An amp cable chart—specifically Table 310.16 in the National Electrical Code (NEC)—defines the maximum continuous current a specific wire gauge and insulation type can safely carry without exceeding its thermal limits. For standard residential copper wiring (THHN/THWN-2), the baseline ampacities are 15A for 14 AWG, 20A for 12 AWG, and 30A for 10 AWG when evaluated under the 60°C column. However, reading the chart correctly requires understanding termination ratings, ambient temperature corrections, and conductor bundling derations.
The Standard Amp Cable Chart (NEC Table 310.16)
This table provides the allowable ampacities for insulated copper conductors rated up to 2000 volts. Before using these numbers, understand the baseline assumptions baked into the data: these values assume an ambient air temperature of 30°C (86°F) and no more than three current-carrying conductors bundled together in a single raceway or cable. If your installation deviates from these two parameters, the base numbers below must be mathematically derated.
Source: NFPA 70 National Electrical Code (NEC) Table 310.16. For extended aluminum wire data and larger gauges, refer to the Cerrowire Ampacity Chart or the NFPA NEC documentation.
| Copper AWG / kcmil | 60°C (140°F) Column | 75°C (167°F) Column | 90°C (194°F) Column |
|---|---|---|---|
| 14 AWG | 15 Amps | 20 Amps | 25 Amps |
| 12 AWG | 20 Amps | 25 Amps | 30 Amps |
| 10 AWG | 30 Amps | 35 Amps | 40 Amps |
| 8 AWG | 40 Amps | 50 Amps | 55 Amps |
| 6 AWG | 55 Amps | 65 Amps | 75 Amps |
| 4 AWG | 70 Amps | 85 Amps | 95 Amps |
| 3 AWG | 85 Amps | 100 Amps | 115 Amps |
| 2 AWG | 95 Amps | 115 Amps | 130 Amps |
| 1 AWG | 110 Amps | 130 Amps | 145 Amps |
| 1/0 AWG | 125 Amps | 150 Amps | 170 Amps |
Selecting the Correct Temperature Column
The most common mistake DIYers and junior electricians make is defaulting to the 90°C column because modern THHN wire is rated for 90°C. This is a code violation that leads to overheated terminals. To determine which column applies to your installation, you must follow the 'weakest link' rule established in NEC 110.14(C).
The allowable ampacity is limited by the lowest temperature rating of any connected component, termination, or conductor in the circuit. Most modern circuit breakers, receptacles, and switches are tested and rated for 75°C terminations. Older equipment (pre-1990s) or specific lighting fixtures may only be rated for 60°C.
For conductors 8 AWG and larger, you can typically use the 75°C column for sizing your overcurrent breaker, provided the lugs on your breaker and disconnect are marked 75°C (which almost all modern Square D, Eaton, and Siemens residential panels are). You use the 90°C column exclusively for calculating derating factors (like ambient temperature corrections), but the final derated ampacity must still be compared against the 75°C or 60°C base limits for termination sizing.
Derating Factors and Chart Limitations
The base amp cable chart assumes ideal conditions: a cool 30°C room and plenty of physical space around the wires. Real-world jobsites rarely offer this. When conditions change, the base ampacity must be multiplied by a derating factor, effectively reducing the current the wire can safely carry.
How Derating Modifies the Base Value
There are two primary derating scenarios you must calculate:
- Ambient Temperature (NEC Table 310.15(B)(1)): If you run THHN wire through an attic that reaches 45°C (113°F) in the summer, you must apply a correction factor of 0.87 to the 90°C column. For a 10 AWG wire (base 40A at 90°C), the math is 40A × 0.87 = 34.8A. Since 34.8A is still higher than the 30A termination limit (60°C column), a 30A breaker is still acceptable.
- Conductor Bundling (NEC Table 310.15(C)(1)): If you pull more than three current-carrying conductors through a single conduit, they trap each other's heat. For 4 to 6 conductors, you multiply the base ampacity by 80%. For 7 to 9 conductors, the multiplier drops to 70%.
War Story Example: Running a 240V baseboard heater circuit using 10 AWG THHN. You pull two hots and a neutral (for a multi-wire branch circuit or specific 120/240V appliance) plus a ground through a conduit. That is 3 current-carrying conductors. No bundling deration needed. But if you add a second circuit to the same conduit (now 4 current-carrying hots/neutrals), the 90°C ampacity of 10 AWG (40A) drops to 32A (40 × 0.80). You must now verify this 32A derated value against your termination limits.
What This Amp Cable Chart Cannot Tell You
Ampacity only addresses thermal limits under continuous load. It completely ignores voltage drop, which is a function of distance, not just heat. The NEC recommends a maximum 3% voltage drop for branch circuits (Informational Note to 210.19).
Consider a 120V, 20A receptacle located 100 feet from the panel. Using 12 AWG copper, the voltage drop formula is VD = (2 × K × I × L) / CM. Using K=12.9 (copper), I=20A, L=100ft, and CM=6530 (circular mils for 12 AWG), the drop is 7.9V. That is a 6.5% drop on a 120V circuit—more than double the recommended limit. The amp cable chart says 12 AWG is perfectly safe from a fire perspective (it won't melt), but your power tools or electronics at the end of the run will suffer from brownouts and reduced torque. To fix this, you must upsize to 10 AWG or 8 AWG purely for voltage drop management, even though the overcurrent breaker remains 20A.
Finally, the chart cannot tell you if a wire will physically fit into a lug. A 1/0 AWG wire rated for 150A might be the correct thermal choice, but if you are terminating into a 100A residential breaker with small mechanical lugs, you may need to use a reducing pin or pigtail, which introduces new resistance and heat variables not captured in a simple lookup table.
