0766 | 0.251 |
| 4/0 | 211,600 | 0.0608 | 0.199 |
Source: NFPA 70 (National Electrical Code), Chapter 9, Table 8. Values are for uncoated, stranded copper.
Decision Path: Picking the Right Gauge for Voltage Drop
Ampacity tables (NEC 310.16) tell you what size wire prevents a fire. Resistance tables tell you what size wire prevents your tools from bogging down or your inverters from throwing low-voltage faults. Use this decision matrix to lock in your gauge.
The Scenario: You are wiring a 120V, 20A dedicated circuit for a workshop table saw. The one-way distance from the panel to the receptacle is 125 feet. The NEC recommends a maximum voltage drop of 3% for branch circuits (3.6V maximum drop).
| Step | Calculation / Check | Result |
|---|---|---|
| 1. Define Max Drop | 120V × 0.03 (3%) | 3.6 Volts |
| 2. Apply VD Formula | VD = 2 × Current × (Resistance per ft) × Distance VD = 2 × 20A × (R_1000 / 1000) × 125 ft VD = 5 × R_1000 |
Target: 5 × R_1000 ≤ 3.6 Therefore, R_1000 must be ≤ 0.72 Ω/kft |
| 3. Check 12 AWG | Table value: 1.98 Ω/kft 1.98 × 5 = 9.9V drop |
FAIL (Exceeds 3.6V, motor will stall) |
| 4. Check 10 AWG | Table value: 1.24 Ω/kft 1.24 × 5 = 6.2V drop |
FAIL (Exceeds 3.6V) |
| 5. Check 8 AWG | Table value: 0.778 Ω/kft 0.778 × 5 = 3.89V drop |
BORDERLINE FAIL (3.89V is > 3.6V) |
| 6. Check 6 AWG | Table value: 0.490 Ω/kft 0.490 × 5 = 2.45V drop |
PASS (2.45V is well under 3.6V) |
The Concrete Pick: For a 125-foot, 120V, 20A run, you must use 6 AWG copper wire. While 8 AWG is incredibly close and might run fine in practice, 6 AWG guarantees you stay strictly under the 3% NEC recommendation. Note that you will need to pigtail the 6 AWG wire down to 12 AWG or 10 AWG to physically fit it into the terminals of a standard 20A receptacle, or use a 50A receptacle configuration if the equipment allows.
How Temperature and Derating Modify Base Resistance
The values in the table above assume the copper is at 75°C. Copper has a positive temperature coefficient of resistance (roughly 0.00393 per °C). As the wire heats up from carrying current or sitting in a hot attic, its resistance increases.
- Cold Environments (e.g., 20°C / 68°F): Resistance drops to the baseline values found in the 20°C column (e.g., 12 AWG drops from 1.98 Ω/kft down to 1.588 Ω/kft). Your voltage drop will be lower than calculated.
- Hot Environments (e.g., 90°C / 194°F): If the wire is routed through a 120°F attic and carrying a heavy load, the conductor temperature can easily reach 90°C. At 90°C, 12 AWG resistance climbs to roughly 2.15 Ω/kft.
Derating Rule of Thumb: If your ambient temperature exceeds 86°F (30°C) and the wire is bundled with other current-carrying conductors, the wire will run hotter. Add a 5% to 10% penalty to your calculated voltage drop to account for this thermal resistance increase. For critical runs, use a dedicated voltage drop calculator that factors in ambient temperature and conduit fill.
What This Table Cannot Tell You
While this AWG wire resistance table is the definitive lookup for DC and standard 60Hz AC branch circuits, it has three strict limitations you must account for on the jobsite:
- It Does Not Dictate Ampacity: A 14 AWG wire might have low enough resistance for a short 10-foot run, but NEC 240.4(D) strictly limits 14 AWG copper to a 15A breaker, regardless of voltage drop. Always cross-reference NEC 310.16 ampacity tables first to satisfy fire codes, then use this resistance table to satisfy performance.
- It Ignores AC Skin Effect for Large Conductors: For alternating current, electrons tend to travel on the outer surface (skin) of the wire. For 14 through 2 AWG at 60Hz, this "skin effect" is negligible, and DC resistance equals AC resistance. However, once you move to 1/0 AWG and larger, AC resistance becomes measurably higher than the DC values listed here. For large feeders, you must consult NEC Chapter 9, Table 9, which provides AC resistance and reactance based on whether the conduit is magnetic (steel) or non-magnetic (PVC).
- It Is Useless for Aluminum Wire: If you are pulling SER cable or aluminum THHN for a subpanel feeder, do not use this table. AA-8000 series aluminum has roughly 61% of the conductivity of copper. To find the resistance of an aluminum wire, multiply the copper value in this table by 1.6, or simply look two AWG sizes higher on this chart (e.g., 2 AWG Aluminum has roughly the same resistance as 4 AWG Copper).
