When sizing conductors for long runs like solar arrays, subpanel feeders, or EV charger circuits, guessing the resistance leads to excessive voltage drop, overheated insulation, and nuisance breaker trips. The exact ohms chart value—defined as the DC resistance per 1,000 feet of conductor—is your mandatory baseline for calculating voltage drop and verifying wire ampacity. Below is the complete reference data, derating formulas, and application rules you need to size your next run correctly.
How to Read the NEC Chapter 9 Ohms Chart Value Table
The definitive source for conductor resistance in the United States is NFPA 70 (National Electrical Code), specifically Chapter 9, Table 8. This table lists the physical properties of conductors, but the most critical column for circuit design is the DC Resistance at 75°C (167°F), measured in ohms per 1,000 feet (Ω/kft).
Which column applies to your installation? Table 8 provides resistance values for Uncoated Copper, Coated (Tinned) Copper, and Aluminum. For 95% of standard residential and commercial branch circuits using THHN, THWN-2, or XHHW-2 wire, you must use the Uncoated Copper column. Use the Coated Copper column only if you are using tinned marine-grade wire or specific solar PV wire (like PV Wire or USE-2 with tinned strands). Use the Aluminum column for SER cable or XHHW-2 aluminum feeders. The 75°C baseline is used because most modern breakers, lugs, and receptacles are rated for 75°C terminations, even if the wire insulation itself is rated for 90°C.
The Complete Wire Resistance Reference Chart
Before scrolling to the full matrix, here are the bookmark-friendly quick-jump rows for the most queried AWG sizes used in DIY and professional electrical work:
- 14 AWG (15A circuits): 2.57 Ω/kft
- 12 AWG (20A circuits): 1.98 Ω/kft
- 10 AWG (30A circuits): 1.24 Ω/kft
- 8 AWG (40A/50A circuits): 0.778 Ω/kft
- 6 AWG (60A subpanels): 0.491 Ω/kft
- 2 AWG (100A feeders): 0.194 Ω/kft
| AWG / kcmil Size | Copper Uncoated (Ω/kft) | Copper Coated/Tinned (Ω/kft) | Aluminum (Ω/kft) |
|---|---|---|---|
| 14 AWG | 2.57 | 2.63 | 4.25 |
| 12 AWG | 1.98 | 2.03 | 3.25 |
| 10 AWG | 1.24 | 1.28 | 2.04 |
| 8 AWG | 0.778 | 0.809 | 1.28 |
| 6 AWG | 0.491 | 0.510 | 0.808 |
| 4 AWG | 0.308 | 0.320 | 0.508 |
| 3 AWG | 0.245 | 0.254 | 0.403 |
| 2 AWG | 0.194 | 0.201 | 0.319 |
| 1 AWG | 0.154 | 0.160 | 0.253 |
| 1/0 AWG | 0.122 | 0.127 | 0.200 |
| 2/0 AWG | 0.0967 | 0.101 | 0.159 |
| 3/0 AWG | 0.0766 | 0.0797 | 0.126 |
| 4/0 AWG | 0.0608 | 0.0626 | 0.100 |
How Temperature Derating Modifies the Base Ohms Chart Value
The values in the table above are locked at 75°C. However, copper is a positive temperature coefficient (PTC) material; as it gets hotter, its resistance increases. If your wire is routed through a 120°F (49°C) attic in the summer, or if you are calculating cold-weather resistance for a solar array at 14°F (-10°C), you must adjust the base ohms chart value.
To find the adjusted resistance, use the standard temperature coefficient formula for copper:
R_adjusted = R_base × [1 + α(T_actual - 75)]
Where α (alpha) is the temperature coefficient of copper at 75°C, which is approximately 0.00393 per °C. (For a deeper dive into the physics of conductor resistivity, refer to the All About Circuits DC textbook chapter on wire resistance).
Worked Numeric Example:
You are running a 10 AWG copper circuit through an attic that reaches 50°C (122°F) in the summer. The base ohms chart value for 10 AWG at 75°C is 1.24 Ω/kft.
1. Temperature difference: 50°C - 75°C = -25°C.
2. Multiplier: 1 + [0.00393 × (-25)] = 1 - 0.09825 = 0.90175.
3. Adjusted Resistance: 1.24 Ω/kft × 0.90175 = 1.118 Ω/kft.
Conversely, if that same attic hits 90°C, the resistance jumps to 1.31 Ω/kft, increasing your voltage drop by over 5% compared to the baseline table.
What the Resistance Table Cannot Tell You
While the NEC Chapter 9 table is the gold standard for DC resistance, it has blind spots that cause real-world failures if ignored:
- AC Impedance and Skin Effect: For circuits larger than 1/0 AWG carrying alternating current, the effective resistance (impedance) is higher than the DC ohms chart value due to the skin effect and proximity effect. For precise AC voltage drop on large feeders, you must use NEC Chapter 9, Table 9 (AC Resistance and Reactance), which factors in power factor and magnetic conduit interactions.
- Termination Resistance: The table assumes a perfect, continuous conductor. It does not account for the micro-ohms of resistance added by loose lugs, oxidized aluminum connections, or undersized wire nuts. A 12 AWG copper wire torqued to 15 in-lbs will have significantly less termination heating than one hand-tightened to 5 in-lbs.
- Stranded vs. Solid Variance: While Table 8 lists both, stranded wire has a slightly higher DC resistance than solid wire of the same AWG due to the air gaps between the individual strands and the longer helical path the electrons must travel. Always use the stranded column if you are pulling THHN from a spool.
Frequently Asked Questions About Ohms Chart Values
What is the exact ohms chart value for 12 AWG copper wire?
According to NEC Chapter 9, Table 8, the DC resistance for 12 AWG uncoated solid copper wire at 75°C is 1.98 ohms per 1,000 feet. If you are using 12 AWG stranded copper, the value is slightly higher at 2.01 ohms per 1,000 feet. For a standard 100-foot total circuit run (50 feet out, 50 feet back), this equates to roughly 0.198 ohms of total loop resistance.
How do I find the ohms chart value for stranded vs. solid wire?
NEC Table 8 separates these into distinct sub-columns under the main material headings. Solid wire is listed as 'Solid' under the stranding column, while stranded is listed by the number of strands (e.g., 7 or 19). Stranded wire will always show a marginally higher ohms chart value (usually 1% to 3% higher) than its solid counterpart because the physical cross-sectional area of the metal is slightly reduced by the spiral lay of the strands.
Why does my multimeter read a different ohms chart value than the table?
Standard digital multimeters (DMMs) inject a very low test current, which is highly susceptible to contact resistance at the probe tips and the alligator clips. If you measure a 50-foot spool of 14 AWG wire and read 0.4 ohms instead of the expected 0.128 ohms, the discrepancy is almost entirely caused by the resistance of your test leads and the probe-to-copper contact point. To accurately verify the ohms chart value on the bench, you must use a 4-wire Kelvin measurement or subtract the shorted-lead resistance from your final reading.
Does the ohms chart value change if I use aluminum instead of copper?
Yes, dramatically. Aluminum has roughly 61% of the conductivity of copper. If you look at the table above, a 2 AWG copper wire has a resistance of 0.194 Ω/kft, while a 2 AWG aluminum wire has a resistance of 0.319 Ω/kft. To achieve the same voltage drop and resistance characteristics as copper, you must step up aluminum wire by one to two AWG sizes (e.g., using 1/0 AWG aluminum to replace 2 AWG copper for a 100A feeder).
