When you need to calculate voltage drop, size a feeder, or verify a wire run on the bench, the baseline figure you are looking for is DC resistance per unit length. For standard 12 AWG solid copper wire, that baseline is 1.93 ohms per 1,000 feet at 75°C. For 10 AWG copper, it is 1.21 ohms per 1,000 feet.
An ohms chart (formally derived from NEC Chapter 9, Table 8) maps American Wire Gauge (AWG) sizes to their exact DC resistance. While ampacity tables tell you how much current a wire can carry before melting its insulation, the ohms chart tells you how much voltage you will lose pushing that current through the metal. Below is the definitive reference data, how to apply temperature derating, and the physical limitations of these baseline numbers.
The AWG to Ohms Chart (NEC Chapter 9, Table 8 Data)
How to read this table: The data below is sourced directly from the National Electrical Code (NFPA 70), Chapter 9, Table 8. The Area column lists the cross-sectional area in circular mils (cmil). The Copper and Aluminum columns provide the uncoated DC resistance in ohms per 1,000 feet. Crucially, these base values are standardized at a conductor temperature of 75°C (167°F), which aligns with the standard temperature rating for most residential and commercial terminations.
Quick-Jump Reference for Most Queried Sizes:
- 14 AWG Copper: 3.07 Ω/kft (Max 15A circuits)
- 12 AWG Copper: 1.93 Ω/kft (Max 20A circuits)
- 10 AWG Copper: 1.21 Ω/kft (Max 30A circuits)
- 6 AWG Copper: 0.491 Ω/kft (Common for 50A-60A EV chargers/subpanels)
| AWG Size | Area (cmil) | Copper (Ω / 1,000 ft) | Aluminum (Ω / 1,000 ft) |
|---|---|---|---|
| 14 | 4,110 | 3.070 | 5.170 |
| 12 | 6,530 | 1.930 | 3.250 |
| 10 | 10,380 | 1.210 | 2.040 |
| 8 | 16,510 | 0.764 | 1.280 |
| 6 | 26,240 | 0.491 | 0.828 |
| 4 | 41,740 | 0.308 | 0.519 |
| 2 | 66,360 | 0.194 | 0.327 |
| 1/0 | 105,600 | 0.122 | 0.206 |
Applying Temperature Derating to Your Base Values
The most common mistake makers and apprentices make with an ohms chart is assuming the 75°C baseline applies to all operating conditions. Wire resistance is not static; it scales linearly with temperature. If your wire is sitting in a cold garage at 20°C (68°F), its resistance is significantly lower. If it is bundled tightly in an attic at 90°C (194°F) under full load, the resistance climbs, which in turn generates more heat—a compounding thermal loop.
Which Column and Temperature Applies to Your Installation?
If you are calculating voltage drop for a standard branch circuit terminating on a standard receptacle or breaker, use the 75°C column. Most residential terminals (like those on a Square D QO breaker or a Leviton outlet) are rated for 75°C. Even if you pull 90°C THHN wire through the conduit, the termination point limits the system to the 75°C baseline for resistance and ampacity calculations.
However, if you are designing a high-current DC system (like a 48V solar battery bank) where the ambient temperature is strictly controlled at 20°C, using the 75°C chart will overestimate your voltage drop by roughly 20%.
To adjust the base 75°C resistance ($R_{75}$) to a different operating temperature ($T$) in Celsius, use the copper temperature coefficient approximation:
R_T = R_75 × [1 + 0.00364 × (T - 75)]Example: For 10 AWG copper at 20°C:
1.21 × [1 + 0.00364 × (20 - 75)] = 1.21 × 0.799 = 0.967 Ω/kft.
Worked Example: 240V EV Charger Voltage Drop
Let’s apply this to a real-world scenario. You are wiring a 48A continuous load EV charger using a 60A breaker. The one-way distance from the panel to the charger is 80 feet. You plan to use 6 AWG copper THHN.
- Find Base Resistance: 6 AWG Copper = 0.491 Ω / 1,000 ft.
- Calculate Run Resistance: 0.491 × (80 / 1000) = 0.03928 Ω (one way).
- Account for the Return Path: Multiply by 2 for the complete circuit = 0.07856 Ω total loop resistance.
- Calculate Voltage Drop: $V_{drop} = I × R = 48A × 0.07856Ω = 3.77V$.
- Check Percentage: (3.77V / 240V) × 100 = 1.57%.
A 1.57% drop is excellent (the NEC recommends keeping branch circuit drop under 3%). If you had mistakenly chosen 8 AWG wire to save money, the drop would jump to 2.44%, still technically compliant but running much hotter in the walls. For a deeper look at how wire sizing interacts with thermal limits, reference the Southwire Voltage Drop Calculator and ampacity tables.
What the Ohms Chart Cannot Tell You
While the NEC Chapter 9 ohms chart is the gold standard for DC resistance, treating it as a universal truth for all electrical parameters will lead to design failures in specific edge cases. Here is what the table leaves out.
1. AC Impedance and Skin Effect
The chart provides DC resistance. For standard residential wiring (14 AWG to 2 AWG) at 60Hz, DC resistance and AC impedance are virtually identical. However, once you move into large feeders (1/0 AWG and larger) or high-frequency applications (like variable frequency drives or inverter outputs), skin effect and proximity effect force the AC current to travel only on the outer edge of the conductor. This effectively reduces the cross-sectional area, raising the AC impedance above the DC resistance listed in the chart. For large AC feeders, you must consult NEC Chapter 9, Table 9 for AC impedance values, which factor in reactance.
2. Termination and Connection Resistance
The chart assumes a continuous, unbroken length of perfect metal. In reality, every lug, wire nut, crimp, and terminal block introduces contact resistance. A poorly torqued breaker terminal or an oxidized aluminum lug can easily add 0.05 to 0.10 ohms of resistance to a circuit. On a 50A load, a 0.10-ohm bad connection will dissipate 250 watts of heat ($I^2R$) directly at the terminal—enough to melt the breaker bus bar and start a fire, regardless of how perfectly you sized the wire based on the ohms chart. Always use a calibrated torque screwdriver (like the Wera VDE series) set to the manufacturer's specified inch-pound rating.
3. Ampacity and Thermal Limits
Do not confuse the ohms chart with an ampacity chart (NEC Table 310.16). The ohms chart tells you that 14 AWG wire has 3.07 ohms per 1,000 feet. It does not tell you that 14 AWG is legally and thermally limited to 15 amps in most residential applications. If you push 30 amps through 14 AWG wire, the ohms chart will accurately tell you your voltage drop, but it won't warn you that the insulation is actively melting. Always cross-reference your resistance calculations with the appropriate ampacity and derating tables for the specific insulation type (THHN, XHHW, NM-B) and ambient temperature of your installation.
Bench Tip: When measuring the actual resistance of a short wire run with a standard digital multimeter, subtract the resistance of your test leads. Touch the probes together, note the lead resistance (usually 0.2Ω to 0.5Ω on cheap leads), and subtract it from your final reading. For measurements under 1 ohm, switch to a milliohm meter or use a 4-wire Kelvin measurement setup to eliminate lead resistance entirely.
