The definitive resistance values chart for electrical wiring in the United States is published in the National Electrical Code (NEC) Chapter 9, specifically Tables 8 (DC resistance) and 9 (AC resistance and reactance). If you need the quick answer for the most common residential branch circuit sizes: standard 12 AWG solid copper wire has a DC resistance of 1.93 ohms per 1,000 feet (at 75°C), and 10 AWG copper is 1.24 ohms per 1,000 feet.
While ampacity tables tell you how much current a wire can carry before melting its insulation, the resistance values chart tells you how much voltage you will lose over distance. This is critical for sizing long feeder runs, low-voltage solar arrays, and motor circuits where excessive voltage drop causes equipment failure.
How to Read the Wire Resistance Values Chart
Before pulling wire or calculating voltage drop, you must understand how the NEC tables are structured. The chart is divided into several distinct columns, and picking the wrong one is the most common cause of voltage drop miscalculations on the jobsite.
- AWG or kcmil: The physical size of the wire. American Wire Gauge (AWG) is used for standard branch circuits (14 through 1 AWG), while thousand circular mils (kcmil) is used for large feeders (1/0 and larger).
- Area (Circular Mils): The cross-sectional area of the conductor. Resistance is inversely proportional to this area.
- DC Resistance at 75°C (Ω/kft): This is your primary column for DC circuits (solar, battery banks, automotive) and for estimating basic AC voltage drop. Assumption: This column assumes a conductor temperature of 75°C (167°F), which aligns with standard termination ratings for modern breakers and receptacles.
- AC Resistance and Reactance (Table 9): Used for precise AC mains calculations, factoring in the magnetic properties of the conduit (PVC vs. steel) and the NEC installation method.
If you are wiring a 12V/24V/48V DC solar array or battery bank, use the DC Resistance column. If you are calculating voltage drop for a 120V/240V AC mains feeder over 100 feet, you should technically use the AC Resistance column from NEC Table 9, as alternating current experiences higher effective resistance than direct current. However, for residential AC runs under 100 feet, the DC column provides a sufficiently conservative estimate.
Complete Wire Resistance Values Chart (NEC Table 8 Extract)
Below is the extracted DC resistance data for uncoated copper and aluminum conductors at 75°C, sourced directly from industry-standard manufacturer sizing guides and NEC Chapter 9, Table 8. We have highlighted the most queried residential and light-commercial sizes for quick bookmarking.
| AWG / kcmil | Area (cmil) | Copper DC Resistance (Ω/1000 ft) | Aluminum DC Resistance (Ω/1000 ft) |
|---|---|---|---|
| 14 AWG | 4,110 | 3.070 | 5.170 |
| 12 AWG | 6,530 | 1.930 | 3.250 |
| 10 AWG | 10,380 | 1.240 | 2.000 |
| 8 AWG | 16,510 | 0.778 | 1.260 |
| 6 AWG | 26,240 | 0.491 | 0.794 |
| 4 AWG | 41,740 | 0.308 | 0.498 |
| 3 AWG | 52,620 | 0.245 | 0.396 |
| 2 AWG | 66,360 | 0.194 | 0.314 |
| 1 AWG | 83,690 | 0.154 | 0.249 |
| 1/0 AWG | 105,600 | 0.122 | 0.197 |
| 2/0 AWG | 133,100 | 0.0967 | 0.156 |
| 3/0 AWG | 167,800 | 0.0766 | 0.124 |
| 4/0 AWG | 211,600 | 0.0608 | 0.0983 |
Source: NEC Chapter 9, Table 8 (Conductor Properties). Values represent uncoated conductors at 75°C. Local AHJ has final authority on code compliance.
Temperature Derating and What the Table Cannot Tell You
The values in the chart above are locked to a specific baseline: 75°C (167°F). But wire in a cold basement or a hot attic will not behave exactly as the table predicts. Resistance increases as temperature increases. For copper, the resistance increases by roughly 0.323% for every 1°C rise in temperature.
How derating modifies the base value:
If you are running a DC solar array in a cold environment where the wire temperature will only reach 20°C (68°F), the actual resistance will be lower than the chart. Use this formula to find the true resistance:
R_actual = R_chart × [1 + 0.00323 × (T_actual - 75)]
Where T_actual is the expected operating temperature of the wire in Celsius.
Worked Example: You are pulling 12 AWG copper wire for a 50-foot, 15-amp load in an environment where the wire will operate at 40°C.
1. Base resistance (from chart): 1.93 Ω/kft.
2. Derating math: 1.93 × [1 + 0.00323 × (40 - 75)] = 1.93 × [1 - 0.113] = 1.71 Ω/kft.
3. Total loop resistance (100 ft round trip): 0.171 Ω.
4. Voltage drop at 15A (Ohm's Law): 15A × 0.171 Ω = 2.56 volts dropped.
What the Table Cannot Tell You
While the Southwire voltage drop calculators and NEC tables are excellent starting points, they have blind spots:
- Skin Effect: In AC circuits, skin effect is the tendency of alternating current (AC) to distribute itself within a conductor such that the current density is largest near the surface of the conductor and decreases exponentially with greater depths. This effectively reduces the cross-sectional area carrying current, raising AC resistance above the DC values in Table 8.
- Terminal Contact Resistance: The chart assumes perfect connections. A loose lug or a corroded breaker terminal can add 0.5 Ω or more of resistance—completely wiping out the math on a long wire run.
- Conduit Fill Heating: If you pull nine current-carrying conductors through a single PVC conduit in the sun, the ambient temperature inside that conduit can exceed 60°C before the load is even turned on, drastically shifting your derating baseline.
Frequently Asked Questions
What is the standard resistance values chart for 12 AWG and 10 AWG copper wire?
According to NEC Chapter 9, Table 8, the standard DC resistance at 75°C for 12 AWG uncoated copper wire is 1.93 ohms per 1,000 feet. For 10 AWG uncoated copper wire, the resistance is 1.24 ohms per 1,000 feet. If you are using aluminum wire instead, the resistance jumps to 3.25 Ω/kft for 12 AWG and 2.00 Ω/kft for 10 AWG.
How do I use the resistance values chart for a 24V solar panel run?
For low-voltage DC solar runs, voltage drop is critical because a 2V drop on a 24V system is an 8.3% loss (well above the recommended 3% maximum). Use the DC Resistance column. Multiply the Ω/kft value by your total wire length (out and back, so double the physical distance), then multiply that result by your maximum power point current (Imp). If the resulting voltage drop exceeds 0.72V (3% of 24V), you must step up to the next wire gauge.
Does the wire resistance values chart apply to both solid and stranded wire?
Yes, but with a minor caveat. NEC Table 8 provides an average value that is generally acceptable for both solid and stranded conductors of the same AWG. However, stranded wire has a slightly higher resistance than solid wire of the exact same AWG because the spiraling of the strands makes the actual path of the current slightly longer than the linear length of the cable, and there are tiny air gaps between the strands reducing the pure copper cross-section. For standard residential and commercial voltage drop calculations, this difference is negligible and the table values apply to both.
Why is the AC resistance higher than the DC resistance in the NEC chart?
If you look at NEC Table 9 for AC resistance, you will notice the numbers are higher than the DC resistance in Table 8. This is due to the skin effect (mentioned above) and the proximity effect, where magnetic fields from adjacent conductors force the current into a smaller cross-sectional area of the wire. Additionally, Table 9 factors in the magnetic hysteresis and eddy current losses induced in steel conduits, which do not exist in DC circuits or non-magnetic PVC conduits.
