If you are sizing copper conductors for a residential or commercial branch circuit, the definitive reference is NEC Table 310.16 (formerly 310.15(B)(16)). This table dictates the maximum allowable ampacity for insulated copper and aluminum wires based on their gauge and insulation temperature rating.
For quick bookmark-friendly reference on standard residential branch circuits (assuming standard 60°C/75°C terminations and no more than 3 current-carrying conductors in a raceway), here are the baseline maximums before applying breaker limits:
- 14 AWG: 15 Amps
- 12 AWG: 20 Amps
- 10 AWG: 30 Amps
- 8 AWG: 40 Amps
- 6 AWG: 55 Amps (60°C column) / 65 Amps (75°C column)
The Master Wire Gauge Amp Table (NEC Table 310.16)
Before using the table below, you need to understand how to read the temperature columns. The numbers represent the maximum current the wire can carry continuously without the insulation degrading. Modern thermoplastic insulations like THHN and XHHW-2 are rated for 90°C (194°F). However, as explained in the next section, you rarely get to use the 90°C column for your final sizing due to termination limits.
Assumptions for this table: Copper conductors, ambient temperature of 30°C (86°F), and no more than three current-carrying conductors bundled in a single raceway or cable.
| Wire Size (AWG/kcmil) | 60°C (140°F) Column | 75°C (167°F) Column | 90°C (194°F) Column | Common Insulation Types |
|---|---|---|---|---|
| 14 AWG | 15A | — | — | TW, UF-B |
| 12 AWG | 20A | 25A | 30A | THHW, THW, THHN, XHHW |
| 10 AWG | 30A | 35A | 40A | THHW, THW, THHN, XHHW |
| 8 AWG | 40A | 50A | 55A | THHW, THW, THHN, XHHW |
| 6 AWG | 55A | 65A | 75A | THHW, THW, THHN, XHHW |
| 4 AWG | 70A | 85A | 95A | THHW, THW, THHN, XHHW |
| 3 AWG | 85A | 100A | 115A | THHW, THW, THHN, XHHW |
| 2 AWG | 95A | 115A | 130A | THHW, THW, THHN, XHHW |
| 1 AWG | 110A | 130A | 145A | THHW, THW, THHN, XHHW |
| 1/0 AWG | 125A | 150A | 170A | THHW, THW, THHN, XHHW |
| 2/0 AWG | 145A | 175A | 195A | THHW, THW, THHN, XHHW |
| 3/0 AWG | 165A | 200A | 225A | THHW, THW, THHN, XHHW |
| 4/0 AWG | 195A | 230A | 260A | THHW, THW, THHN, XHHW |
Source: NFPA 70: National Electrical Code (NEC), Table 310.16. Always verify against the specific code year adopted by your local Authority Having Jurisdiction (AHJ).
Which Temperature Column Actually Applies to Your Install?
The most common 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 number. This violates NEC 110.14(C), which enforces the "weakest link" rule for terminations.
Your wire is only as good as the device it connects to. Most modern residential circuit breakers (like Square D Homeline or Siemens QP) and standard receptacles have terminals rated for 75°C. Older equipment or specific small-device lugs may only be rated for 60°C. Therefore, even if your wire insulation can handle 90°C, the heat dissipation at the breaker terminal cannot. You must use the temperature column of the lowest-rated component in the circuit to determine your base ampacity.
How Derating Rows Modify the Base Value
Ampacity drops when wires are bundled together (trapping heat) or when the ambient temperature exceeds 30°C (86°F). This is where the 90°C column finally becomes useful. According to Electrical Contractor Magazine (ECMAG) code interpretations, you are permitted to use the 90°C column as your starting point for applying derating factors, provided the final derated ampacity does not exceed the ampacity of the termination temperature column.
Worked Derating Example:
You are pulling four 10 AWG THHN current-carrying conductors through a single EMT conduit to feed a multi-wire branch circuit.
- Find the base 90°C ampacity: Table 310.16 lists 10 AWG at 40A in the 90°C column.
- Apply the bundling derating factor: NEC Table 310.15(C)(1) states that 4 to 6 current-carrying conductors in a raceway require an 80% adjustment factor.
- Calculate derated ampacity: 40A × 0.80 = 32A.
- Check against termination limits: Your breaker terminals are rated 75°C. The 75°C column for 10 AWG is 35A. Since 32A is less than 35A, the wire is thermally safe.
What the Wire Gauge Amp Table Cannot Tell You
Table 310.16 is a thermal limit chart, not a complete system design tool. Relying on it blindly will lead to failed inspections or poorly performing circuits. Here is what the table leaves out:
- Voltage Drop Over Distance: The table assumes relatively short runs. If you are running a 240V circuit to a detached garage 150 feet away, a 6 AWG wire might be thermally rated for 55A, but the resistance over that distance will cause a voltage drop exceeding the recommended 3% limit (NEC Informative Annex B). You must upsize the wire (e.g., to 4 AWG) purely for voltage drop mitigation, even if the breaker size remains the same.
- Continuous Load Requirements: If a load will run for 3 hours or more (like an EV charger, baseboard heater, or commercial lighting), NEC 210.20(A) requires the branch circuit to be sized at 125% of the continuous load. A 32A continuous EV charger requires a circuit rated for 40A (32 × 1.25), meaning you must use 8 AWG wire and a 40A breaker, not 10 AWG.
- Wet vs. Dry Location Ratings: If your THHN wire is pulled through an underground conduit where condensation occurs, it must be treated as a wet location. The "W" in THHN does not stand for wet; you need THWN-2 or XHHW-2 for wet locations, which may alter the allowable temperature column if the specific insulation variant lacks a wet-location 90°C rating.
- Local AHJ Amendments: Some municipalities mandate minimum wire sizes regardless of NEC minimums (e.g., banning 14 AWG entirely and requiring 12 AWG minimum for all 15A and 20A lighting/receptacle circuits to reduce voltage drop and fire risk). Always check local amendments before buying wire.
Use this wire gauge amp table as your thermal baseline, apply the termination and derating rules, and then verify your final selection against voltage drop and continuous load requirements to ensure a safe, code-compliant installation.
