The ampacity of a cable is the maximum continuous current it can carry without exceeding its insulation temperature rating. For standard residential copper wiring, the quick-reference baseline values are 15A for 14 AWG, 20A for 12 AWG, and 30A for 10 AWG. However, pulling a single number from an ampacity of cable table without understanding the temperature columns and derating factors is the most common cause of failed electrical inspections, overheated terminals, and nuisance breaker trips.
The Core Ampacity of Cable Table (NEC 310.16)
The definitive source for wire sizing in the United States is Table 310.16 in the National Electrical Code (NFPA 70), published by the National Fire Protection Association (NFPA). This table establishes the baseline ampacities assuming an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors bundled in a single raceway or cable.
How to read this table: The columns are divided by the temperature rating of the wire's insulation (60°C, 75°C, and 90°C) and the conductor material (Copper vs. Aluminum). The rows represent the American Wire Gauge (AWG) size. The most common modern building wire, THHN/THWN-2, is rated for 90°C, but as you will learn below, you rarely get to use the full 90°C ampacity for breaker sizing.
| Copper AWG Size | 60°C Column (TW, UF) | 75°C Column (THW, THWN, XHHW) | 90°C Column (THHN, THWN-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 |
| 2 AWG | 95A | 115A | 130A |
| 1/0 AWG | 125A | 150A | 170A |
• Standard 15A/20A Receptacle Circuits: Use 14 AWG (15A max) or 12 AWG (20A max).
• Electric Dryer / Range (30A-50A): Use 10 AWG (30A), 8 AWG (40A), or 6 AWG (50A).
• 100A Subpanel Feeder: Use 3 AWG Copper or 1 AWG Aluminum (not listed above, but standard practice).
Which Column Applies to Your Installation?
The most frequent mistake DIYers and junior electricians make is looking at a spool of 90°C-rated THHN wire, finding the 90°C column, and sizing the breaker based on that higher number. This violates NEC Article 110.14(C), which dictates the 'Weakest Link' rule.
Your circuit's allowable ampacity is limited by the lowest temperature rating of any connected component, including the breaker terminals, receptacles, switches, and lugs. In residential and light commercial panels, standard breakers (like Square D Homeline or Siemens QP) and standard 15A/20A duplex receptacles are typically rated for 60°C or 75°C. Very few residential terminals are actually rated for 90°C.
The Rule for Final Sizing: You must use the 60°C column for circuits rated 100A or less (unless the equipment is specifically marked for 75°C). For circuits over 100A, you use the 75°C column. Therefore, even though 12 AWG THHN has a 90°C ampacity of 30A, you must protect it with a 20A breaker because the 60°C column limits 12 AWG copper to 20A. The 90°C column is essentially a 'bonus' pool of thermal capacity that you are only allowed to use for applying derating factors, not for final breaker sizing.
How Derating Modifies the Base Ampacity
The baseline numbers in the ampacity of cable table assume ideal conditions: a cool 30°C room and plenty of space for heat to dissipate. Real-world jobsites rarely match this. Derating reduces the base ampacity to account for excess heat trapped in the wire. There are two primary derating triggers:
- Ambient Temperature Correction: If your conduit runs through a hot attic (e.g., 45°C / 113°F), the wire cannot dissipate heat as efficiently. You must multiply the base ampacity by a correction factor (for 90°C wire at 45°C, the factor is 0.87).
- Bundling (Conductor Count): When you pull more than three current-carrying conductors through a single conduit or cable, they heat each other up. NEC Table 310.15(C)(1) requires you to reduce the ampacity. For 4 to 6 conductors, multiply by 80%. For 7 to 9 conductors, multiply by 70%.
Worked Numeric Example: You are running a multi-wire branch circuit (MWBC) through a conduit that contains four current-carrying 12 AWG THHN conductors.
Step 1: Find the base 90°C ampacity for 12 AWG (30A).
Step 2: Apply the 80% derating factor for four conductors (30A × 0.80 = 24A).
Step 3: Compare this derated value (24A) to the terminal temperature limit from the 60°C column (20A).
Result: You must use the lower of the two values. Your final allowable ampacity is 20A, meaning you must still protect this wire with a 20A breaker. If you had six 10 AWG THHN wires in that same conduit, the 90°C base is 40A. Derated at 80%, it becomes 32A. The 60°C column limit is 30A. Your final allowable ampacity is 30A.
What the Ampacity Table Cannot Tell You
While Table 310.16 is the bible for preventing wire insulation from melting, it is not a complete engineering guide. Relying solely on the ampacity of cable table will leave you blind to three critical installation failures:
1. Voltage Drop: Ampacity only addresses heat. It does not account for the resistance of the copper over long distances. If you run 12 AWG wire 150 feet to a 15A space heater, the wire will not overheat (it is within its 20A ampacity limit), but the voltage at the receptacle will drop below 114V, causing the heater to underperform and potentially damaging the motor. Industry best practice (and NEC informational notes) recommend sizing wire to keep voltage drop under 3% for branch circuits. For long runs, you must use voltage drop calculators and often upsize the wire by one or two AWG sizes beyond what the ampacity table demands.
2. Conduit Fill Limits: The table tells you how many wires you can bundle before derating kicks in, but it does not tell you if they will physically fit inside the pipe. NEC Chapter 9, Table 1 limits conduit fill to 40% for three or more wires. Cramming four 6 AWG THHN wires into a 1/2-inch EMT conduit will result in jammed wires, stripped insulation, and a failed inspection, even if the derating math technically checks out. Always cross-reference your wire count with a conduit fill chart (like those provided by Southwire's support tools) before pulling.
3. Short-Circuit Withstand (Let-Through Current): Ampacity assumes continuous, steady-state loading. It does not guarantee the wire can survive the massive thermal and magnetic forces of a dead short before the breaker trips. For standard residential branch circuits, the breaker trips fast enough that this is rarely an issue. However, in industrial settings or service entrance feeders with high available fault currents, engineers must verify that the wire's cross-sectional area can withstand the let-through current of the specific protective device used.
