The correct amperage chart for wire size in the United States is dictated by NEC Table 310.16. For standard residential and commercial copper conductors, the baseline quick-reference values are: 14 AWG for 15A, 12 AWG for 20A, 10 AWG for 30A, 8 AWG for 40A, and 6 AWG for 55A/65A depending on insulation. However, picking the right wire requires matching your installation method to the correct temperature column and applying derating factors for bundled wires.
The Master Amperage Chart for Wire Size (NEC Table 310.16)
Below is the complete ampacity data for copper conductors, sourced directly from the National Electrical Code (NFPA 70) Table 310.16. This table assumes an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors in a raceway or cable.
| AWG Size | 60°C (140°F) NM-B / Romex |
75°C (167°F) THHN Terminations |
90°C (194°F) THHN Derating Base |
|---|---|---|---|
| 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 | 115A |
| 2 AWG | 95A | 115A | 130A |
| 1 AWG | 110A | 130A | 145A |
| 1/0 AWG | 125A | 150A | 170A |
| 2/0 AWG | 145A | 175A | 195A |
How to Read the Chart: Temperature Columns & Insulation
The most common mistake DIYers and junior apprentices make is reading the 90°C column and assuming they can push that amperage through the wire. You must select the column that matches both your cable type and your termination points.
Which column applies to your installation?
- The 60°C Column: Use this for Non-Metallic Sheathed Cable (NM-B, commonly known as Romex). Per NEC 334.80, the ampacity of NM-B cable is strictly limited to the 60°C column, even though the internal THHN wires might technically be rated for 90°C. Use this column for standard interior wall wiring.
- The 75°C Column: Use this for individual THHN/THWN-2 wires pulled through conduit (EMT, PVC) and for calculating termination limits. Almost all modern breakers, lugs, and receptacles are rated for 75°C terminations. This is your baseline for conduit runs and subpanel feeders.
- The 90°C Column: You cannot use this column to determine your final breaker size. This column exists solely as a mathematical starting point for calculating derating factors (explained below).
Derating: When the Base Amperage Chart Fails
The base amperage chart assumes perfect conditions: an ambient temperature of 86°F (30°C) and a maximum of three current-carrying conductors (CCCs) in a single raceway. When you exceed these parameters, the wire cannot dissipate heat efficiently, and you must apply derating factors to the 90°C column.
How derating rows modify the base value:
If you pull four to six current-carrying conductors through a single conduit, NEC Table 310.15(C)(1) requires you to multiply the 90°C base ampacity by 80%. If you pull seven to nine CCCs, you multiply by 70%.
Worked Numeric Example: You are pulling four 12 AWG THHN wires (two hot, one neutral, one grounded neutral for a multi-wire branch circuit) through a single PVC conduit. 1. Base 90°C ampacity for 12 AWG = 30A. 2. Derating factor for 4 CCCs = 80%. 3. 30A × 0.80 = 24A adjusted ampacity. Because 24A is still greater than the 20A breaker you plan to use, 12 AWG THHN is legally compliant. However, if you added two more wires (6 CCCs total), the derating drops to 70% (30A × 0.70 = 21A). You are now dangerously close to the 20A breaker limit and should upsize to 10 AWG to prevent nuisance tripping and insulation degradation.
Decision Tree: Pick Your Exact Wire Size in 4 Steps
Use this if-then decision path to terminate your wire sizing process with a concrete material pick.
| Step | Condition | Action / Result |
|---|---|---|
| 1. Calculate Load | Is the load continuous (on for 3+ hours, like an EV charger or HVAC)? | If Yes: Multiply load by 1.25. (e.g., 40A EV charger × 1.25 = 50A required). If No: Use the exact nameplate amperage. |
| 2. Select Cable Type | Is the wire running inside a finished, enclosed wall cavity? | If Yes: You must use NM-B (Romex). Go to the 60°C column. If No: You can use THHN in conduit. Go to the 75°C column. |
| 3. Check Derating | Are there more than 3 current-carrying wires in the conduit, or is ambient temp > 86°F? | If Yes: Apply derating math to the 90°C column. Ensure the final adjusted number is higher than your breaker size. If No: Skip derating. |
| 4. Final Pick | Match the calculated required ampacity to the table. | Select the smallest AWG that meets or exceeds the required ampacity in your designated column. |
Concrete Pick Example: You are wiring a 40A continuous hardwired EV charger in an exposed garage ceiling using EMT conduit. Step 1: 40A × 1.25 = 50A required. Step 2: Exposed conduit means THHN wire (75°C column). Step 3: Only 2 current-carrying conductors (240V circuit, no neutral), so no derating. Step 4: Looking at the 75°C column, 8 AWG is rated for 50A. Final Concrete Pick: Buy 8 AWG Copper THHN and protect it with a 50A double-pole breaker.
What This Amperage Chart Cannot Tell You
While NEC Table 310.16 is the definitive amperage chart for wire size regarding thermal limits and fire safety, it is blind to three critical real-world constraints. Always verify these before pulling wire:
- Voltage Drop: The chart assumes the wire is short enough that resistance won't cause a meaningful voltage drop. For runs exceeding 100 feet, the wire will carry the amperage safely without melting, but the voltage at the receptacle may drop below the acceptable 3% threshold (e.g., dropping to 114V on a 120V circuit), causing motors to overheat or electronics to brown out. Rule of thumb: Bump up one AWG size for every 100 feet of run past 50 feet on 240V circuits.
- Conduit Fill Capacity: The chart tells you how much current the wire can handle, but NEC Chapter 9, Table 1 dictates how many wires physically fit inside a conduit. You cannot legally stuff six 6 AWG THHN wires into a 1/2-inch EMT conduit, regardless of what the ampacity chart says.
- Short-Circuit Withstand Ratings: Ampacity charts measure steady-state thermal limits. They do not indicate how well a wire withstands the extreme magnetic and thermal forces of a dead short before the breaker trips. For high-fault-current environments (like main service entrances feeding 200A+), equipment and busbar bracing ratings take precedence over standard branch-circuit wire sizing.
For further reading on conductor sizing and official code updates, refer to the National Fire Protection Association's NEC resources and manufacturer-specific installation guides like the Southwire Ampacity Chart. Always consult your local Authority Having Jurisdiction (AHJ), as local amendments can supersede baseline NEC tables.
