The Master Electrical Chart for Wire Sizing (NEC Table 310.16)
When you need to know exactly how much current a copper wire can safely carry, the National Electrical Code (NEC) Table 310.16 is the definitive source. This table dictates the allowable ampacities for insulated conductors rated up to 2000 volts.
| Wire Size (AWG/kcmil) | 60°C (140°F) Column | 75°C (167°F) Column | 90°C (194°F) Column |
|---|---|---|---|
| 14 AWG | 15A | 20A | 25A |
| 12 AWG | 20A | 25A | 30A |
| 10 AWG | 30A | 35A | 40A |
| 8 AWG | 40A | 50A | 55A |
| 6 AWG | 55A | 65A | 75A |
| 4 AWG | 70A | 85A | 95A |
| 3 AWG | 85A | 100A | 110A |
| 2 AWG | 95A | 115A | 130A |
| 1 AWG | 110A | 130A | 145A |
| 1/0 AWG | 125A | 150A | 170A |
| 2/0 AWG | 145A | 175A | 195A |
| 3/0 AWG | 165A | 200A | 225A |
| 4/0 AWG | 195A | 230A | 260A |
Bookmark Quick-Jump Rows: For standard residential branch circuits, the most queried values are 14 AWG (15A at 60°C), 12 AWG (20A at 60°C), 10 AWG (30A at 60°C), and 6 AWG (55A at 60°C, commonly used for 50A ranges and EV chargers after applying the 80% continuous load rule).
Which Temperature Column Applies to Your Installation?
The most common mistake DIYers make is looking at the 90°C column because it offers the highest ampacity. You cannot simply pick the highest number. NEC Article 110.14(C) enforces the weakest link rule: your final ampacity is limited by the lowest temperature rating of any component in the circuit, including the wire insulation, the breaker lugs, and the receptacle terminals.
- The 60°C Column: Use this for 14, 12, and 10 AWG circuits. Standard NM-B cable (Romex) is rated for 90°C, but NEC 334.80 explicitly restricts its final ampacity to the 60°C column. Furthermore, most standard 15A and 20A receptacles are only rated for 60°C terminations.
- The 75°C Column: Use this for 8 AWG and larger conductors. Most modern circuit breakers and heavy-duty receptacles (like NEMA 14-50 dryer/range outlets) feature 75°C rated lugs. If you pull 8 AWG THHN through conduit to a 75°C breaker, you can use the 50A value.
- The 90°C Column: Never use this column for your final breaker sizing. The 90°C column exists almost exclusively as a starting point for calculating derating factors.
Derating Factors: When the Base Chart Isn't Enough
The base electrical chart for wire sizing assumes perfect conditions: a cool 30°C (86°F) environment and a maximum of three current-carrying conductors in a raceway. When conditions worsen, the wire cannot dissipate heat as efficiently, and you must reduce (derate) the allowable ampacity.
1. Ambient Temperature Derating (NEC Table 310.15(B)(1))
If your conduit runs through an attic that reaches 50°C (122°F) in the summer, you must multiply the base ampacity by a correction factor. For 90°C THHN wire at 50°C ambient, the correction factor is 0.82.
2. Bundling Derating (NEC Table 310.15(C)(1))
If you pull four to six current-carrying conductors through a single conduit, you must multiply the 90°C base ampacity by 80%.
Worked Numeric Example:
You are pulling four 10 AWG THHN current-carrying conductors through a conduit in a 30°C attic.
1. Start at the 90°C column for 10 AWG: 40A.
2. Apply the bundling derating (4-6 conductors = 80%): 40A × 0.80 = 32A.
3. Check the termination limit: Your breaker lugs are 75°C, and the 75°C column for 10 AWG is 35A.
4. The weakest link is the derated 90°C value (32A). Since 32A is greater than your 30A breaker, 10 AWG is safe and code-compliant for this run.
Quick-Decision Path: Pick Your Exact Wire Gauge
Stop guessing. Use this decision-tree-table to terminate your search and pick the exact wire gauge and type for standard residential single-phase (120V/240V) applications.
| If your breaker is... | And your installation method is... | Then your exact wire pick is... |
|---|---|---|
| 15A | Interior dry walls (NM-B) | 14 AWG Copper NM-B (12 AWG acceptable for long runs) |
| 20A | Interior dry walls (NM-B) | 12 AWG Copper NM-B |
| 20A | Conduit (THHN/THWN-2) | 12 AWG Copper THHN |
| 30A | Conduit or NM-B | 10 AWG Copper |
| 40A | Conduit (THHN/THWN-2) | 8 AWG Copper THHN |
| 50A | Conduit (THHN/THWN-2) | 6 AWG Copper THHN (Required for 50A continuous/80% rule) |
| 60A | Conduit (THHN/THWN-2) | 6 AWG Copper THHN (Rated 65A at 75°C) |
| 100A (Subpanel) | Conduit or SER Cable | 3 AWG Copper or 1 AWG Aluminum |
What This Wire Sizing Chart Cannot Tell You
While NEC 310.16 is the bible for thermal limits (ampacity), it completely ignores three critical physical realities of your installation. If you ignore these, your circuit might not melt, but it will fail to perform.
1. Voltage Drop
Ampacity charts do not account for distance. If you run 12 AWG copper to a 20A outlet 150 feet away, the wire won't overheat, but the voltage at the receptacle will drop below 114V, causing motors to stall and electronics to brown out. According to Penn State Extension's electrical sizing guidelines, you should size wire to keep voltage drop under 3% for branch circuits. For a 120V, 20A load at 150 feet, you must step up to 8 AWG copper to maintain acceptable voltage, even though 12 AWG is technically rated for 20A.
2. Conduit Fill Capacity (NEC Chapter 9)
The ampacity chart tells you the wire is safe, but it doesn't tell you if the wire will physically fit in your pipe. NEC Chapter 9, Table 1 limits conduit fill to 40% for three or more wires. You cannot stuff six 6 AWG THHN wires into a 1/2-inch EMT conduit, no matter what the ampacity chart says. You must consult a conduit fill calculator to ensure physical clearance.
3. Short-Circuit Interrupting Ratings
Wire sizing protects against continuous overload heating. It does not protect against the magnetic and thermal forces of a dead short. Your breaker's AIC (Ampere Interrupting Capacity) rating—typically 10,000A for standard residential panels—must be coordinated with the available fault current from your utility transformer.
