For standard residential branch circuits, the baseline rule of thumb is simple: 14 AWG copper handles 15 amps, 12 AWG handles 20 amps, and 10 AWG handles 30 amps. However, these numbers are derived specifically from the 60°C column of the National Electrical Code (NEC) Table 310.16. If you are pulling wire through conduit, terminating at modern breakers, or running long feeder lines, relying solely on these three numbers will lead to oversized breakers, tripped thermal protections, or failed inspections. This guide provides the complete wire size and amp chart, explains how to read the temperature columns, and details the derating math required for real-world installations.

SAFETY WARNING: Any work involving mains voltage (>50V AC) requires de-energizing the circuit, locking out the breaker, and verifying the wires are dead with a tested non-contact voltage tester or multimeter. Local codes may require a licensed electrician for panel and feeder work.

How to Read the NEC Wire Size and Amp Chart

The definitive source for conductor ampacity in the United States is NEC Table 310.16 (formerly 310.15(B)(16)), published by the National Fire Protection Association. This table assumes an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors bundled together.

How to read this table: The rows represent the American Wire Gauge (AWG) or circular mil (kcmil) size. The columns are split by conductor material (Copper vs. Aluminum/Copper-Clad) and by the insulation's maximum temperature rating (60°C, 75°C, and 90°C). Always match the column to the lowest temperature rating of any component in your circuit, not just the wire's printed jacket.

Bookmark-Friendly Quick Jumps (Most Queried Residential Values):

  • 15 Amp Circuit: 14 AWG Copper (60°C col) / 12 AWG Aluminum (75°C col)
  • 20 Amp Circuit: 12 AWG Copper (60°C col) / 10 AWG Aluminum (75°C col)
  • 30 Amp Circuit: 10 AWG Copper (60°C col) / 8 AWG Aluminum (75°C col)
  • 50 Amp Circuit: 6 AWG Copper (75°C col) / 4 AWG Aluminum (75°C col)
  • 100 Amp Subpanel: 3 AWG Copper (75°C col) / 1 AWG Aluminum (75°C col)
Table 1: Allowable Ampacities of Insulated Conductors (Source: NEC Table 310.16, 30°C Ambient)
Size (AWG/kcmil) Copper Aluminum / Copper-Clad
60°C (140°F) 75°C (167°F) 90°C (194°F) 60°C (140°F) 75°C (167°F) 90°C (194°F)
1415*2025---
1220*2530152025
1030*3540253035
8405055304045
6556575405060
4708595556575
385100110657585
2951151307590100
111013014585100115
1/0125150170100120135
2/0145175195115135150
3/0165200225130155175
4/0195230260150180205
* The Small Conductor Rule (NEC 240.4(D)): Notice the asterisks on 14, 12, and 10 AWG copper. Even though the 75°C column allows 12 AWG to carry 25A, NEC 240.4(D) strictly limits overcurrent protection for these small conductors to 15A, 20A, and 30A respectively, regardless of the insulation rating.

Which Temperature Column Applies to Your Installation

The most common mistake DIYers make is looking at the wire jacket (e.g., THHN, which is rated 90°C) and sizing the breaker using the 90°C column. This violates NEC 110.14(C), which mandates the 'weakest link' rule. Your final ampacity is limited by the lowest temperature rating of any device, terminal, or splice in the circuit.

Here is how to determine which column to use for your final breaker sizing:

  • The 60°C Column: Use this for circuits rated 100 amps or less, or for conductors sized 14 AWG through 1 AWG, unless the equipment is specifically marked otherwise. Most standard residential receptacles, light switches, and older breakers fall into this category. NM-B (Romex) cable is also strictly limited to the 60°C column, regardless of the individual wire insulation inside the sheath.
  • The 75°C Column: Use this for circuits rated over 100 amps, or for conductors sized larger than 1 AWG. Modern load centers, subpanel lugs, and heavy-duty appliances (like electric ranges and dryers) typically feature 75°C rated terminals.
  • The 90°C Column: You almost never use this column for final breaker sizing. Instead, the 90°C column is used as your starting baseline when you need to apply derating factors for bundling or high ambient temperatures.

How Derating Factors Modify Your Base Ampacity

The ampacities in the wire size and amp chart above assume ideal conditions: an ambient temperature of 30°C (86°F) and a maximum of three current-carrying conductors in a raceway. When you bundle multiple circuits in a single conduit or run wire through a hot attic, the heat cannot dissipate. The NEC requires you to 'derate' the wire's ampacity to prevent the insulation from melting.

Derating is calculated using the 90°C column (for THHN/THWN-2 wire), applying the multiplier, and then checking if the result still satisfies the terminal temperature limits.

Table 2: NEC Adjustment Factors for Bundled Conductors (NEC 310.15(C)(1))
Number of Current-Carrying Conductors Derating Multiplier Practical Example (10 AWG THHN Copper)
1 - 3100%40A × 1.0 = 40A (Base value)
4 - 680%40A × 0.8 = 32A
7 - 970%40A × 0.7 = 28A
10 - 2050%40A × 0.5 = 20A
21 - 3045%40A × 0.45 = 18A

Worked Numeric Example: You are pulling four 12 AWG THHN current-carrying conductors through a single EMT conduit to feed a multi-wire branch circuit.
1. Look at the 90°C column for 12 AWG: 30A.
2. Apply the 4-6 conductor derating factor (80%): 30A × 0.80 = 24A.
3. Check terminal limits: Your breaker terminals are rated 60°C. The 60°C limit for 12 AWG is 20A.
4. The final allowable ampacity is the lower of the derated value (24A) and the terminal limit (20A). Therefore, the wire is good for a 20A breaker. If you had tried to use 14 AWG (25A at 90°C × 0.8 = 20A), you would fail because 14 AWG is hard-capped at 15A by the small conductor rule.

What This Wire Size and Amp Chart Cannot Tell You

While Table 310.16 is the bible for thermal limits, it does not account for the physics of long-distance electron flow or physical space constraints. Before finalizing your materials list, you must calculate three additional factors that the chart ignores:

1. Voltage Drop

Ampacity charts only tell you what the wire can handle before it catches fire; they do not tell you if your equipment will actually run. NEC Chapter 9 recommends keeping voltage drop under 3% for branch circuits and 5% for the total feeder-plus-branch run. For example, if you are wiring a 50-amp RV pedestal 150 feet away from your subpanel using 6 AWG copper (rated for 55A at 60°C), the wire won't melt, but the voltage drop will be roughly 4.5% under full load. Your RV's AC compressor may stall or overheat. To fix this, you must upsize to 4 AWG or 3 AWG copper, even though the ampacity chart says 6 AWG is sufficient.

2. Conduit Fill Capacity

You might calculate that you need three 4/0 AWG aluminum conductors for a 200-amp residential service. However, Chapter 9, Table 1 of the NEC limits conduit fill to 40% of the conduit's internal cross-sectional area for three or more wires. Those massive 4/0 cables will not physically fit inside a standard 1-inch PVC Schedule 80 conduit. You must consult a conduit fill chart to ensure you can actually pull the wire without stripping the insulation.

3. Short-Circuit Withstand Ratings

The ampacity chart assumes steady-state continuous loading. It does not indicate how the wire behaves during a massive, instantaneous short-circuit fault before the breaker trips. For specialized industrial or solar applications, engineers must calculate the available fault current and verify the conductor's thermal withstand rating, a metric entirely absent from standard residential ampacity tables.

Always treat the wire size and amp chart as your starting baseline. Verify your terminal temperatures, apply your derating math for bundled runs, and run a quick voltage drop calculator for any circuit exceeding 50 feet. For complex commercial or industrial feeder calculations, consult the full NEC guidelines and engineering references to ensure compliance with your local Authority Having Jurisdiction (AHJ).