For standard residential and commercial AC branch circuits under 100 amps, you must use the 60°C column of NEC Table 310.16 (copper conductors). The most common baseline sizes are 14 AWG for 15A, 12 AWG for 20A, 10 AWG for 30A, 8 AWG for 40A, and 6 AWG for 50A. Sizing wire is not about guessing; it is a strict mathematical path dictated by the National Electrical Code (NEC) to prevent insulation meltdown and termination fires.
This guide provides the complete ampacity reference, explains the temperature column trap that fails DIYers, and gives you a concrete decision path to pick your exact wire gauge.
The Master AC Wire Sizing Chart (NEC Table 310.16)
The following data is extracted from NFPA 70 (National Electrical Code) Table 310.16. This table applies strictly to copper conductors with an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a raceway or cable.
| AWG Size | 60°C (140°F) Ampacity | 75°C (167°F) Ampacity | 90°C (194°F) Ampacity | Max Standard Breaker (NEC 240.4) |
|---|---|---|---|---|
| 14 | 15A | 20A | 25A | 15A |
| 12 | 20A | 25A | 30A | 20A |
| 10 | 30A | 35A | 40A | 30A |
| 8 | 40A | 50A | 55A | 40A / 50A* |
| 6 | 55A | 65A | 75A | 60A |
| 4 | 70A | 85A | 95A | 70A / 80A* |
| 3 | 85A | 100A | 115A | 100A |
| 2 | 95A | 115A | 130A | 100A / 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 / 250A* |
*Note: NEC 240.4(B) allows rounding up to the next standard breaker size if the ampacity does not correspond to a standard breaker, provided the load is not continuous and the wire is under 800A.
• 15A Lighting/Receptacles: 14 AWG (NM-B or THHN)
• 20A Kitchen/Bath/Appliance: 12 AWG
• 30A Dryer/Water Heater: 10 AWG
• 50A Range/EV Charger: 6 AWG (Use 4 AWG if NM-B cable for 50A continuous)
How to Read the Chart: Picking Your Temperature Column
The most common mistake on the jobsite is looking at a spool of 90°C-rated THHN wire, checking the 90°C column, and assuming a 12 AWG wire can handle 30 amps. It cannot.
NEC Section 110.14(C)(1)(a) dictates that for circuits rated 100 amps or less, you must size the wire based on the 60°C column, unless the equipment terminations are explicitly marked and listed for 75°C. Because most standard residential breakers, receptacles, and switches are not reliably marked for 75°C, the 60°C column is your legal baseline for branch circuits.
So why does the 90°C column exist? It is used exclusively as the starting point for derating calculations (explained below). You start your math at 90°C, apply your penalty multipliers, and then verify that the final derated number is still equal to or greater than the 60°C column requirement for your breaker size.
Derating: When the Chart's Base Values Drop
The chart above assumes two perfect conditions: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors bundled together. When you violate either condition, the wire cannot dissipate heat as effectively, and its ampacity drops.
1. Bundling (More than 3 Current-Carrying Conductors)
If you pull four to six current-carrying conductors through a single conduit, you must multiply the wire's 90°C ampacity by 80% (NEC 310.15(C)(1)).
- Example: You are pulling four 12 AWG THHN wires in a conduit for two separate 20A circuits.
- Base 90°C Ampacity: 30A.
- Derating Math: 30A × 0.80 = 24A.
- Result: 24A is still greater than the 20A required by the 60°C column. 12 AWG is legal and safe.
2. Ambient Temperature (Hot Attics and Garages)
If your conduit runs through an attic that hits 50°C (122°F) in the summer, you apply a temperature correction factor. For 90°C wire at 50°C ambient, the multiplier is 0.82.
- Example: 10 AWG THHN in a hot attic.
- Base 90°C Ampacity: 40A.
- Derating Math: 40A × 0.82 = 32.8A.
- Result: 32.8A is greater than the 30A required for a 30A breaker. 10 AWG passes.
Decision Path: Sizing Your Specific Circuit
Use this decision tree to terminate your sizing process with a single, concrete wire pick. Never leave sizing to guesswork.
| Step | Action & Rule | Concrete Example: 40A EV Charger (Continuous Load) |
|---|---|---|
| 1. Calculate True Load | If the load runs for 3+ hours (continuous), multiply the nameplate amps by 1.25. If non-continuous, use nameplate amps. | 40A × 1.25 = 50A minimum circuit ampacity. |
| 2. Pick Base Wire | Find the calculated amps in the 60°C column (for NM-B cable or standard terminations) or 75°C column (if using THHN in conduit with 75°C rated lugs). | Looking at 60°C column for 50A: 6 AWG Copper (rated 55A). |
| 3. Apply Derating | Check conduit fill and ambient temp. If penalties apply, multiply 90°C ampacity by the factor. Ensure result > Step 1 value. | Only 2 wires in conduit, normal temp. No derating needed. |
| 4. Select Breaker | Breaker must protect the wire. Standard size equal to or less than wire ampacity (unless 240.4(B) rounding applies). | Wire is rated 55A (60°C). Next standard breaker down is 50A. |
| FINAL PICK | Use 6 AWG Copper wire on a 50A breaker. (If using NM-B Romex, 6 AWG is mandatory. If using THHN in conduit with 75°C lugs, 8 AWG THHN is acceptable as it is rated 50A at 75°C). | |
What This Chart Cannot Tell You (Voltage Drop)
Ampacity charts prevent fires; they do not prevent equipment malfunction. The U.S. Department of Energy and NEC informational notes strongly recommend keeping voltage drop under 3% for branch circuits and 5% total from the utility transformer to the furthest outlet.
If you are running a 20A circuit to a shed 150 feet away, the ampacity chart says 12 AWG is perfectly legal. However, pulling 16A through 300 feet of total wire (hot + neutral) will result in a voltage drop of roughly 7.5 volts (over 6%). Your table saw will bog down, overheat, and trip its internal thermal protector.
The Fix: For any run exceeding 75 feet on a 15A/20A circuit, or 50 feet on a 30A+ circuit, run a voltage drop calculation. If the drop exceeds 3%, upsize the wire by one AWG and recalculate. In the shed example, upsizing to 10 AWG drops the loss to roughly 3.8%, and upsizing to 8 AWG brings it down to a safe 2.4%.
