The current wire gauge chart below maps American Wire Gauge (AWG) sizes to their maximum allowable ampacity based on NFPA 70, National Electrical Code (NEC) Table 310.16. For standard residential branch circuits using NM-B (Romex) cable, you must use the 60°C column. For individual THHN/THWN conductors in conduit, use the 75°C or 90°C column depending on terminal ratings and derating requirements.
14 AWG (15A) | 12 AWG (20A) | 10 AWG (30A) | 8 AWG (40A) | 6 AWG (55A) | 4 AWG (70A) | 2 AWG (95A)
The Master Current Wire Gauge Chart (NEC Table 310.16)
How to read this table: The NEC publishes ampacities based on the insulation temperature rating (60°C, 75°C, and 90°C). The column you must use depends on your cable type and termination equipment. Most modern breakers and receptacles are rated for 75°C. However, NEC Article 334.80 strictly limits NM-B (Romex) cable to the 60°C column, regardless of the breaker's rating. Furthermore, NEC 240.4(D) imposes strict overcurrent protection limits on small conductors (14, 12, and 10 AWG), which override the higher values found in the 75°C and 90°C columns.
Source: NFPA 70 National Electrical Code, Table 310.16. Values assume an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors in a raceway.
| AWG Size | Copper 60°C (NM-B) | Copper 75°C (THHN Terminals) | Copper 90°C (Derating Base) | Aluminum 60°C | Aluminum 75°C |
|---|---|---|---|---|---|
| 14 | 15A* | 20A* | 25A* | — | — |
| 12 | 20A* | 25A* | 30A* | — | — |
| 10 | 30A* | 35A* | 40A* | — | — |
| 8 | 40A | 50A | 55A | 30A | 40A |
| 6 | 55A | 65A | 75A | 40A | 50A |
| 4 | 70A | 85A | 95A | 55A | 65A |
| 3 | 85A | 100A | 115A | 65A | 75A |
| 2 | 95A | 115A | 130A | 75A | 90A |
| 1 | 110A | 130A | 145A | 85A | 100A |
| 1/0 | 125A | 150A | 170A | 100A | 120A |
| 2/0 | 145A | 175A | 195A | 115A | 135A |
| 3/0 | 165A | 200A | 225A | 130A | 155A |
| 4/0 | 195A | 230A | 260A | 150A | 180A |
*Note: Per NEC 240.4(D), overcurrent protection for 14 AWG is capped at 15A, 12 AWG at 20A, and 10 AWG at 30A, regardless of the higher ampacities listed in the 75°C and 90°C columns.
Derating Factors: When the Base Chart Isn't Enough
The ampacities in the chart above assume ideal conditions: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors bundled together. When real-world jobsite conditions deviate from this, you must apply derating factors that modify the base value. According to NEC Article 310.15, derating is calculated using the 90°C column for THHN/THWN wire, even if your terminations are only rated for 75°C.
You are pulling four current-carrying THHN conductors through a conduit in an attic where the ambient temperature reaches 40°C (104°F). You need to supply a 40A load.
1. Base Ampacity: 8 AWG Copper at 90°C = 55A.
2. Bundling Derating: 4 conductors requires an 80% adjustment factor (55A × 0.80 = 44A).
3. Temperature Derating: 40°C ambient requires a 91% correction factor (44A × 0.91 = 40.04A).
4. Termination Check: The 75°C column for 8 AWG is 50A. Since our final derated value (40.04A) is less than the termination limit (50A), 8 AWG is legally permitted. Protect it with a 40A breaker.
Always remember that the equipment grounding conductor (bare copper or green) does not count as a current-carrying conductor for bundling derating purposes. However, a grounded neutral (white) carrying unbalanced current from a standard multi-wire branch circuit does count.
What This Chart Cannot Tell You (Voltage Drop & Local Code)
While this current wire gauge chart dictates the maximum safe thermal limit of the wire insulation, it does not account for voltage drop over long distances. The NEC recommends (via Informational Note to 210.19(A)) limiting voltage drop to 3% on branch circuits and 5% overall for feeder and branch circuits combined to ensure reasonable efficiency.
For example, a 12 AWG copper wire is perfectly legal for a 20A, 120V circuit according to the chart. However, if that circuit runs 150 feet from the panel to a receptacle powering a high-draw tool, the voltage drop will exceed 5%, causing motors to run hot and trip internal thermal overloads. In long-run scenarios, you must upsize the wire to 10 AWG or 8 AWG purely for voltage drop mitigation, even though 12 AWG satisfies the ampacity chart.
Furthermore, this chart cannot override local Authority Having Jurisdiction (AHJ) amendments. Some municipalities strictly prohibit aluminum branch circuit wiring regardless of the ampacity chart, or require all residential receptacle circuits to be 12 AWG minimum, eliminating 14 AWG entirely. Always verify local codes before pulling wire.
Frequently Asked Questions
What size wire do I need for a 20-amp breaker?
You must use a minimum of 12 AWG copper wire for a 20-amp breaker. While 12 AWG is rated for 25A in the 75°C column, NEC 240.4(D) caps the overcurrent protection for 12 AWG at exactly 20A. Never use 14 AWG on a 20-amp breaker, as it poses a severe fire hazard; the breaker will not trip before the 14 AWG wire overheats.
Can I use the 90°C column for my home wiring?
You can only use the 90°C column as the starting baseline for calculating derating factors (like conduit bundling or high ambient temperatures). The final ampacity of the circuit cannot exceed the temperature rating of the weakest link in the system, which is almost always the breaker or receptacle terminals, rated at 75°C or 60°C.
How does aluminum wire gauge 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 match the 100A ampacity of 3 AWG copper (75°C column), you must use 1 AWG aluminum. Always use anti-oxidant paste (like Noalox) on aluminum terminations to prevent galvanic corrosion and high-resistance connections.
Does the ground wire count towards conduit fill derating?
No. Per NEC 310.15(C)(1), equipment grounding conductors (bare or green) are not considered current-carrying conductors. They only carry current during a fault condition, which should be brief enough that thermal derating is not required. Therefore, if you pull two hot wires, one neutral, and one ground through a conduit, you only count three current-carrying conductors for your derating calculations.
