When sizing conductors for a residential or commercial branch circuit, guessing leads to tripped breakers, melted insulation, or failed inspections. The definitive reference for copper conductor ampacity in the United States is NEC Table 310.16 (formerly 310.15(B)(16)). This amp rating wire size chart gives you the exact allowable ampacities for copper conductors rated up to 2000 volts, assuming an ambient temperature of 30°C (86°F).
Below is the complete master chart, followed by a decision path to ensure you select the correct temperature column and apply the necessary derating factors for your specific installation.
The Master Amp Rating Wire Size Chart (NEC Table 310.16)
| AWG / kcmil | 60°C Column (NM-B / TW) | 75°C Column (THWN / XHHW) | 90°C Column (THHN / THWN-2) | Max Standard Breaker (No Derating) |
|---|---|---|---|---|
| 14 | 15A | 20A | 25A | 15A (NEC 240.4(D) limits) |
| 12 | 20A | 25A | 30A | 20A (NEC 240.4(D) limits) |
| 10 | 30A | 35A | 40A | 30A (NEC 240.4(D) limits) |
| 8 | 40A | 50A | 55A | 50A |
| 6 | 55A | 65A | 75A | 60A |
| 4 | 70A | 85A | 95A | 80A |
| 3 | 85A | 100A | 115A | 100A |
| 2 | 95A | 115A | 130A | 125A |
| 1 | 110A | 130A | 145A | 125A |
| 1/0 | 125A | 150A | 170A | 150A |
| 2/0 | 145A | 175A | 195A | 175A |
| 3/0 | 165A | 200A | 225A | 200A |
| 4/0 | 195A | 230A | 260A | 225A |
Quick-Jump: Most Queried Wire and Breaker Sizes
For standard residential branch circuits, bookmark these baseline pairings. These assume standard NM-B (Romex) cable in a typical wall cavity with no more than three current-carrying conductors bundled together.
- 15-Amp Breaker (Lighting/General Purpose): 14 AWG Copper (Minimum). 12 AWG recommended for future-proofing.
- 20-Amp Breaker (Kitchen/Bath/Outlets): 12 AWG Copper (Strict minimum).
- 30-Amp Breaker (Dryer/HVAC Condenser): 10 AWG Copper.
- 40-Amp Breaker (Range/Cooktop): 8 AWG Copper.
- 50-Amp Breaker (EV Charger/Hot Tub): 6 AWG Copper.
Decision Path: Which Temperature Column Applies to You?
A common mistake is using the 90°C column for THHN wire simply because the wire jacket is rated for 90°C. The National Electrical Code (NEC) requires that the ampacity of a conductor cannot exceed the lowest temperature rating of any connected device, terminal, or splice. Use this decision tree to find your governing column.
| IF Your Installation Is... | THEN Use This Column... | Why? |
|---|---|---|
| Using NM-B (Romex) cable for any branch circuit. | 60°C Column | NEC 334.80 mandates NM-B ampacity is determined by the 60°C column, regardless of the 90°C rating of the internal conductors. |
| Using THHN/THWN-2 in conduit, terminating on a breaker or receptacle rated 15A to 100A. | 60°C Column | NEC 110.14(C)(1)(a) requires terminations for equipment rated 100A or less to be sized using the 60°C column unless the equipment is specifically marked for 75°C. |
| Using THHN/THWN-2 in conduit, terminating on a panel lug or breaker rated over 100A. | 75°C Column | NEC 110.14(C)(1)(b) allows the 75°C column for equipment rated over 100A, as modern panel lugs are universally rated for 75°C. |
| Calculating voltage drop or applying derating factors before termination. | 90°C Column | The 90°C column is used as the starting baseline for math (derating and voltage drop), but the final adjusted ampacity cannot exceed the 60°C or 75°C termination limits. |
Derating: When Your Base Ampacity Drops
The amp rating wire size chart above assumes ideal conditions: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors (CCCs) in a single raceway or cable. When reality deviates from this, you must derate the 90°C column baseline.
1. Bundling (More than 3 CCCs)
When you pull multiple circuits through a single conduit, the heat generated by adjacent wires cannot dissipate. Per NEC Table 310.15(C)(1), if you have 4 to 6 CCCs in a conduit, you must multiply the 90°C base ampacity by 80%. For 7 to 9 CCCs, multiply by 70%.
Worked Example: You are pulling three 20A circuits (6 CCCs total) through a single EMT conduit using 12 AWG THHN.
• Base 90°C ampacity for 12 AWG = 30A.
• Derating factor for 6 CCCs = 80%.
• Adjusted ampacity = 30A × 0.80 = 24A.
• Because 24A exceeds the 20A breaker, 12 AWG is legally permissible here. However, if you added a fourth circuit (8 CCCs, 70% derating), the adjusted ampacity drops to 21A. You would be forced to upsize to 10 AWG THHN to maintain a safe margin.
2. High Ambient Temperature
If your conduit runs through an attic in a southern climate where temperatures regularly exceed 86°F (30°C), you must apply the temperature correction factors in NEC Table 310.15(B)(1). At 122°F (50°C), the correction factor for 90°C insulation is 0.82. Always multiply the bundling factor and the temperature factor together if both conditions exist.
What This Chart Cannot Tell You (And Your Default Pick)
The NEC ampacity tables dictate the maximum current a wire can carry before its insulation degrades or becomes a fire hazard. They do not account for voltage drop. According to the Copper Development Association, a voltage drop exceeding 3% on a branch circuit can cause motors to overheat, LED drivers to flicker, and electronics to brown out.
For example, a 50-amp EV charger located 120 feet from the panel using 6 AWG copper will experience a voltage drop of roughly 4.1% at full load. The 6 AWG wire will not melt (it satisfies the ampacity chart), but the performance will suffer. To fix this, you must upsize to 4 AWG copper to bring the drop back under 3%.
