The definitive resistance value chart for electrical wiring in the US is NEC Chapter 9, Table 9. For standard 12 AWG uncoated copper wire at 75°C, the baseline DC resistance is 1.98 ohms per 1,000 feet. Whether you are calculating voltage drop for a 50-foot well pump run, sizing battery cables for a 48V solar bank, or verifying conductor integrity before energizing a panel, this chart is your starting point. Below is the complete lookup table, followed by the exact derating math and limitations you need to know for real-world installations.
How to Read the NEC Resistance Value Chart
The resistance value chart is divided into three primary material columns. Choosing the wrong column is a common mistake that leads to undersized feeders and excessive voltage drop.
- Uncoated Copper: This is the column you will use 90% of the time. It applies to standard modern building wire like THHN, THWN-2, and XHHW-2 with bare copper conductors.
- Coated Copper: Use this column for tinned copper wire, commonly found in marine environments (boat wiring), outdoor solar tray cable, or older NM-B Romex where the copper was tinned to resist oxidation.
- Aluminum: Use this for XHHW-2 or USE-2 aluminum service entrance conductors and feeder wires (e.g., 2-2-2-4 SER cable for subpanels).
Complete AWG Resistance Data Table (NEC Table 9)
The following data is sourced directly from the National Fire Protection Association (NFPA 70), Chapter 9, Table 9. Values represent DC resistance at 75°C in ohms per 1,000 feet (Ω/kft).
Bookmark quick-jump links for the most queried residential and DIY sizes: 14 AWG | 12 AWG | 10 AWG | 8 AWG | 6 AWG
| AWG Size | Uncoated Copper (Ω/kft) | Coated Copper (Ω/kft) | Aluminum (Ω/kft) |
|---|---|---|---|
| 14 | 3.14 | 3.26 | 5.17 |
| 12 | 1.98 | 2.05 | 3.25 |
| 10 | 1.24 | 1.29 | 2.04 |
| 8 | 0.778 | 0.809 | 1.28 |
| 6 | 0.491 | 0.510 | 0.808 |
| 4 | 0.308 | 0.321 | 0.508 |
| 3 | 0.245 | 0.254 | 0.403 |
| 2 | 0.194 | 0.201 | 0.319 |
| 1 | 0.154 | 0.160 | 0.253 |
| 1/0 | 0.122 | 0.127 | 0.201 |
| 2/0 | 0.0967 | 0.101 | 0.159 |
| 3/0 | 0.0766 | 0.0797 | 0.126 |
| 4/0 | 0.0608 | 0.0632 | 0.100 |
Derating, Voltage Drop, and What the Table Cannot Tell You
The chart above provides a baseline, but real-world installations rarely operate at exactly 75°C in a vacuum. Here is how derating rows and environmental factors modify the base value.
How Temperature Derating Modifies the Base Value
Copper has a positive temperature coefficient of resistance, meaning as the wire heats up from ambient temperature or I²R (current squared times resistance) losses, its resistance increases. According to Georgia State University's HyperPhysics database, the temperature coefficient for copper is approximately 0.00393 per °C.
If you are running wire through a hot attic at 50°C (122°F) ambient, and the wire heats up to 90°C under load, you must adjust the chart value using this formula:
R_actual = R_chart × [1 + 0.00393 × (T_actual - 75)]
For 10 AWG copper at 90°C: 1.24 × [1 + 0.00393 × (90 - 75)] = 1.31 Ω/kft. This 5.6% increase can push a marginal voltage drop calculation over the NEC recommended 3% limit.
What the Table Cannot Tell You
- AC Impedance and Reactance: Table 9 only lists DC resistance. For AC circuits, especially with conductors larger than 1/0 AWG or when wires are pulled through steel conduit, inductive reactance becomes significant. You must use NEC Chapter 9, Table 9's AC impedance columns (which factor in power factor and conduit material) for large AC feeder calculations.
- High-Frequency Skin Effect: If you are designing high-frequency inverter outputs or RF links, current travels primarily on the outer surface of the conductor (skin effect), effectively reducing the cross-sectional area and raising the AC resistance well above the DC chart value.
- Termination Contact Resistance: The chart assumes a perfect, continuous conductor. It does not account for the micro-ohms of resistance added by lug crimps, terminal block screws, or oxidation at the breaker connection.
Frequently Asked Questions
How do I use the resistance value chart to calculate voltage drop?
To calculate single-phase voltage drop, use the formula: VD = (2 × L × I × R) / 1000, where L is the one-way length in feet, I is the current in amps, and R is the resistance value from the chart (Ω/kft). For example, a 15A load on a 100-foot run of 12 AWG uncoated copper: VD = (2 × 100 × 15 × 1.98) / 1000 = 5.94V. On a 120V circuit, that is a 4.95% drop, which exceeds the 3% NEC recommendation, meaning you should upsize to 10 AWG for that specific run.
Does the resistance value chart apply to DC solar and battery cables?
Yes, but the stakes are much higher. In a 12V or 24V DC battery system, even a 0.5V drop represents a massive percentage of your system voltage, leading to inverter brownouts and inefficient charging. Always use the uncoated copper column for standard battery cables, but verify your lugs are crimped with a proper hydraulic or ratchet crimper. A poorly crimped 2/0 AWG lug can introduce more resistance than 50 feet of the cable itself.
Why is my multimeter reading a different resistance than the chart?
If you measure a 100-foot spool of 12 AWG wire and your multimeter reads 0.45Ω instead of the expected 0.198Ω (1.98 Ω/kft × 0.1 kft), you are likely measuring the resistance of your test leads and the contact resistance of the probes. Standard multimeter leads can add 0.2Ω to 0.5Ω of resistance. To get an accurate reading, short your probes together, note the baseline resistance, and subtract that from your final measurement. For highly accurate low-resistance measurements, use a milliohm meter or a Kelvin (4-wire) measurement setup.
