The amp capacity of wire chart (officially NEC Table 310.16) dictates the maximum continuous current a conductor can carry based on its gauge, material, and insulation temperature rating. For standard residential branch circuits using copper wire, the baseline capacities are: 14 AWG is 15A, 12 AWG is 20A, and 10 AWG is 30A when referencing the 60°C column for NM-B (Romex) cable.
How to Read the Amp Capacity of Wire Chart
The NEC ampacity table is divided into three main temperature columns for copper: 60°C (140°F), 75°C (167°F), and 90°C (194°F). Understanding which column applies to your specific installation is the most common point of failure for DIYers and junior apprentices.
Which Column Applies to Your Installation?
Under NEC 110.14(C)(1), you must use the 60°C column for circuits rated 100 amps or less, or for conductors sized 14 AWG through 1 AWG, unless the equipment (breaker, lug, or receptacle) is specifically listed and marked for 75°C terminations. Furthermore, standard NM-B (Romex) and UF-B cables are strictly limited to the 60°C column by NEC 334.80, even though the individual THHN wires inside the jacket technically have 90°C insulation. You use the 75°C or 90°C columns primarily for THHN/THWN-2 wires pulled in conduit terminating into industrial or heavy-duty commercial panels rated for those temperatures.
What the Table Cannot Tell You
The amp capacity of wire chart only provides thermal limits for the insulation. It does not account for voltage drop over long distances. A 12 AWG wire can safely carry 20A at 150 feet without melting, but the voltage drop will exceed the NEC-recommended 3% limit, causing motors to overheat or lights to dim. It also does not override the terminal temperature limits of the specific breaker or lug you are terminating into.
Bookmark-Friendly Quick-Jump Reference
- 15A Breaker: 14 AWG (Lighting, general receptacles)
- 20A Breaker: 12 AWG (Kitchen, bathroom, laundry receptacles)
- 30A Breaker: 10 AWG (Dryers, heavy window ACs)
- 40A Breaker: 8 AWG (Electric ranges, large EV chargers)
- 50A Breaker: 6 AWG (Hot tubs, subpanels, welders)
- 100A Breaker: 3 AWG (Subpanel feeders)
- 200A Breaker: 2/0 AWG (Main residential service entrance)
Complete Copper Ampacity Reference (NEC Table 310.16)
The following data is sourced directly from the NFPA 70 National Electrical Code, specifically Table 310.16 for copper conductors with up to three current-carrying conductors in a raceway or cable, at an ambient temperature of 30°C (86°F). For more detailed engineering data, refer to the Copper Development Association wiring manuals.
| AWG / kcmil Size | 60°C (140°F) TW, UF, NM-B |
75°C (167°F) RHW, THHW, THWN |
90°C (194°F) THHN, XHHW-2 |
Standard Max 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 | 80A / 90A* |
| 2 | 95A | 115A | 130A | 100A |
| 1 | 110A | 130A | 145A | 110A |
| 1/0 | 125A | 150A | 170A | 125A |
| 2/0 | 145A | 175A | 195A | 150A / 175A* |
| 3/0 | 165A | 200A | 225A | 200A |
| 4/0 | 195A | 230A | 260A | 225A |
*Note: NEC 240.4(B) allows you to round up to the next standard breaker size (e.g., using a 50A breaker for an 8 AWG THHN wire rated at 40A in the 60°C column, provided the calculated load does not exceed the wire's base ampacity. This exception does not apply to circuits under 800A, but specific local amendments may restrict rounding up. Always default to the exact match or lower if in doubt.
Derating Factors: When the Chart Doesn't Tell the Whole Story
The baseline numbers in the amp capacity of wire chart assume an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors bundled together. When you deviate from these conditions, you must apply derating factors that modify the base value.
Ambient Temperature Derating
If you are pulling wire through an attic in a hot climate where ambient temperatures routinely hit 50°C (122°F), you cannot use the base chart value. You must multiply the 90°C ampacity by the temperature correction factor. For 50°C ambient, the factor is 0.82. A 10 AWG THHN wire (base 40A at 90°C) derates to 32.8A (40 x 0.82).
Conduit Fill (Bundling) Derating
When you pull 4 to 6 current-carrying conductors in a single conduit, the heat cannot dissipate. You must multiply the 90°C ampacity by 80%. If you pull 7 to 9 conductors, the factor drops to 70%.
Worked Example: You are pulling four 12 AWG THHN current-carrying conductors in a conduit for two separate 20A circuits. The base 90°C ampacity for 12 AWG is 30A. Applying the 80% bundling derating factor (30A x 0.80) yields 24A. Because 24A is still greater than your 20A breaker, the wire is safe to use. However, if you added a third circuit (6 current-carrying conductors), the ampacity remains 24A. If you added a fourth circuit (8 conductors, 70% derating), the ampacity drops to 21A (30 x 0.70), which is dangerously close to the continuous load limits of a 20A breaker.
Frequently Asked Questions
What size wire do I need for a 50-amp circuit?
For a standard 50-amp circuit (like a hot tub or EV charger), you need 6 AWG copper wire if you are using NM-B cable or terminating into standard 60°C-rated residential breakers. If you are pulling individual THHN/THWN-2 wires in conduit and your breaker/lugs are explicitly rated for 75°C, you can use 8 AWG copper (which is rated 50A in the 75°C column). Always check the terminal markings on your specific breaker and receptacle before downsizing.
Can I use the 90°C column to size my NM-B (Romex) cable?
No. While the individual conductors inside NM-B cable have 90°C insulation, NEC 334.80 strictly mandates that the ampacity of NM-B cable be determined using the 60°C column. The 90°C rating of the internal wires is only useful as a starting point for applying derating factors (like ambient temperature or conduit bundling) before the final ampacity is capped at the 60°C value.
Does the ground wire count towards conduit fill derating?
No. According to NEC 310.15(C)(1), equipment grounding conductors (bare copper or green insulated) are not considered "current-carrying conductors" for the purpose of conduit fill derating. They only carry current during a fault condition. However, they do count toward the physical maximum fill percentage of the conduit (NEC Chapter 9, Table 1) to ensure you can physically pull the wires without damaging the insulation.
How does voltage drop affect the amp capacity of wire chart?
Voltage drop does not change the thermal ampacity (fire safety limit) of the wire, but it dictates practical performance. NEC 210.19(A) Informational Note recommends a maximum 3% voltage drop for branch circuits and 5% total for feeder and branch circuits combined. If you are running a 20A circuit 150 feet from the panel, a 12 AWG wire will safely carry the 20A without overheating, but it will experience a ~6% voltage drop. To fix this, you must upsize to 10 AWG or 8 AWG wire to reduce resistance, even though the breaker remains 20A.
