When sizing conductors for a new branch circuit or calculating voltage drop for a long feeder run, guessing the resistance will lead to undersized wire, excessive heat, and dimming lights. The definitive reference for this is the wire resistance ohm chart, derived directly from NEC Chapter 9, Table 8. For a quick baseline: standard 12 AWG solid copper wire has a DC resistance of 1.588 ohms per 1,000 feet at 75°C.
Below is the complete master reference combining the resistance data with the ampacity limits from NEC 310.16, followed by the exact math you need to translate these raw numbers into real-world voltage drop calculations.
How to Read the Wire Resistance and Ohm Chart
This chart merges two distinct NEC standards: Chapter 9, Table 8 (which dictates physical properties and DC resistance) and Table 310.16 (which dictates allowable ampacity). To use it correctly, you must understand which column applies to your specific installation and how environmental factors alter the base values.
Which Column Applies to Your Installation?
- 60°C Ampacity Column: Use this for non-metallic sheathed cable (NM-B / Romex) and any termination rated for 60°C. Even if the wire insulation is rated higher, the 60°C column limits the circuit.
- 75°C Ampacity Column: Use this for THHN/THWN-2 wires in conduit terminating at standard modern breakers, lugs, and receptacles, which are almost universally rated for 75°C.
- 90°C Ampacity Column: You cannot use this column for final overcurrent protection sizing. It is used strictly as the starting baseline for applying ambient temperature and bundling derating factors.
- Ohms/1,000 ft Column: This is your raw DC resistance at 75°C. Use this for voltage drop calculations.
How Derating Modifies the Base Values
Derating applies only to the ampacity columns, not the resistance column. If you pull four current-carrying conductors in a single conduit, NEC Table 310.15(C)(1) requires you to derate the ampacity to 80%. You take the 90°C column value, multiply it by 0.80, and then verify the result does not exceed the 75°C column limit for your terminations.
However, resistance does change with temperature. If your wire operates at a higher temperature due to high load or hot attics, the resistance increases. According to All About Circuits, copper's temperature coefficient is roughly 0.00393 per °C. If your wire heats to 90°C, the resistance increases by about 5.8% over the 75°C baseline.
The Master AWG Ohm and Ampacity Reference Table
Use the quick-jump links below to find the most commonly queried sizes for residential and commercial branch circuits.
- 14 AWG (Lighting)
- 12 AWG (Receptacles)
- 10 AWG (Water Heaters / AC)
- 8 AWG (Ranges / EV Chargers)
- 6 AWG (Subpanels / Heavy EV)
| AWG Size | Area (kcmil) | 60°C Amps (NM-B) | 75°C Amps (THHN Term) | 90°C Amps (Derating Base) | DC Ohms / 1,000 ft (75°C) |
|---|---|---|---|---|---|
| 14 | 4.11 | 15 | 20 | 25 | 3.070 |
| 12 | 6.53 | 20 | 25 | 30 | 1.930 |
| 10 | 10.4 | 30 | 35 | 40 | 1.210 |
| 8 | 16.5 | 40 | 50 | 55 | 0.764 |
| 6 | 26.3 | 55 | 65 | 75 | 0.491 |
| 4 | 41.7 | 70 | 85 | 95 | 0.308 |
| 3 | 52.6 | 85 | 100 | 115 | 0.245 |
| 2 | 66.4 | 95 | 115 | 130 | 0.194 |
| 1 | 83.7 | 110 | 130 | 145 | 0.154 |
| 1/0 | 106 | 125 | 150 | 170 | 0.122 |
| 2/0 | 133 | 145 | 175 | 195 | 0.0967 |
| 3/0 | 168 | 165 | 200 | 225 | 0.0766 |
| 4/0 | 212 | 195 | 230 | 260 | 0.0608 |
Source: Resistance values from NEC Chapter 9, Table 8 (Copper, Uncoated). Ampacity values from NEC Table 310.16 (Copper, 60°C/75°C/90°C columns, 30°C ambient). Always defer to your local AHJ for final code compliance.
What This Ohm Chart Cannot Tell You
While the chart above is the gold standard for DC resistance and thermal limits, it has blind spots that cause errors in complex or large-scale installations.
1. AC Impedance and Skin Effect
The ohms/1,000 ft column lists DC resistance. For wire sizes 1/0 AWG and larger carrying alternating current, the magnetic field pushes electrons toward the outer skin of the conductor (skin effect). This effectively reduces the cross-sectional area, increasing the actual AC resistance. For large feeders, you must consult NEC Chapter 9, Table 9, which provides AC impedance (Z) values that factor in reactance and skin effect based on conduit material (PVC vs. Steel).
2. Actual Voltage Drop Without Math
The chart gives you resistance per 1,000 feet, but it does not tell you if your circuit will fail. You must apply the single-phase voltage drop formula:
VD = (2 × L × I × R) / 1000
Where L = one-way length in feet, I = current in amps, R = ohms/1000ft from the chart.
The NEC recommends a maximum 3% voltage drop on branch circuits and 5% total from the service entrance to the furthest outlet.
3. Stranded vs. Solid Wire Variance
Table 8 lists values that are technically an average, but physically, stranded wire has a slightly higher resistance than solid wire of the same AWG. Because the individual strands spiral (lay length), the actual path the electrons travel is roughly 1% to 2% longer than the physical length of the cable. For precision low-voltage DC electronics, always measure stranded wire directly with a milliohm meter.
Frequently Asked Questions
How do I use an ohm chart to calculate voltage drop for a 50-foot 12 AWG circuit?
First, find 12 AWG on the chart: the resistance is 1.930 ohms/1000ft. If your load draws 15 amps and the one-way distance is 50 feet, plug it into the formula: VD = (2 × 50 × 15 × 1.930) / 1000. This equals 2.895 volts. On a 120V circuit, that is a 2.41% drop, which is well within the NEC recommended 3% limit. If the run was 100 feet, the drop would be 5.79V (4.8%), requiring an upsizing to 10 AWG.
Why does the resistance on my multimeter differ from the ohm chart?
Three factors cause this discrepancy. First, your multimeter leads have their own resistance (often 0.2 to 0.5 ohms); you must short the probes together and subtract this baseline. Second, the chart assumes a wire temperature of 75°C (167°F). If you are measuring a spool in a 65°F basement, the copper will read roughly 4% lower resistance than the chart. Third, if the wire is heavily oxidized or the termination is loose, contact resistance will artificially inflate your reading.
Does this ohm chart apply to aluminum wire?
No. This chart is strictly for uncoated copper. Aluminum has roughly 1.6 times the resistance of copper for the same AWG size. To find the aluminum equivalent, you generally need to step up two AWG sizes (e.g., 2 AWG Aluminum has a similar resistance and ampacity to 4 AWG Copper). Always consult NEC Chapter 9, Table 8 specifically for the 'Aluminum' column when sizing AL/CU feeders.
What is the difference between DC resistance and AC impedance on the chart?
DC resistance (the column in our chart) is the pure opposition to electron flow based on the wire's material and cross-section. AC impedance includes that base resistance plus inductive reactance caused by the alternating magnetic field around the wire. For small wires (14 to 2 AWG), the reactance is negligible, so DC resistance is perfectly fine for voltage drop math. For 1/0 AWG and larger, the inductive reactance becomes significant, especially if the wires are run in steel conduit, which amplifies the magnetic field. In those cases, AC impedance must be used.
