The UL wire ampacity chart you need for almost all residential and commercial wiring in the United States is based directly on NEC Table 310.16 (formerly 310.15(B)(16)). When you buy UL-listed wire—whether it is THHN in conduit or NM-B (Romex) in a wall cavity—the maximum continuous current it can safely carry is dictated by this exact table, cross-referenced with the insulation temperature rating tested under UL 44 and UL 83 standards.
Below is the complete master chart, followed by the critical rules for reading it, the variables it intentionally leaves out, and answers to the most common installer questions.
The Master UL Wire Ampacity Chart (NEC Table 310.16)
This table provides the allowable ampacities for insulated conductors rated up to 2000 volts, assuming an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors in a raceway or cable. The data is sourced directly from the NFPA 70 (National Electrical Code).
- 14 AWG (Cu): 15A (Max breaker 15A per NEC 240.4(D))
- 12 AWG (Cu): 20A (Max breaker 20A per NEC 240.4(D))
- 10 AWG (Cu): 30A (Max breaker 30A per NEC 240.4(D))
- 8 AWG (Cu): 40A (60°C col) / 50A (75°C col)
- 6 AWG (Cu): 55A (60°C col) / 65A (75°C col)
- 4 AWG (Cu): 70A (60°C col) / 85A (75°C col)
- 2 AWG (Cu): 95A (60°C col) / 115A (75°C col)
| AWG / kcmil | Copper (THHN/THWN-2, XHHW-2, etc.) | Aluminum (XHHW-2, THWN-2, etc.) | ||||
|---|---|---|---|---|---|---|
| 60°C (140°F) | 75°C (167°F) | 90°C (194°F) | 60°C (140°F) | 75°C (167°F) | 90°C (194°F) | |
| 14 | 15 | 20 | 25 | - | - | - |
| 12 | 20 | 25 | 30 | 15 | 20 | 25 |
| 10 | 30 | 35 | 40 | 25 | 30 | 35 |
| 8 | 40 | 50 | 55 | 30 | 40 | 45 |
| 6 | 55 | 65 | 75 | 40 | 50 | 60 |
| 4 | 70 | 85 | 95 | 55 | 65 | 75 |
| 3 | 85 | 100 | 110 | 65 | 75 | 85 |
| 2 | 95 | 115 | 130 | 75 | 90 | 100 |
| 1 | 110 | 130 | 145 | 85 | 100 | 115 |
| 1/0 | 125 | 150 | 170 | 100 | 120 | 135 |
| 2/0 | 145 | 175 | 195 | 115 | 135 | 150 |
| 3/0 | 165 | 200 | 225 | 130 | 155 | 175 |
| 4/0 | 195 | 230 | 260 | 150 | 180 | 205 |
How to Read the Chart: Columns, Temperatures, and Derating
The most common mistake DIYers and junior apprentices make is blindly using the 90°C column because it offers the highest ampacity. In the real world, the column you are legally allowed to use depends entirely on the weakest link in your circuit.
Which Column Applies to Your Installation?
Under NEC 110.14(C), the temperature column you must use for sizing the breaker and wire is determined by the terminal temperature ratings of the equipment (breakers, lugs, receptacles) you are connecting to, combined with a strict 100-amp threshold:
- Circuits 100A or less (or 14 AWG through 1 AWG): You must use the 60°C column unless the equipment is explicitly marked otherwise. Most standard residential breakers and receptacles are rated for 75°C, which allows you to use the 75°C column for sizing. However, NM-B (Romex) cable is legally capped at the 60°C column ampacity regardless of its physical insulation rating.
- Circuits over 100A (or larger than 1 AWG): You default to the 75°C column unless the equipment is specifically listed and marked for 90°C terminations (which is rare in standard residential panels).
- The 90°C Column's True Purpose: The 90°C column is almost never used for final breaker sizing. It is used strictly as the base starting number for calculating derating adjustments.
How Derating Modifies the Base Value
The chart assumes a 30°C (86°F) ambient environment and a maximum of three current-carrying conductors (CCCs) in a single conduit. If your installation deviates from this, you must apply adjustment factors to the 90°C column value (for THHN/THWN-2) before comparing it to your breaker size.
Worked Example: You are pulling four current-carrying conductors (two hots, two neutrals for a multi-wire branch circuit) through a single conduit for a 30A circuit using 10 AWG THHN.
- Base ampacity from 90°C column for 10 AWG Cu = 40A.
- NEC Table 310.15(C)(1) dictates an 80% adjustment factor for 4-6 CCCs.
- Derated ampacity = 40A × 0.80 = 32A.
- Because 32A is greater than your 30A load and 30A breaker, 10 AWG THHN is perfectly legal and safe here, even though the 60°C column only lists 30A.
What This Chart Cannot Tell You
An ampacity chart only tells you the thermal limit of the wire's insulation before it begins to degrade or melt. It does not guarantee the circuit will function properly. Here are the three critical variables missing from the table:
1. Voltage Drop Over Distance
Ampacity is about heat; voltage drop is about resistance. If you run 6 AWG copper to a detached garage 150 feet away to feed a 50A subpanel, the wire will not overheat (it is rated for 65A in the 75°C column). However, the resistance of 300 feet of total conductor (hot + neutral) will cause a voltage drop exceeding 5% under heavy load, causing motors to stall and lights to dim. For runs over 100 feet, you must consult NEC Chapter 9, Table 8 for DC resistance and calculate voltage drop, often forcing you to upsize the wire by one or two AWG sizes beyond what the ampacity chart demands.
2. Local AHJ Overrides and Specific Insulation Types
The chart assumes standard UL-listed wire. If you are using specialized cable like UF-B (Underground Feeder), NEC 310.104 restricts its ampacity to the 60°C column regardless of physical markings. Furthermore, your local Authority Having Jurisdiction (AHJ) may have local amendments that restrict conduit fill or require larger grounding conductors than NEC Table 250.122 dictates.
3. Continuous vs. Non-Continuous Loads
The ampacities in the chart are absolute maximums. If your load is considered 'continuous' by the NEC (operating for 3 hours or more, like a commercial lighting circuit or an EV charger), you must derate the load by 125%. A continuous 40A EV charger requires a circuit rated for 50A (40 × 1.25), meaning you must step up to 6 AWG copper on a 50A breaker, even though 8 AWG is technically rated for 50A in the 75°C column.
Frequently Asked Questions About UL Wire Ampacity
Why is 12 AWG wire rated for 25A in the 75°C column but I can only use a 20A breaker?
This is due to NEC 240.4(D), a specific safety rule for small conductors. Regardless of the insulation temperature rating or the math in Table 310.16, the NEC strictly caps the overcurrent protection for 14 AWG at 15A, 12 AWG at 20A, and 10 AWG at 30A. This prevents DIYers from using thin wires on high-amperage breakers in standard branch circuits where physical damage or poor terminations could cause a fire before the breaker trips.
Does the UL ampacity chart apply to both THHN in conduit and NM-B (Romex) cable?
Yes, but with a major caveat. While THHN/THWN-2 wire in conduit allows you to utilize the 75°C or 90°C columns for derating and termination sizing, NM-B cable is legally restricted by NEC 310.15(B) to the 60°C column. This means a 10 AWG NM-B cable is permanently capped at 30A, whereas a 10 AWG THHN wire in conduit can be used for higher calculated loads after derating adjustments are applied from its 90°C base of 40A.
How do I calculate ampacity for a wire running through a hot attic?
If your attic ambient temperature exceeds the chart's baseline of 30°C (86°F), you must apply a temperature correction factor from NEC Table 310.15(B)(1). For example, if your attic reaches 50°C (122°F) in the summer, and you are using 90°C rated THHN wire, you must multiply the base 90°C ampacity by 0.82. If you are using 6 AWG THHN (base 75A), the corrected ampacity drops to 61.5A. You must then ensure your breaker and continuous loads do not exceed this corrected thermal limit.
