For standard residential branch circuits using copper wire, the baseline ampacities are: 14 AWG is 15A, 12 AWG is 20A, 10 AWG is 30A, 8 AWG is 40A, and 6 AWG is 55A. These values come directly from the 60°C column of NEC Table 310.16, which governs nonmetallic-sheathed cable (NM-B/Romex). However, pulling a single number from a wire gauges chart without understanding the temperature columns and derating factors is the most common way DIYers end up with melted terminals or tripped breakers.

How to Read the NEC Wire Gauges Chart

The definitive wire gauges chart for North American electrical installations is NEC Table 310.16 (formerly 310.15(B)(16)). This table lists the allowable ampacities of insulated conductors rated up to 2000 volts. To read it correctly, you must cross-reference the American Wire Gauge (AWG) size on the left with the specific temperature rating of your wire's insulation on the top.

Bookmark Quick-Jump: The most queried residential sizes are 14 AWG (15A lighting circuits), 12 AWG (20A general receptacles), 10 AWG (30A dryers and water heaters), and 6 AWG (55A/60A ranges and subpanel feeders).

Below is the data-dense extraction for copper conductors, including the maximum standard overcurrent protection (breaker) size permitted under NEC 240.4.

NEC Table 310.16 Copper Wire Gauges Chart (Source: NFPA 70, 2023 Edition)
AWG Size 60°C Column (NM-B) 75°C Column (Terminations) 90°C Column (THHN Derating) Max Standard Breaker
14 AWG15A20A25A15A *
12 AWG20A25A30A20A *
10 AWG30A35A40A30A *
8 AWG40A50A55A40A
6 AWG55A65A75A60A
4 AWG70A85A95A80A
3 AWG85A100A110A100A
2 AWG95A115A130A125A

* Note: NEC 240.4(D) strictly limits small conductors (14, 12, and 10 AWG) to 15A, 20A, and 30A breakers respectively, regardless of the 75°C or 90°C column values.

Which Temperature Column Applies to Your Installation

The most frequent mistake made when reading a wire gauges chart is picking the highest number in the row. You cannot simply use the 90°C column because the wire jacket says 'THHN'. Under NEC 110.14(C), the allowable ampacity is dictated by the lowest temperature rating of any connected component, splice, or termination in the circuit.

  • The 60°C Column: This applies if you are using NM-B (Romex) cable, UF-B underground feeder, or if you are terminating into older devices and panels not explicitly marked with a higher temperature rating. Because the PVC jacket of NM-B cannot dissipate heat as effectively as individual wires in open air, the NEC restricts its final ampacity to the 60°C column.
  • The 75°C Column: This applies to most modern THHN/THWN-2 wires pulled through conduit when terminating into modern breakers, lugs, and receptacles. Almost all equipment manufactured after 1985 and rated 100A or less is tested and marked for 75°C terminations. If you pull 8 AWG THHN to a 50A breaker, you use the 50A value from the 75°C column.
  • The 90°C Column: This column is almost never used for final ampacity. Its primary purpose is to provide a higher baseline for derating calculations before you apply the termination limit.

For a deeper understanding of termination rules, refer to the EC&M guide on sizing conductors and the official NFPA NEC hub.

How Derating and Bundling Modify Base Ampacity

The base values in the wire gauges chart assume an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors bundled together in a single raceway or cable. When you exceed these conditions, the wire cannot shed heat efficiently, and you must derate the ampacity.

Derating modifies the base value by applying a percentage multiplier from NEC Table 310.15(C)(1). Crucially, you apply this multiplier to the 90°C column, not the 60°C or 75°C column. After derating, you compare the result to the termination column (usually 75°C) and use the lower of the two numbers.

Worked Derating Example:
Imagine you are pulling four separate 12 AWG THHN circuits (8 current-carrying conductors total) through a single 1-inch EMT conduit to a subpanel.
  1. Base 90°C ampacity for 12 AWG is 30A.
  2. NEC Table 310.15(C)(1) states that 7-9 conductors require an 80% derating factor (actually 70% for 7-9, let's use 4-6 conductors for 80% to be precise: 4 circuits = 8 wires. 8 wires = 70% factor).
  3. 30A x 0.70 = 21A.
  4. Compare 21A to the 75°C termination limit (25A). The derated value (21A) is lower.
  5. Because 21A is greater than the 20A breaker limit for 12 AWG under 240.4(D), you can still use a 20A breaker, but you have lost your safety margin. If you added a ninth wire, you would drop to 60% (18A), forcing you to upsize to 10 AWG wire.

Always count the neutral wire as a current-carrying conductor if it carries unbalanced current (like in a multi-wire branch circuit), but do not count a bare equipment grounding conductor.

What the Wire Gauges Chart Cannot Tell You

While the wire gauges chart is the foundation of circuit design, it only addresses thermal limits under ideal conditions. It completely ignores three critical real-world factors that will ruin an installation if overlooked:

1. Voltage Drop Over Distance

The chart assumes the wire is short enough that resistance is negligible. If you are running a 120V circuit to a detached garage 150 feet away, a 12 AWG wire carrying 15A will experience a voltage drop of roughly 5.8%. This exceeds the NEC's recommended 3% limit for branch circuits, causing motors to run hot and lights to dim. For long runs, you must upsize the wire (e.g., to 10 AWG or 8 AWG) strictly to mitigate voltage drop, even if the breaker size remains 20A.

2. Conduit Fill Capacity

The chart tells you how much current a wire can carry, but not how many wires physically fit inside a pipe. NEC Chapter 9, Table 1 limits conduit fill to 40% for three or more wires. You can easily fit three 6 AWG THHN wires in a 3/4-inch conduit, but trying to pull four will result in jammed wires, damaged insulation, and a failed inspection. Always cross-reference your wire gauge with a conduit fill calculator.

3. Physical Termination Limits

A wire gauges chart might tell you that 4 AWG copper is rated for 85A at 75°C, allowing you to protect it with an 80A breaker. However, if the physical lug on your 80A breaker is only manufactured to accept a maximum of 6 AWG wire, you cannot physically terminate the 4 AWG without pigtailing it down—a practice that introduces unnecessary resistance and failure points. Always check the manufacturer's datasheet for the physical wire range of the specific breaker or lug you are using.