The ampacity wire chart—specifically Table 310.16 in the National Electrical Code (NEC)—defines the maximum continuous current a conductor can carry before its insulation begins to thermally degrade. For standard residential branch circuits using copper wire, the baseline rule of thumb is simple: 14 AWG is rated for 15 amps, 12 AWG for 20 amps, and 10 AWG for 30 amps. However, those numbers only apply if you are reading the correct temperature column and haven't triggered a derating penalty.
Misreading this chart is one of the most common reasons DIYers and junior apprentices end up with melted terminal lugs or failed inspections. Below is the complete reference data, the rules for reading it, and the edge cases the chart leaves out.
How to Read the NEC Ampacity Wire Chart (Table 310.16)
The most critical mistake made when looking up wire sizes is defaulting to the highest number on the page. The chart is divided into temperature columns (60°C, 75°C, and 90°C) based on the thermal rating of the wire's insulation. Here is how to determine which column applies to your installation:
- Use the 60°C Column: For circuits rated 100 amps or less, or using wire sizes 14 AWG through 1 AWG, unless the equipment is explicitly marked otherwise.
- Use the 75°C Column: For circuits rated over 100 amps, or using wire sizes larger than 1 AWG (e.g., 1/0 AWG and up), assuming modern 75°C-rated terminations.
- Use the 90°C Column: Almost exclusively as the starting baseline for derating calculations (adjusting for heat or bundled wires), not for final overcurrent protection sizing.
Furthermore, NEC 240.4(D) places hard limits on small conductors regardless of their insulation rating: 14 AWG copper is capped at 15A, 12 AWG at 20A, and 10 AWG at 30A for standard overcurrent protection.
The Complete Copper Ampacity Data Table
The following table covers the most queried copper wire sizes for residential and light commercial use. Source: NFPA 70, National Electrical Code (NEC), Table 310.16 (Allowable Ampacities of Insulated Conductors).
Quick-Jump Bookmarks: 14 AWG | 12 AWG | 10 AWG | 8 AWG | 6 AWG | 4 AWG | 2 AWG
| Wire Size (AWG/kcmil) | 60°C Column (TW, UF, NM-B) | 75°C Column (THW, THWN, XHHW) | 90°C Column (THHN, THWN-2) |
|---|---|---|---|
| 14 AWG | 15A * | 20A * | 25A * |
| 12 AWG | 20A * | 25A * | 30A * |
| 10 AWG | 30A * | 35A * | 40A * |
| 8 AWG | 40A | 50A | 55A |
| 6 AWG | 55A | 65A | 75A |
| 4 AWG | 70A | 85A | 95A |
| 3 AWG | 85A | 100A | 115A |
| 2 AWG | 95A | 115A | 130A |
| 1 AWG | 110A | 130A | 145A |
| 1/0 AWG | 125A | 150A | 170A |
| 2/0 AWG | 145A | 175A | 195A |
| 3/0 AWG | 165A | 200A | 225A |
| 4/0 AWG | 195A | 230A | 260A |
* Note: Per NEC 240.4(D), overcurrent protection for 14, 12, and 10 AWG copper is limited to 15A, 20A, and 30A respectively, even if the 75°C or 90°C columns show higher allowable ampacities.
Derating Factors: When the Base Value Drops
The ampacities listed above assume an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors bundled together. When you exceed these conditions, you must apply derating factors from NEC Table 310.15(B)(2)(a) and Table 310.15(B)(1).
Adjustment Factors for Bundled Conductors (NEC Table 310.15(B)(2)(a)):
- 4 to 6 conductors: Multiply 90°C base ampacity by 80%.
- 7 to 9 conductors: Multiply 90°C base ampacity by 70%.
- 10 to 20 conductors: Multiply 90°C base ampacity by 50%.
Worked Example: You are pulling four current-carrying 12 AWG THHN (90°C) wires in a single conduit for a multi-wire branch circuit. The 90°C base ampacity for 12 AWG is 30A. Applying the 80% derating factor (30A × 0.80) gives you 24A. Because 24A is higher than the 20A limit in the 60°C column (which governs your standard receptacle terminations), you are legally permitted to protect this circuit with a standard 20-amp breaker. If you had pulled seven wires (70% derating: 30A × 0.70 = 21A), you would still be safe for a 20A breaker. But if you pulled ten wires (50% derating: 30A × 0.50 = 15A), you would be forced to downsize to a 15-amp breaker or upsize to 10 AWG wire.
What the Ampacity Chart Cannot Tell You
While Table 310.16 is the bible for thermal limits, it is not a comprehensive wire-sizing tool. Relying on it blindly will cause failures in the field. Here is what the chart leaves out:
- Voltage Drop: The NEC ampacity chart does not account for distance. A 12 AWG wire is rated for 20 amps, but if you run it 150 feet to a window AC unit, the resistance will cause a severe voltage drop, leading to motor burnout. For runs over 50 feet, use a voltage drop calculator and expect to upsize by at least one AWG step to maintain a 3% drop limit.
- Physical and Mechanical Strength: The chart will tell you that 14 AWG can carry 15 amps, but NEC 230.23 and general mechanical durability practices prohibit using wire smaller than 8 AWG for overhead service entrance spans due to wind and ice loading.
- Short-Circuit Withstand: Ampacity measures continuous thermal loading. It does not tell you if the wire can survive the instantaneous magnetic and thermal forces of a 10,000-amp short circuit before the breaker clears the fault. That requires coordinating the breaker's AIC (Ampere Interrupting Capacity) rating and I²t let-through energy.
For deeper dives into code-compliant installations and field applications, reference the NFPA National Electrical Code documentation and practical field guides from EC&M's NEC resources.
Frequently Asked Questions
What size wire do I need for a 50-amp breaker according to the ampacity wire chart?
For a standard 50-amp circuit (like an EV charger or RV outlet), you need 6 AWG copper wire. Looking at the chart, 6 AWG in the 60°C column is rated for 55A, and in the 75°C column, it is rated for 65A. Both safely exceed the 50A requirement. If you are using aluminum wire, you must step up to 4 AWG (rated 55A at 60°C / 65A at 75°C).
Why does my THHN wire show a lower ampacity than the 90°C column on the chart?
THHN insulation is indeed rated for 90°C in dry locations, but the terminations (breakers, lugs, receptacles) are typically only rated for 60°C or 75°C. Per NEC 110.14(C), the weakest link dictates the rule. You use the 90°C column only to calculate derating penalties; the final adjusted ampacity must still be compared against the 60°C or 75°C column to ensure you aren't melting the breaker lugs.
Does the ground wire count when calculating derating on the ampacity chart?
No. Per NEC 310.15(B)(3)(a), equipment grounding conductors (bare copper or green insulated) are not considered current-carrying conductors. They only carry current during a fault condition, which should trip the breaker instantly. Therefore, if you pull two hots, one neutral, and one ground in a conduit, you only count three current-carrying conductors, meaning no bundling derating penalty applies.
How does the ampacity wire chart apply to aluminum vs. copper conductors?
Table 310.16 contains separate sections for copper and aluminum. Aluminum has higher electrical resistance and expands/contracts more under heat, meaning it requires a larger cross-sectional area to carry the same current. As a general rule, aluminum wire must be upsized by two AWG steps compared to copper. For example, a 100-amp subpanel feeder requires 3 AWG copper, but demands 1/0 AWG aluminum. Always ensure your terminations are explicitly marked 'AL/CU' before terminating aluminum.
