When sizing conductors for residential or commercial branch circuits, the direct answer for standard 120V/240V copper NM-B (Romex) cable relies on the 60°C column of the NEC ampacity tables: 14 AWG = 15A, 12 AWG = 20A, 10 AWG = 30A, 8 AWG = 40A, and 6 AWG = 55A. If you are pulling individual THHN wires in conduit, your base ampacity is higher, but your final breaker size is usually bottlenecked by the 75°C rating of the panel and device terminals.
- 15A Breaker: 14 AWG copper
- 20A Breaker: 12 AWG copper
- 30A Breaker: 10 AWG copper
- 40A Breaker: 8 AWG copper
- 50A Breaker: 6 AWG copper (Note: 8 AWG is 50A in the 75°C column for THHN, but NM-B 8 AWG is capped at 40A).
How to Read the NEC 310.16 Wire Gauge Chart Amp Table
The most common mistake DIYers and junior electricians make is looking at the highest number in the table and sizing the breaker to match it. The National Electrical Code (NEC) Table 310.16 (formerly 310.15(B)(16)) splits ampacity into three temperature columns: 60°C, 75°C, and 90°C. Here is exactly which column applies to your installation:
- 60°C Column: Use this for all Nonmetallic-Sheathed Cable (NM-B / Romex). Per NEC 334.80, the ampacity of NM-B is strictly limited to the 60°C column, regardless of the fact that the wire insulation inside the jacket is technically rated for 90°C. Also use this column if you are connecting to older devices or terminals not explicitly marked with a temperature rating.
- 75°C Column: Use this for sizing the final breaker and wire when terminating in modern panels, receptacles, and breakers. NEC 110.14(C) assumes modern terminals are rated for 75°C. If you pull THHN in conduit for a 50A circuit, you use this column to find that 8 AWG is sufficient.
- 90°C Column: Use this only as the starting base number for derating calculations (bundling wires or high ambient temperatures). You cannot use the 90°C ampacity to size your breaker unless every single termination point in the entire circuit is explicitly rated and marked for 90°C, which is virtually nonexistent in standard residential/commercial gear.
Complete Copper Wire Gauge Chart Amp Table (THHN/THWN & NM-B)
The following data is extracted directly from NFPA 70 (NEC) Table 310.16 for copper conductors with an ambient temperature of 30°C (86°F). This table covers the standard sizes you will pull from the spool for feeders, subpanels, and heavy branch circuits.
| AWG / kcmil | 60°C (NM-B / Romex) | 75°C (THWN / Terminals) | 90°C (THHN / Derating Base) |
|---|---|---|---|
| 14 | 15A | 20A | 25A |
| 12 | 20A | 25A | 30A |
| 10 | 30A | 35A | 40A |
| 8 | 40A | 50A | 55A |
| 6 | 55A | 65A | 75A |
| 4 | 70A | 85A | 95A |
| 3 | 85A | 100A | 110A |
| 2 | 95A | 115A | 130A |
| 1 | 110A | 130A | 145A |
| 1/0 | 125A | 150A | 170A |
| 2/0 | 145A | 175A | 195A |
| 3/0 | 165A | 200A | 225A |
| 4/0 | 195A | 230A | 260A |
Source: National Electrical Code (NEC) Table 310.16. For aluminum conductors, ampacities are significantly lower; refer to the right side of the NEC table or Cerrowire's official ampacity charts for aluminum-specific data.
Derating Factors: When the Base Ampacity Drops
The table above assumes perfect conditions: an ambient temperature of 86°F (30°C) and no more than three current-carrying conductors bundled together in a raceway. When you violate either condition, you must apply derating factors to the 90°C column base value.
How derating modifies the base value: You multiply the 90°C ampacity by the correction factors. If the final derated number is still higher than your required load, the wire is acceptable, but your breaker size remains limited by the 75°C terminal rating.
Worked Numeric Example: You are running a 20A receptacle circuit through an attic where the ambient temperature hits 110°F (43°C), and you are pulling four current-carrying conductors (two 120V circuits sharing a neutral is not applicable here; assume 4 distinct hot wires for two 240V circuits) in a single EMT conduit using 12 AWG THHN.
- Base 90°C Ampacity: 12 AWG = 30A.
- Temperature Correction (105°F - 113°F): Multiply by 0.87.
- Bundling Adjustment (4 to 6 wires): Multiply by 0.80.
- Calculation: 30A × 0.87 × 0.80 = 20.88A.
Because 20.88A is greater than the 20A breaker protecting the circuit, 12 AWG THHN is legally and safely compliant. However, you cannot upsize the breaker to 25A, because the breaker terminals are rated for 75°C (which caps 12 AWG at 25A, but standard practice and device ratings limit it to 20A). For a deeper breakdown on temperature limitations, Mike Holt's guide on 60°C vs 75°C vs 90°C terminations is the industry benchmark reference.
Decision Tree: Pick Your Exact AWG and Breaker Size
Use this decision path to terminate your sizing process with a concrete parts list. This example solves a common modern upgrade: installing a hardwired Level 2 EV charger.
| Step | Condition / Question | Action / Calculation |
|---|---|---|
| 1. Determine Continuous Load | EV charger draws 40A continuously (over 3 hours). | Multiply by 125% per NEC 210.20(A). Target Circuit Ampacity = 50A. |
| 2. Select Breaker Size | Target ampacity is 50A. | Pick next standard breaker size up if exact match isn't available. Pick: 50A Double-Pole Breaker. |
| 3. Identify Wire Type & Termination | Pulling individual wires in flexible metallic conduit (FMC) to a hardwired junction box. | Use THHN/THWN-2. Terminals in modern EVSE and 50A breakers are rated 75°C. |
| 4. Check Base Ampacity Table | Need 50A in the 75°C column. | Look at Table 310.16. 8 AWG copper in the 75°C column is exactly 50A. |
| 5. Apply Derating (If Applicable) | Only 3 current-carrying conductors (Hot, Hot, Ground). Normal basement temp. | No bundling or temperature derating required. Base ampacity holds. |
| FINAL PICK | Buy: 8 AWG Copper THHN (Black, Red, Green) + 50A 240V Double-Pole Breaker. | |
What This Wire Gauge Chart Amp Table Cannot Tell You
The NEC 310.16 ampacity chart is a thermal limit table—it tells you the point at which the wire insulation will melt or degrade under a specific current. It completely ignores the physics of voltage drop over distance.
Voltage Drop Blindspot: If you are running a 50A circuit to a detached garage 150 feet away using 8 AWG copper, the ampacity table says 8 AWG is perfectly safe from a fire/melting perspective. However, pushing 50A through 300 feet of total wire length (hot + neutral/ground return path equivalent) will result in a voltage drop of roughly 4.5% on a 240V circuit. This exceeds the NEC's recommended 3% maximum for branch circuits (NEC 210.19 Informational Note No. 4). Your EV charger may fault out or charge slower due to low voltage at the terminals.
The Fix for Distance: The ampacity chart will not tell you to upsize for distance. You must manually upsize to 6 AWG copper for that 150-foot run to keep voltage drop under 3%, even though your breaker remains 50A and the 8 AWG ampacity was technically sufficient for the thermal load. Always calculate voltage drop for any run exceeding 75 feet at high currents.
