The copper amp chart, formally codified as NEC Table 310.16, defines the allowable ampacity (current-carrying capacity) of insulated copper conductors based on wire gauge and insulation temperature rating. For standard residential and light commercial branch circuits, the baseline quick-jump reference values are:
- 15A Circuit: 14 AWG (using the 60°C column)
- 20A Circuit: 12 AWG (using the 60°C column)
- 30A Circuit: 10 AWG (using the 60°C column)
- 40A Circuit: 8 AWG (using the 75°C column)
- 50A Circuit: 6 AWG (using the 75°C column)
- 100A Feeder: 3 AWG (using the 75°C column)
While these quick-jump rows cover 90% of typical DIY and residential rough-in scenarios, relying solely on the base chart without understanding temperature columns and derating factors is a primary cause of overheated terminations and failed inspections. Below is the complete breakdown of how to apply this data on the jobsite.
How to Read the NEC Copper Amp Chart
The most common mistake when reading a copper amp chart is defaulting to the highest ampacity number available for a given wire size. The chart is divided into three temperature columns: 60°C (140°F), 75°C (167°F), and 90°C (194°F). These columns correspond to the thermal rating of the wire's insulation (e.g., TW/UF for 60°C, THWN/THHN for 90°C).
Which Column Applies to Your Installation?
To determine which column dictates your final wire size, you must apply the weakest link rule outlined in NEC 110.14(C). Your allowable ampacity is limited by the lowest temperature rating of any connected component in the circuit.
- Use the 60°C column if you are using NM-B (Romex) cable, UF-B cable, or connecting to older equipment and small breakers (15A-50A) that are not explicitly marked with a higher temperature rating.
- Use the 75°C column for most modern commercial/industrial equipment, larger residential breakers (60A and above), and THHN wire terminating in panels or disconnects explicitly rated for 75°C.
- Use the 90°C column almost exclusively for calculating derating adjustments (explained below) or for high-temperature environments where the equipment lugs are specifically rated for 90°C (rare in standard construction).
The Complete Copper Ampacity Chart (NEC Table 310.16)
The following data is extracted directly from the 2023/2026 NEC Table 310.16 for copper conductors. It assumes an ambient air temperature of 30°C (86°F) and not more than three current-carrying conductors in a raceway or cable. For standard reference and safety compliance, always cross-reference with OSHA electrical safety guidelines and your local Authority Having Jurisdiction (AHJ).
| AWG / kcmil | 60°C (140°F) TW, UF |
75°C (167°F) RHW, THHW, THW, THWN, XHHW |
90°C (194°F) THHN, THHW, THW-2, THWN-2, XHHW-2 |
|---|---|---|---|
| 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 |
Derating Factors: When the Base Chart Fails
The ampacities listed in the chart above assume ideal conditions: exactly three current-carrying conductors bundled together in an ambient temperature of 30°C (86°F). When you pull more wires through a single conduit or route them through a hot attic, the wires cannot dissipate heat effectively. This is where derating modifies the base value.
How Derating Rows Modify the Base Value
When bundling more than three current-carrying conductors in a raceway, you must apply the adjustment factors from NEC Table 310.15(C)(1). You always start your derating math using the 90°C column (assuming you are using THHN/THWN-2 wire), and then verify the final derated number against the terminal temperature limits.
Worked Numeric Example:
You are pulling a multi-wire branch circuit through a 3/4-inch EMT conduit. You have 4 current-carrying 12 AWG THHN copper conductors (two hot, two neutral for two separate 120V circuits). The ambient temperature is a standard 30°C.
- Base Ampacity: Look at the 90°C column for 12 AWG. The base value is 30A.
- Adjustment Factor: For 4 to 6 current-carrying conductors, the NEC requires an 80% adjustment factor.
- Calculation: 30A × 0.80 = 24A.
- Final Verification: The derated ampacity is 24A. Because 24A is greater than the 20A breaker protecting the circuit, the 12 AWG wire is perfectly legal and safe to use. Furthermore, 24A exceeds the 75°C termination limit (25A base, but breaker is 20A), satisfying NEC 110.14(C).
If you had pulled 10 conductors in that same pipe, the adjustment factor drops to 50%. Your 12 AWG wire would derate to 15A (30A × 0.50), forcing you to upsize to 10 AWG to maintain a 20A circuit capacity.
Copper Amp Chart FAQ
What size copper wire do I need for a 50 amp breaker?
For a standard 50A circuit (like an electric range or EV charger), you need 6 AWG copper wire if your terminations and insulation are rated for 75°C (such as THHN in conduit terminating at a 75°C rated breaker). However, if you are running NM-B (Romex) cable inside a wall cavity, NM-B is strictly limited to the 60°C column. In the 60°C column, 6 AWG is only rated for 55A. Therefore, if using NM-B for a 50A circuit, you must upsize to 4 AWG copper to safely meet the requirement without violating the 60°C thermal limit.
Can I use the 90°C column to size my entire residential branch circuit?
No. While you can buy 90°C rated wire (like THHN-2) and it is the standard for conduit pulls, you cannot use the 90°C ampacity column to determine your final wire size for breaker terminations. Almost all residential breakers and panelboard lugs are rated for 75°C maximum. The 90°C column is legally reserved as a starting point for calculating derating adjustments (like conduit fill or high ambient temperatures). Once derated, the final ampacity must still meet or exceed the 75°C or 60°C requirements of the connected equipment.
Does the copper amp chart account for voltage drop over long distances?
No. NEC Table 310.16 strictly dictates ampacity—the maximum current the wire can carry before the insulation degrades from heat. It does not factor in voltage drop. If you are running a 50A circuit to a detached garage 150 feet away, 6 AWG copper will safely handle the thermal load, but the voltage at the far end may drop below the acceptable 3% to 5% threshold under full load. For long runs, you must calculate voltage drop using the direct-current resistance values found in NEC Chapter 9, Table 8, and typically upsize the wire by one or two gauges to compensate.
Why is 14 AWG copper missing from some commercial amp charts?
Many commercial and industrial reference charts omit 14 AWG because the NEC heavily restricts its use. While the 90°C column shows 14 AWG can theoretically handle 25A, NEC 240.4(D) hard-caps the overcurrent protection for 14 AWG copper at 15A. Furthermore, many commercial specifications and local municipal codes ban 14 AWG entirely, requiring a minimum of 12 AWG for all branch circuits to ensure mechanical durability and reduce voltage drop in larger facilities.
