When sizing wire for a circuit, the direct answer depends on your load, insulation type, and termination temperature. For standard residential copper branch circuits (NM-B / Romex) protected by standard breakers, here are the most queried baseline sizes:

  • 15 Amps: 14 AWG Copper
  • 20 Amps: 12 AWG Copper
  • 30 Amps: 10 AWG Copper
  • 40 Amps: 8 AWG Copper
  • 50 Amps: 6 AWG Copper
Bench Rule of Thumb: Never size wire solely by the breaker rating. Size the wire for the load, then size the breaker to protect the wire. The chart below provides the legal ampacity limits that dictate this relationship.

How to Read This Cable Size and Amps Chart

The most common mistake DIYers and junior electricians make is reading the wrong temperature column. This chart is derived directly from NFPA 70 (National Electrical Code) Table 310.16. To use it correctly, you must understand which column applies to your specific installation:

  • 60°C Column: Use this for residential branch circuits rated 100A or less, or when using NM-B (Romex) cable. Even if the wire insulation is rated 90°C, NEC 110.14(C) forces you to use the 60°C column because standard residential receptacles and breakers are only tested and rated for 60°C terminations.
  • 75°C Column: Use this for circuits over 100A, feeders, and commercial installations where the terminals on both ends (breaker and lug) are explicitly marked for 75°C. THHN/THWN-2 wire in conduit often utilizes this column for feeder calculations.
  • 90°C Column: Never use this column for final ampacity. The 90°C column exists almost exclusively for calculating derating factors (ambient temperature and conduit bundling). You start your derating math here, but your final adjusted ampacity cannot exceed the 60°C or 75°C termination limits.

The Master Cable Size and Amps Chart (NEC Table 310.16)

The following table covers the most common copper and aluminum wire sizes used in residential and light commercial work. Values assume an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors in a raceway.

AWG / kcmil Copper 60°C (NM-B) Copper 75°C (THHN Feeders) Copper 90°C (Derating Base) Aluminum 75°C
1415A-25A-
1220A25A30A-
1030A35A40A-
840A50A55A40A
655A65A75A50A
470A85A95A65A
385A100A110A75A
295A115A130A90A
1110A130A145A100A
1/0125A150A170A120A
2/0145A175A195A135A
3/0165A200A225A155A
4/0195A230A260A180A

Source: Adapted from NFPA 70 (NEC) Table 310.16. For comprehensive wire specifications, refer to the Southwire Ampacity Chart or manufacturer datasheets.

Derating Factors: When the Chart Lies

The ampacities listed above assume ideal conditions: an ambient temperature of 30°C (86°F) and a maximum of three current-carrying conductors bundled together. When you deviate from these conditions, the physical insulation heats up faster than it can dissipate heat, requiring you to reduce (derate) the allowable current.

Conduit Bundling (More than 3 Conductors)

If you pull 4 to 6 current-carrying conductors through a single conduit, you must multiply the base ampacity by 80%. For 7 to 9 conductors, multiply by 70%. Note that a neutral wire carrying only the unbalanced load from a standard 120/240V split-phase circuit does not count as a current-carrying conductor, but a neutral on a 3-phase wye circuit with non-linear loads (like LED drivers or computers) does.

Worked Example: The 12 AWG THHN Trap

Suppose you pull four current-carrying 12 AWG THHN wires through a conduit to feed two 20A circuits.
Step 1: Look at the 90°C column for derating. 12 AWG at 90°C is 30A.
Step 2: Apply the 80% bundling factor. 30A × 0.80 = 24A.
Step 3: Check termination limits. Your breaker and receptacles are rated 60°C. The 60°C limit for 12 AWG is 20A.
Result: You must use the lowest value between the derated 90°C calculation (24A) and the termination limit (20A). Your maximum breaker size remains 20A. The 90°C insulation bought you enough thermal headroom to survive the bundling without forcing you to upsize to 10 AWG.

Decision Tree: Picking Your Exact Wire Size

Use this logical sequence to terminate your wire sizing process with a single, concrete part number. We will use a real-world scenario: Sizing a feeder for a 40A continuous EV charger installed in a garage where the conduit runs through an attic reaching 110°F (43°C).

Step Action & Calculation Result for EV Charger Scenario
1. Base Load Identify the maximum continuous amperage of the load. 40 Amps
2. Continuous Factor Multiply continuous loads (on for 3+ hours) by 125% per NEC 210.20(A). 40A × 1.25 = 50A minimum circuit ampacity
3. Termination Column Select the 75°C column (assuming standard 75°C rated lugs on the EV charger and subpanel). Look for a value ≥ 50A. 8 AWG Copper is 50A.
4. Ambient Derating Attic is 43°C. Use the 90°C column and the NEC Table 310.15(B)(1) correction factor for 41-45°C (0.87). 8 AWG at 90°C is 55A.
55A × 0.87 = 47.85A
5. Final Verification Does the derated ampacity (47.85A) meet the minimum required circuit ampacity (50A)? NO. 47.85A is less than 50A. We must upsize.
6. Upsize & Repeat Move to the next wire size (6 AWG) and re-run Step 3 and 4. 6 AWG at 75°C = 65A (Passes 50A req).
6 AWG at 90°C (75A) × 0.87 = 65.25A (Passes).
Concrete Pick: For this 40A continuous EV charger in a hot attic, buy 6 AWG Copper THHN/THWN-2. Do not use 8 AWG, even though the base 75°C chart says it is good for 50A; the ambient heat derating will cause the insulation to degrade prematurely and trip the breaker under sustained load.

What This Chart Cannot Tell You

While NEC Table 310.16 dictates the thermal limits of wire insulation, it ignores physics that affect power delivery over distance. Keep these three blind spots in mind before pulling wire:

  1. Voltage Drop: The chart assumes the wire is infinitely short. On a 120V circuit, a 50-foot run of 14 AWG carrying 15A will drop roughly 3.8 volts (3.1%). If your run exceeds 100 feet, or you are powering sensitive electronics or motors, you must calculate voltage drop. Default action: Bump up one AWG size for every 50 feet beyond the standard 50-foot baseline to maintain a drop under 3%.
  2. Short-Circuit Let-Through Current: Ampacity charts measure sustained heat. They do not tell you if the wire can survive the violent magnetic and thermal forces of a 10,000A short circuit for the 2 milliseconds it takes the breaker to trip. For standard residential breakers, the chart sizes are inherently safe, but in industrial settings with high available fault currents, you must verify short-circuit withstand ratings.
  3. Local AHJ Amendments: The National Electrical Code is a baseline standard. Your local Authority Having Jurisdiction (AHJ) may have strict amendments. For example, some municipalities mandate a minimum of 12 AWG for all 15A and 20A residential branch circuits, effectively banning 14 AWG entirely to reduce fire risk and voltage drop. Always verify with your local building department.

When in doubt, copper costs more upfront but saves money in labor and longevity. If your calculation lands exactly on the boundary between two wire sizes, the definitive best practice is to select the larger AWG (smaller number). It runs cooler, drops less voltage, and provides headroom for future load additions.