How to Read the NEC Wire Size Table Ampacity Chart
The ampacity of a conductor is its maximum continuous current-carrying capacity under specific conditions. When you look up a wire size table ampacity value, you are looking at the thermal limit of the wire's insulation before it begins to degrade. The definitive source for these values in the United States is NFPA 70 (National Electrical Code), specifically Table 310.16.
Before jumping to the chart, you must understand how to read the columns. The table is divided by conductor material (Copper vs. Aluminum) and insulation temperature rating (60°C, 75°C, and 90°C). Here is the most critical rule that trips up DIYers and junior apprentices: the wire's insulation rating does not automatically dictate the circuit's ampacity.
Which column applies to your installation?
- 60°C Column: Use for 14, 12, and 10 AWG branch circuits (NEC 240.4(D) specifically limits these to 15A, 20A, and 30A respectively, regardless of insulation). Also use for older equipment or NM-B (Romex) cable assemblies.
- 75°C Column: Use for feeders and branch circuits 8 AWG and larger, provided the terminations at both ends (breaker and lugs) are rated 75°C. Most modern commercial and residential load centers meet this requirement.
- 90°C Column: Use exclusively for calculating derating adjustments (ambient temperature or conduit fill), then verify the final derated value does not exceed the 60°C or 75°C termination limits.
The Master Wire Size Table Ampacity Chart (NEC Table 310.16)
Below is the complete reference table for the most queried residential and commercial conductor sizes. Data is sourced directly from the 2023/2026 NEC Table 310.16 for up to three current-carrying conductors in a raceway at an ambient temperature of 30°C (86°F). For manufacturer-specific insulation thicknesses and exact diameters, refer to the Cerro Wire Ampacity Chart.
Bookmark these quick-jump links for the most common residential sizes:
14 AWG | 12 AWG | 10 AWG | 8 AWG | 6 AWG | 4 AWG | 2 AWG | 1/0 AWG | 4/0 AWG
| AWG / kcmil | Copper 60°C (140°F) | Copper 75°C (167°F) | Copper 90°C (194°F) | Aluminum 60°C (140°F) | Aluminum 75°C (167°F) | Aluminum 90°C (194°F) |
|---|---|---|---|---|---|---|
| 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 | 40A | 50A | 60A |
| 4 AWG | 70A | 85A | 95A | 55A | 65A | 75A |
| 3 AWG | 85A | 100A | 115A | 65A | 75A | 85A |
| 2 AWG | 95A | 115A | 130A | 75A | 90A | 100A |
| 1 AWG | 110A | 130A | 145A | 85A | 100A | 115A |
| 1/0 AWG | 125A | 150A | 170A | 100A | 120A | 135A |
| 2/0 AWG | 145A | 175A | 195A | 115A | 135A | 150A |
| 3/0 AWG | 165A | 200A | 225A | 130A | 155A | 170A |
| 4/0 AWG | 195A | 230A | 260A | 150A | 180A | 205A |
Derating and Adjustments: When the Table Lies to You
The values in the wire size table ampacity chart above assume ideal conditions: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors bundled together in a single raceway. When real-world jobsite conditions deviate from this, the base ampacity must be modified.
How Derating Rows Modify the Base Value
When you pull more than three current-carrying conductors through a single conduit, the heat generated by adjacent wires cannot dissipate. NEC Table 310.15(C)(1) requires you to apply a derating factor.
1. Base 90°C ampacity for 12 AWG = 30A.
2. Derating factor for 4-6 conductors = 80%.
3. Derated ampacity = 30A × 0.80 = 24A.
Because 24A is still greater than the 20A breaker protecting the circuit, 12 AWG is legally compliant. If you had mistakenly started with the 60°C column (20A), 20A × 0.80 = 16A, which would force you to upsize to 10 AWG unnecessarily.
What the Table Cannot Tell You
While the wire size table ampacity chart dictates thermal limits, it is blind to other critical electrical physics. It will not tell you:
- Voltage Drop: A 10 AWG wire might be rated for 30A thermally, but if you run it 150 feet to a well pump, the resistance will cause a massive voltage drop. For voltage drop calculations, you must reference NEC Chapter 9, Table 8 (Conductor Properties) and use the formula: VD = (2 × K × I × D) / CM.
- Short-Circuit Let-Through Current: Ampacity is about continuous thermal loading. It does not indicate whether a wire can withstand the magnetic and thermal stress of a 10,000-amp fault current for the milliseconds it takes a breaker to trip.
- Physical Pull Tension: The table won't warn you that pulling 4/0 aluminum through three 90-degree sweeps exceeds the maximum sidewall pressure and pulling tension limits, risking stretched insulation or snapped conductors.
Frequently Asked Questions About Wire Size Table Ampacity
What wire size table ampacity applies to a 50-amp breaker?
For a standard 50-amp circuit (like an EV charger or subpanel feeder), you must use 6 AWG Copper (rated 65A in the 75°C column) or 4 AWG Aluminum (rated 65A in the 75°C column). While 8 AWG copper has a 90°C ampacity of 55A, you cannot use it because standard 50A breakers and receptacles are not rated for 90°C terminations, and the 75°C column for 8 AWG is only 50A, which does not meet the continuous load requirements if the load runs for 3+ hours.
Why does the wire size table ampacity for THHN show higher numbers than my breaker allows?
THHN wire has a 90°C insulation rating, which yields high ampacity numbers on paper (e.g., 40A for 10 AWG). However, NEC 110.14(C) termination rules mandate that the circuit ampacity cannot exceed the lowest temperature rating of any connected device. Since most residential breakers and receptacles are rated 60°C or 75°C, you are legally capped at the lower column values (30A for 10 AWG), regardless of the THHN wire's superior thermal tolerance.
How does ambient temperature change the wire size table ampacity values?
Table 310.16 assumes an ambient temperature of 30°C (86°F). If you are routing wire through a hot attic that reaches 110°F (43°C), you must apply a temperature correction factor from NEC Table 310.15(B)(1). For 90°C wire at 41-45°C ambient, the correction factor is 0.87. You multiply the base 90°C ampacity by 0.87 to find your new maximum current limit before applying any conduit fill derating.
Can I use the 90°C wire size table ampacity column for residential branch circuits?
Only for the mathematical step of derating (adjusting for conduit fill or high ambient heat). The final calculated ampacity after derating must still be compared against the 60°C or 75°C column limits to ensure you are not overheating the physical brass or copper lugs inside your breakers and outlets. The 90°C column is a calculation tool, not a final sizing authority for standard terminations.
