The wiring ampacity chart is the master reference for matching wire gauge to overcurrent protection. Ampacity defines the maximum continuous current a conductor can carry before its insulation degrades or it becomes a fire hazard. Sizing wire based on guesswork or online forum advice is how terminals melt and panels catch fire. The table below provides the exact, code-compliant numbers you need to pull the right wire and install the correct breaker.
The NEC 310.16 Wiring Ampacity Chart (Copper)
The following data is extracted from the NFPA 70 National Electrical Code (NEC), specifically Article 310.16. This table applies to copper conductors with up to three current-carrying conductors in a raceway or cable, in an ambient temperature of 30°C (86°F).
| AWG / kcmil | 60°C (140°F) TW, UF | 75°C (167°F) RHW, THHW, THW, THWN, XHHW | 90°C (194°F) THHN, THHW, THW-2, THWN-2, XHHW-2 |
|---|---|---|---|
| 14 AWG | 15 A | 20 A | 25 A |
| 12 AWG | 20 A | 25 A | 30 A |
| 10 AWG | 30 A | 35 A | 40 A |
| 8 AWG | 40 A | 50 A | 55 A |
| 6 AWG | 55 A | 65 A | 75 A |
| 4 AWG | 70 A | 85 A | 95 A |
| 3 AWG | 85 A | 100 A | 115 A |
| 2 AWG | 95 A | 115 A | 130 A |
| 1 AWG | 110 A | 130 A | 145 A |
| 1/0 AWG | 125 A | 150 A | 170 A |
| 2/0 AWG | 145 A | 175 A | 195 A |
| 3/0 AWG | 165 A | 200 A | 225 A |
| 4/0 AWG | 195 A | 230 A | 260 A |
How to Read the Table and Choose Your Column
The most common mistake DIYers and junior apprentices make is looking at the 90°C column because they bought THHN/THWN-2 wire, seeing a higher number, and sizing their breaker to that higher number. This violates NEC 110.14(C) and creates a severe fire hazard at the termination points.
Here is the rule for selecting the correct column: Your circuit's ampacity is limited by the weakest temperature rating of any component in the system.
- The 60°C Column: Use this for older equipment, specific types of non-metallic sheathed cable (NM-B / Romex is rated 90°C but its ampacity must be determined from the 60°C column per NEC 334.80), and circuits rated 100A or less where the equipment termination temperature is not explicitly marked.
- The 75°C Column: This is the standard for most modern residential and commercial breaker panels, lugs, and disconnects rated over 100A. If your breaker lug is stamped 75°C, you use this column to determine your final breaker size.
- The 90°C Column: You almost never use this column to size your breaker. It is used strictly as the starting point for derating calculations (explained below).
Derating: What the Base Chart Cannot Tell You
The wiring ampacity chart assumes ideal conditions: exactly three current-carrying conductors in a pipe, and an ambient air temperature of 30°C (86°F). When real-world conditions deviate, you must apply adjustment factors found in NEC 310.15(C)(1) and 310.15(B)(1). The base table cannot tell you these modified values.
Bundling (More than 3 Current-Carrying Conductors)
When you pull multiple circuits through a single conduit, the wires heat each other up. If you have 4 to 6 current-carrying conductors, you must multiply the base ampacity by 80%. If you have 7 to 9 conductors, multiply by 70%.
Worked Example: You are pulling three 20A circuits (which means 6 current-carrying conductors: three hots, three neutrals) in a single EMT conduit using 12 AWG THHN.
1. Base ampacity from the 90°C column for 12 AWG = 30A.
2. Apply 80% derating factor: 30A × 0.80 = 24A.
3. The derated ampacity is 24A. Since 24A is greater than the 20A breaker protecting the circuit, 12 AWG is still compliant. If you added a fourth circuit (8 conductors, 70% factor), 30A × 0.70 = 21A. You would be forced to upsize to 10 AWG wire to maintain a safe margin for a 20A breaker.
Ambient Temperature Corrections
If your conduit runs through an attic in a southern climate where temperatures routinely hit 40°C (104°F), you must apply a temperature correction factor. For 90°C wire at 40°C ambient, the correction factor is 0.91. You multiply your 90°C base ampacity by 0.91 before applying any bundling derating.
Wiring Ampacity Chart FAQ
What size wire do I need for a 50 amp breaker?
For a standard 50A circuit (like an EV charger or welder outlet), you need a minimum of 8 AWG copper wire if your terminations are rated 75°C (which yields exactly 50A in the chart). However, if the run exceeds 50 feet, you should upsize to 6 AWG copper (65A at 75°C) to mitigate voltage drop and ensure the equipment receives adequate voltage under heavy load. If you are using NM-B (Romex) cable, you are forced into the 60°C column, where 8 AWG is only rated 40A, meaning you must use 6 AWG (55A) to legally protect it with a 50A breaker.
Can I use the 90°C column for sizing my breaker?
No. The 90°C column is almost exclusively used as the mathematical baseline for derating calculations when adjusting for ambient temperature or conduit fill. The final, adjusted ampacity must never exceed the value listed in the 60°C or 75°C column, depending on the temperature rating of your breaker lugs and terminals. For a deep dive on termination limits, refer to industry resources like Copper.org's building wire guidelines or the NEC handbook.
How does voltage drop affect the wiring ampacity chart?
It doesn't. The wiring ampacity chart is strictly a thermal limit designed to prevent fires and insulation melting. It does not account for the resistance of the wire over long distances. NEC recommends a maximum 3% voltage drop on branch circuits and 5% total from the service entrance to the furthest outlet. For example, a 12 AWG wire on a 120V, 16A load is perfectly safe thermally according to the chart, but if the run is 150 feet long, the voltage drop will be roughly 7.5% (9 volts), which can cause motors to overheat and electronics to brown out. Always calculate voltage drop separately for any run exceeding 75 to 100 feet.
