When sizing conductors for a branch circuit or feeder, a gauge to amp chart is your primary reference for determining the maximum allowable ampacity of a wire. In the United States, the definitive source for these values is the National Electrical Code (NEC). While older editions referenced this data under Table 310.15(B)(16) or 310.16, the 2020 and 2023 NEC revisions relocated the standard ampacity table to NEC Table 310.17. Regardless of the article number your local jurisdiction currently enforces, the core physics and ampacity values for standard building wire remain identical.
This reference provides the complete chart, explains how to select the correct temperature column, and details the derating factors that modify these base numbers on the jobsite.
How to Read the Gauge to Amp Chart (and Pick Your Column)
The most common mistake DIYers and junior apprentices make is looking at the 90°C column for THHN wire and sizing the breaker to that higher number. You cannot do this for standard terminations. To read the chart correctly, you must understand NEC 110.14(C), which governs terminal temperature limitations.
• 60°C Column: Use this for circuits rated 100A or less, or for conductors sized 14 AWG through 1 AWG, unless the equipment is specifically marked otherwise. Most standard residential receptacles and breakers fall here.
• 75°C Column: Use this for circuits rated over 100A, or for conductors sized larger than 1 AWG. Most commercial panels, subpanels, and heavy appliance terminations are rated 75°C.
• 90°C Column: Use this only as the starting point for derating calculations (like bundling or high ambient heat). You almost never use the 90°C ampacity for final breaker sizing because the lugs on the breaker and the device are rarely rated for 90°C.
The Complete NEC Gauge to Amp Chart (Copper & Aluminum)
Below is the complete ampacity table for standard building wires (THHN, THWN, XHHW, NM-B) in an ambient temperature of 30°C (86°F), based on NEC Table 310.17.
• 15A Circuit: 14 AWG Copper (60°C col)
• 20A Circuit: 12 AWG Copper (60°C col)
• 30A Circuit: 10 AWG Copper (60°C col)
• 50A Circuit: 6 AWG Copper or 4 AWG Aluminum (75°C col)
| AWG / kcmil | Cu 60°C | Cu 75°C | Cu 90°C | Al 60°C | Al 75°C | Al 90°C |
|---|---|---|---|---|---|---|
| 14 | 15 | 20 | 25 | — | — | — |
| 12 | 20 | 25 | 30 | 15 | 20 | 25 |
| 10 | 30 | 35 | 40 | 25 | 30 | 40 |
| 8 | 40 | 50 | 55 | 30 | 40 | 45 |
| 6 | 55 | 65 | 75 | 40 | 50 | 60 |
| 4 | 70 | 85 | 95 | 55 | 65 | 75 |
| 3 | 85 | 100 | 115 | 65 | 75 | 85 |
| 2 | 95 | 115 | 130 | 75 | 90 | 100 |
| 1 | 110 | 130 | 145 | 85 | 100 | 120 |
| 1/0 | 125 | 150 | 170 | 100 | 120 | 135 |
| 2/0 | 145 | 175 | 195 | 115 | 135 | 155 |
| 3/0 | 165 | 200 | 225 | 130 | 155 | 180 |
| 4/0 | 195 | 230 | 260 | 150 | 180 | 205 |
Source: NFPA 70 National Electrical Code, Table 310.17 (formerly 310.16). Values assume not more than three current-carrying conductors in a raceway or cable.
Derating and What This Table Cannot Tell You
The numbers in the chart above are base ampacities. They assume an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors bundled together. Real-world installations frequently violate these assumptions, requiring you to apply derating multipliers.
How Derating Modifies the Base Value
When you pull four or more current-carrying conductors through a single conduit, the heat generated by the wires cannot dissipate efficiently. According to NEC 310.15(C)(1), you must multiply the base ampacity by a correction factor.
Worked Example: You are pulling four 12 AWG THHN current-carrying conductors in a conduit for a multi-wire branch circuit.
1. Start with the 90°C column for THHN: 30A.
2. Apply the bundling derating factor for 4-6 conductors (80%): 30A × 0.80 = 24A.
3. Check the terminal temperature limit (60°C column for 12 AWG): 20A.
4. The final allowable ampacity is the lower of the two: 20A. You must protect this wire with a 20A breaker.
What the Gauge to Amp Chart Cannot Tell You
An ampacity chart only tells you the thermal limit of the wire's insulation. It does not account for:
- Voltage Drop: A 12 AWG wire can safely carry 20A thermally, but if the run is 150 feet long, the resistance will cause a voltage drop exceeding the recommended 3% limit. You must upsize to 10 AWG or 8 AWG to maintain 114V-126V at the load, even if the breaker remains 20A.
- Short-Circuit Withstand: The chart does not indicate how the wire will survive a massive fault current before the breaker trips. This is governed by the breaker's let-through current and the wire's circular mil area.
- Physical Fit: The chart won't tell you if three 4/0 AWG aluminum feeders will physically fit into the lugs of a 200A residential main breaker. Always verify terminal wire-range markings on the equipment.
Gauge to Amp Chart FAQ
What size wire do I need for a 20 amp breaker?
For a standard 20A residential branch circuit, you need 12 AWG copper wire. Looking at the 60°C column of the gauge to amp chart, 12 AWG copper is rated for exactly 20A. While 12 AWG THHN has a 90°C rating of 30A, NEC 110.14(C) requires you to use the 60°C column for standard terminations on circuits 100A or less. If you are using aluminum, you must step up to 10 AWG, which is rated 25A at 60°C and 30A at 75°C.
Can I use the 90°C column for my home wiring?
No, you cannot use the 90°C column for final breaker sizing in typical home wiring. The 90°C rating applies to the insulation itself (like THHN), but the breakers, receptacles, and switches you connect the wire to are almost universally rated for 60°C or 75°C. The NEC requires the ampacity to be based on the lowest temperature rating of any connected component. You only use the 90°C column as a mathematical starting point before applying derating factors for heat or bundling.
How does voltage drop change my gauge to amp calculation?
Voltage drop does not change the ampacity (thermal limit) of the wire, but it dictates the minimum practical size for long runs. The NEC recommends a maximum 3% voltage drop for branch circuits and 5% overall. For example, a 240V, 30A load located 120 feet from the panel requires 10 AWG copper for thermal ampacity, but calculating the resistance (using NEC Chapter 9, Table 8) reveals a 4.2% drop. To keep the drop under 3%, you must upsize to 8 AWG copper, even though the breaker remains 30A.
Why is aluminum wire sized differently than copper?
Aluminum has a higher electrical resistance than copper—roughly 61% of the conductivity of copper by volume. Therefore, an aluminum conductor must have a larger cross-sectional area (a lower AWG number) to carry the same current without overheating. Additionally, aluminum expands and contracts more than copper under thermal cycling, which historically caused loose connections and fires if not installed with proper torque and antioxidant paste. Modern AA-8000 series aluminum alloy wire is safe and code-compliant when sized correctly via the aluminum columns in the gauge to amp chart and torqued to manufacturer specifications.
