For a standard 15-amp branch circuit, use 14 AWG copper wire. For a 20-amp circuit, use 12 AWG. For a 30-amp circuit, use 10 AWG. While these are the baseline rules for residential wiring, the actual legal ampacity of a wire depends entirely on its insulation temperature rating, the termination temperature of your devices, and how many wires are bundled in the conduit. Sizing wire strictly by memory leads to tripped breakers, melted lugs, and failed inspections.
How to Read the NEC Wire Sizes Table
The National Electrical Code (NEC) Table 310.16 is the master reference for conductor ampacity. When you look at the table, you will see three primary temperature columns for copper: 60°C (140°F), 75°C (167°F), and 90°C (194°F). Choosing the correct column is where most DIYers and junior apprentices make critical errors.
Which column applies to your installation? NEC Article 110.14(C) dictates that for circuits rated 100 amps or less, you must use the 60°C column unless the equipment (breakers, lugs, receptacles) is explicitly marked for 75°C. Most modern residential breakers and receptacles are rated for 75°C, but standard NM-B (Romex) cable is legally limited to the 60°C column for final ampacity, regardless of the fact that its internal THHN conductors have 90°C insulation. The 90°C column is almost exclusively used as the starting baseline for derating calculations, not for final breaker sizing.
The Complete Copper Wire Sizes Table (NEC Table 310.16)
The following data is sourced directly from NEC Table 310.16 for copper conductors with an ambient temperature of 30°C (86°F). For comprehensive manufacturer specifications and metric conversions, refer to the Southwire Technical Resources documentation.
| AWG / kcmil | 60°C Ampacity (NM-B / Romex) | 75°C Ampacity (THHN Terminations) | 90°C Ampacity (Derating Base) | Max Standard Breaker (NEC 240.4) |
|---|---|---|---|---|
| 14 AWG | 15A | 20A | 25A | 15A (Strict Limit) |
| 12 AWG | 20A | 25A | 30A | 20A (Strict Limit) |
| 10 AWG | 30A | 35A | 40A | 30A (Strict Limit) |
| 8 AWG | 40A | 50A | 55A | 50A |
| 6 AWG | 55A | 65A | 75A | 60A |
| 4 AWG | 70A | 85A | 95A | 80A |
| 3 AWG | 85A | 100A | 115A | 100A |
| 2 AWG | 95A | 115A | 130A | 125A |
| 1 AWG | 110A | 130A | 145A | 150A |
| 1/0 AWG | 125A | 150A | 170A | 150A |
| 2/0 AWG | 145A | 175A | 195A | 175A |
| 3/0 AWG | 165A | 200A | 225A | 200A |
| 4/0 AWG | 195A | 230A | 260A | 225A |
Note on NEC 240.4(D): The NEC places a hard cap on small conductors. Even if your terminations are rated 75°C and you are using THHN wire, 14 AWG is legally capped at a 15A breaker, 12 AWG at 20A, and 10 AWG at 30A. You cannot use the higher column values to justify a larger breaker on these specific sizes.
Decision Tree: Picking the Right Wire and Breaker
Stop guessing. Follow this decision path to terminate your sizing debate and pick the exact materials for your next pull.
- IF you are wiring standard bedroom/living room 120V lighting and receptacles:
THEN pick 14 AWG NM-B with a 15A breaker. (Pro-tip: Upgrade to 12 AWG NM-B and a 20A breaker for minimal extra cost and better future-proofing). - IF you are wiring kitchen small-appliance circuits, bathroom GFCI receptacles, or laundry rooms:
THEN pick 12 AWG NM-B with a 20A breaker. Code mandates 20A for these specific locations. - IF you are wiring a standard electric dryer or a 30A RV receptacle:
THEN pick 10 AWG (NM-B for short runs, THHN in conduit for longer runs) with a 30A breaker. - IF you are wiring a 50A EV Level 2 charger or a 50A subpanel feeder under 50 feet:
THEN pick 6 AWG THHN/THWN-2 in conduit with a 50A breaker. (If using NM-B cable instead of conduit, you must upsize to 4 AWG because NM-B is locked to the 60°C column, which caps 6 AWG at 55A). - IF you are wiring a 100A subpanel feeder:
THEN pick 3 AWG THHN copper (or 1/0 AWG aluminum) in conduit with a 100A breaker.
Derating Factors: When the Base Table Lies
The ampacity table above assumes you have no more than three current-carrying conductors in a raceway and an ambient temperature of 30°C (86°F). When you violate either assumption, the wire cannot dissipate heat as efficiently, and you must apply derating factors to the 90°C column.
Example: Bundling in Conduit
Imagine you are pulling four current-carrying 12 AWG THHN wires through a single EMT conduit to feed two separate 20A circuits.
- Start with the 90°C base ampacity for 12 AWG: 30A.
- Consult NEC Table 310.15(C)(1) for 4 to 6 current-carrying conductors. The adjustment factor is 80%.
- Multiply: 30A × 0.80 = 24A.
- Since 24A is still greater than your 20A load and breaker size, 12 AWG is legally acceptable.
Example: High Ambient Heat
Now imagine those same wires are running through an attic where the ambient temperature reaches 50°C (122°F) in the summer. The temperature correction factor for 90°C insulation at 50°C ambient is 0.82.
30A × 0.82 = 24.6A. Still safe for a 20A breaker. But if you bundle nine wires in that hot attic (50% derating factor), your 12 AWG wire drops to 15A. You would be forced to upsize to 10 AWG or 8 AWG to maintain a 20A circuit.
What This Table Cannot Tell You
Ampacity is only half the battle. The NEC wire sizes table dictates thermal limits, but it is blind to three critical real-world variables:
- Voltage Drop: The table assumes the wire can carry the current safely, but it doesn't guarantee the voltage will arrive at the destination. For runs exceeding 50 feet on a 120V circuit, or 100 feet on a 240V circuit, you must calculate voltage drop. A 3% maximum drop is the standard target. If you are running a 50A EV charger 150 feet from the panel, 6 AWG wire will overheat the voltage drop limit; you must upsize to 4 AWG or 3 AWG purely to maintain voltage, even though 6 AWG is thermally rated for 50A.
- Aluminum Conductors: This table is strictly for copper. If you are using aluminum wire (common for service entrances and large subpanel feeders due to cost), you must use the aluminum columns in NEC 310.16. As a general rule, aluminum must be sized two AWG steps larger than copper for the same ampacity (e.g., use 2/0 AWG aluminum where you would use 1/0 AWG copper for a 150A service).
- Conduit Fill Capacity: Just because the thermal derating math allows you to pull ten 12 AWG wires through a 1/2-inch EMT conduit doesn't mean they will physically fit. NEC Chapter 9, Table 1 limits conduit fill to 40% for three or more wires to prevent jamming and insulation damage during the pull. Always check physical fill tables alongside thermal ampacity tables.
Use this table as your thermal baseline, apply your derating math, verify your voltage drop, and always defer to your local Authority Having Jurisdiction (AHJ) for final code compliance.
