How to read the ampacity table
Find the conductor size in the left column, then read across to the column that matches both the metal (copper or aluminum) and the temperature rating you are allowed to use. The number is the allowable ampacity in amps for not more than three current-carrying conductors in a raceway, cable or directly buried, with the surrounding air at 30°C (86°F).
The temperature column you may use is set by the lowest-rated part of the circuit, not just the wire. A THHN conductor is rated 90°C, but if the breaker or receptacle terminal is rated 60°C, the 60°C column governs at that connection. For circuits of 100 A or less, or conductors 14 through 1 AWG, NEC 110.14(C) assumes 60°C terminations unless the equipment is listed and marked for 75°C. Most modern breakers and panels are marked 60/75°C.
The rule behind the small sizes
The table shows 14 AWG copper at 20 A in the 75°C column, yet no one puts it on a 20 A breaker. Section 240.4(D) sets a ceiling on overcurrent protection for small conductors: 15 A for 14 AWG copper, 20 A for 12 AWG and 30 A for 10 AWG. The second table on this page lists those limits next to the 60°C ampacity so the two can be compared at a glance.
When the table number is not the final answer
Table 310.16 is a starting point. Two adjustments are applied before choosing the breaker:
- Ambient temperature correction (310.15(B)): conductors in hot attics, on rooftops or near boilers carry less current. The correction starts from the insulation's own column, which is why 90°C wire is useful even when terminals are 75°C.
- More than three current-carrying conductors (310.15(C)(1)): bundling four or more conductors in one raceway or cable reduces each one's ampacity.
After derating, the conductor must still be protected per 240.4, and its final ampacity must not exceed the termination rating. Long runs may also need a larger size to keep voltage drop down; see the voltage drop chart.
Common ampacity mistakes
- Reading the 90°C column for a breaker size on an ordinary branch circuit.
- Using a copper value for aluminum. Aluminum needs roughly two sizes larger for the same current.
- Forgetting that NM cable is limited to the 60°C column.
- Skipping voltage drop on long feeders to detached garages and outbuildings.
This chart reproduces values from NFPA 70, National Electrical Code, 2023 edition. States and cities adopt the NEC with local amendments, so confirm with your authority having jurisdiction.