According to the National Electrical Code (NEC), the standard wire AWG ampacity chart dictates that for residential copper wiring, 14 AWG handles 15 amps, 12 AWG handles 20 amps, 10 AWG handles 30 amps, and 6 AWG handles 55 amps. These baseline values assume copper conductors, an ambient temperature of 30°C (86°F), and no more than three current-carrying conductors in a raceway or cable. However, picking the right wire requires understanding which temperature column applies to your specific breakers and how bundling wires reduces their actual current-carrying capacity.
The Master Wire AWG Ampacity Chart (NEC Table 310.16)
The definitive source for conductor sizing in the US is NEC Table 310.16 (formerly 310.15(B)(16)). Before reading the table, understand its core assumptions: the values apply to copper conductors with insulation types like THHN, THWN, or XHHW, installed in an ambient temperature of 30°C, with a maximum of three current-carrying conductors bundled together.
How to read this table: The 60°C column is your baseline for smaller branch circuits and older equipment. The 75°C column applies to most modern residential breakers, lugs, and receptacles. The 90°C column is strictly used as a starting point for derating calculations, not for final breaker sizing.
| AWG / kcmil | 60°C Column (140°F) | 75°C Column (167°F) | 90°C Column (194°F) | Common Application |
|---|---|---|---|---|
| 14 AWG | 15A * | 20A * | 25A | Lighting, general receptacles |
| 12 AWG | 20A * | 25A * | 30A | Kitchen/bath receptacles, 20A circuits |
| 10 AWG | 30A * | 35A * | 40A | Dryers, water heaters, AC disconnects |
| 8 AWG | 40A | 50A | 55A | Electric ranges, 50A RV outlets |
| 6 AWG | 55A | 65A | 75A | 50A circuits (long runs), subpanel feeders |
| 4 AWG | 70A | 85A | 95A | 60A-70A subpanels, EV chargers |
| 3 AWG | 85A | 100A | 115A | 100A subpanel feeders |
| 2 AWG | 95A | 115A | 130A | 100A-125A service entrances |
| 1 AWG | 110A | 130A | 145A | 125A subpanels |
| 1/0 AWG | 125A | 150A | 170A | 150A residential service |
| 2/0 AWG | 145A | 175A | 195A | 150A-200A service entrances |
| 3/0 AWG | 165A | 200A | 225A | 200A residential service (standard) |
| 4/0 AWG | 195A | 230A | 260A | 200A service (long runs/voltage drop) |
* Note on Asterisks: Per NEC 240.4(D), overcurrent protection for 14, 12, and 10 AWG copper is strictly capped at 15A, 20A, and 30A respectively, regardless of the 75°C or 90°C column values.
Which Column Applies to Your Installation?
The most common mistake DIYers make is looking at the 90°C column because they bought THHN wire, seeing a higher ampacity, and sizing their breaker accordingly. This is a code violation and a fire hazard. To determine your legal ampacity, you must follow the termination rules outlined in NEC 110.14(C).
The Golden Rule of Terminations: Your circuit's ampacity is limited by the lowest temperature rating of any connected component. Most modern breakers, lugs, and receptacles are rated for 75°C. Therefore, you can use the 75°C column for 8 AWG and larger.
However, there is a massive exception for small conductors. NEC 240.4(D) explicitly limits the overcurrent device (breaker) for 14 AWG to 15A, 12 AWG to 20A, and 10 AWG to 30A. Even if your 12 AWG THHN wire is technically capable of 30A at 90°C, and your breaker terminals are rated for 75°C (25A), you must use a 20A breaker. For 14, 12, and 10 AWG, the 60°C column effectively dictates your maximum breaker size.
How Derating Modifies the Base Ampacity
The chart above assumes ideal conditions: 30°C ambient temperature and no more than three current-carrying conductors in a conduit. When you deviate from this, you must apply derating factors to the 90°C column (for THHN/XHHW-2) to find your adjusted ampacity.
Worked Numeric Example:
You are pulling four current-carrying conductors (two hots, two neutrals for two multi-wire branch circuits) through a conduit in an attic where the ambient temperature reaches 45°C (113°F). You want to use 12 AWG THHN on a 20A breaker.
- Base 90°C Ampacity: 12 AWG THHN = 30A.
- Bundling Factor: 4 to 6 current-carrying conductors = 80% derating. (30A × 0.80 = 24A).
- Temperature Factor: 41-45°C ambient for 90°C wire = 82% derating. (24A × 0.82 = 19.68A).
Your final derated ampacity is 19.68A. Because this is less than the 20A breaker requirement, 12 AWG fails this installation. You must upsize to 10 AWG THHN (Base 40A × 0.80 × 0.82 = 26.24A) to safely use a 20A breaker in this specific conduit.
What the Chart Cannot Tell You
While NEC Table 310.16 prevents wires from melting, it completely ignores voltage drop. Ampacity charts assume a theoretical zero-distance run. In reality, wire has resistance.
If you are wiring a detached garage 150 feet away from your main panel on a 20A circuit, 12 AWG copper has the legal ampacity to handle the load without overheating. However, at 150 feet, a 16A continuous load will cause a voltage drop of roughly 4.5% (exceeding the NEC recommended 3% limit for branch circuits). Your power tools will run hot and inefficiently, and LED lights may flicker. To fix this, you must upsize to 10 AWG or 8 AWG purely for voltage drop mitigation, even though your breaker remains 20A.
Frequently Asked Questions
What size wire do I need for a 50 amp breaker?
For a standard 50-amp breaker (like an electric range or RV outlet), you need 6 AWG copper if your terminations are rated for 75°C (which yields 65A). If you are using older equipment rated only for 60°C, or if you are using aluminum wire, you must upsize to 4 AWG. Always verify the temperature rating stamped on the breaker lugs before pulling wire.
Can I use the 90°C THHN ampacity for my breaker sizing?
No. The 90°C column is almost never used for final breaker sizing in residential work because standard breakers and receptacles are not rated for 90°C terminations. The 90°C column is strictly a mathematical starting point for calculating derating adjustments when bundling wires or routing them through hot environments. Your final adjusted ampacity must still be protected by a breaker sized to the 60°C or 75°C column limits.
Does the ground wire count towards ampacity derating?
No. When calculating bundling derating factors (e.g., applying the 80% rule for 4-6 conductors), you only count current-carrying conductors. Equipment grounding conductors (bare copper or green) do not carry current under normal operation and are explicitly excluded from the count per NEC 310.15(C)(1). However, if you are using a metallic conduit as your ground path, the wires inside are still subject to the heat generated by the circuit.
How does aluminum wire ampacity compare to copper?
Aluminum has a lower conductivity than copper, meaning it generates more heat for the same current. As a general rule, aluminum wire must be two AWG sizes larger than copper to achieve the same ampacity. For example, to safely carry 100 amps on a 75°C termination, you need 3 AWG copper, but you must use 1 AWG aluminum. Aluminum is highly cost-effective for large feeder cables (like 2/0 or 4/0 for 200A services) but requires specialized anti-oxidant paste and precise torque settings to prevent loose, arcing connections.
