The Complete Electrical Wire Chart for Amperage: NEC Ampacity & Derating Guide

If you need the direct answer for standard residential branch circuits using copper wire: 14 AWG is rated for 15 amps, 12 AWG for 20 amps, 10 AWG for 30 amps, and 8 AWG for 40 amps. These values assume a standard 60°C temperature column limit, which applies to most residential devices and NM-B (Romex) cable assemblies.

However, pulling individual THHN conductors in conduit for a subpanel or a 240V appliance changes the math. The definitive electrical wire chart for amperage is based on NFPA 70 (National Electrical Code) Table 310.16. Reading this chart correctly requires understanding which temperature column applies to your specific installation, how derating factors shrink your baseline ampacity, and where the table's limits end and voltage drop calculations begin.

How to Read the NEC Ampacity Chart (and Which Column Applies)

Table 310.16 is divided by conductor material (Copper vs. Aluminum) and insulation temperature rating (60°C, 75°C, and 90°C). The most common mistake DIYers and junior apprentices make is looking at the 90°C column because they bought THHN wire (which is rated for 90°C) and sizing the breaker to that higher number. This is a code violation and a fire hazard.

The Master Ampacity Data Table

The following table reproduces the core values from NEC Table 310.16 for ambient temperatures of 30°C (86°F). Source Standard: NEC 2023/2026 Table 310.16.

Which Column Applies to Your Installation?

The column you must use for sizing your breaker and wire is dictated by NEC 110.14(C), which governs terminal temperature limitations. The rule is simple: your ampacity is limited by the lowest temperature rating of any connected component in the circuit.

• The 60°C Column: You must use this column for circuits rated 100 amps or less, or for wires sized 14 AWG through 1 AWG, unless the equipment (breaker lugs, receptacles, switches) is explicitly marked and listed for 75°C. Furthermore, NM-B (Romex) cable is always limited to the 60°C column by NEC 334.80, regardless of the fact that the individual THHN conductors inside the jacket have 90°C insulation.
• The 75°C Column: This is the standard for most modern commercial equipment, subpanel feeders, and circuits rated over 100 amps (or wire sizes 1/0 AWG and larger). If you are pulling individual THHN/THWN-2 wires in conduit to a 75°C-rated breaker and a 75°C-rated lug, you can use this column.
• The 90°C Column: You almost never use this column to size a breaker. It exists primarily to give you a higher baseline number for derating calculations (explained below).

Derating Factors: When the Base Chart Value Shrinks

The ampacities in the table above assume two things: an ambient air temperature of 30°C (86°F), and no more than three current-carrying conductors bundled together in a raceway or cable. When either of those assumptions fails, the base value shrinks.

1. Conduit Bundling (Table 310.15(C)(1))

When current flows through a wire, it generates heat. If you bundle multiple wires tightly inside a conduit, they cannot dissipate that heat, which degrades the insulation over time and causes premature failure. If you have 4 to 6 current-carrying conductors in a single conduit, you must multiply the wire's base ampacity by 80%. For 7 to 9 conductors, the multiplier drops to 70%.

Pro-Tip on Counting Conductors: Equipment grounding conductors (bare copper or green) do not count toward the bundling derating limit. However, grounded (neutral) conductors that carry unbalanced current do count. In a standard 120/240V single-phase feeder, the two ungrounded (hot) wires and the neutral all count, but the ground does not.

2. Ambient Temperature Corrections (Table 310.15(B)(1))

If your conduit runs through an attic in the summer, or alongside a hot water line, the ambient temperature exceeds 30°C. For example, if the attic ambient temperature reaches 113°F (45°C), you must multiply the 90°C column ampacity by 0.87. If you are running 8 AWG THHN (55A at 90°C), the corrected ampacity becomes 47.8A. You would then apply the terminal temperature limits to finalize your breaker size.

What This Electrical Wire Chart for Amperage Cannot Tell You

While Table 310.16 is the bible for thermal limits of wire insulation, it is entirely blind to two critical real-world factors: voltage drop and continuous load dynamics. Relying solely on the ampacity chart for long runs or heavy machinery will result in poorly performing circuits.

The Voltage Drop Blindspot

Ampacity charts only tell you the maximum current a wire can carry before its insulation melts. They do not account for the resistance of the copper over distance. According to U.S. Department of Energy guidelines on wire sizing, excessive resistance causes voltage drop, which leads to dim lights, tripped motor overloads, and inefficient appliance operation.

The NEC recommends (via Informational Notes in Article 210 and 215) that branch circuits be sized to limit voltage drop to 3%, and the total feeder plus branch circuit drop to 5%. If you are running a 120V, 20A circuit to a detached garage 150 feet away, 12 AWG wire is perfectly legal per the ampacity chart, but it will suffer a 5.8% voltage drop at full load. You must upsize to 10 AWG or even 8 AWG to maintain voltage stability, even though your breaker remains 20A.

Continuous Loads and the 125% Rule

The ampacity chart assumes a varied load profile. If a load is expected to run at its maximum current for 3 hours or more, the NEC defines it as a "continuous load" (Article 100). Examples include EV chargers, server racks, commercial lighting, and baseboard heaters.

For continuous loads, you must multiply the load by 125% to size the wire and the breaker. If you are installing a 30A continuous EV charger, you cannot use 10 AWG wire on a 30A breaker. The math requires 30A × 1.25 = 37.5A. You must upsize to 8 AWG wire (rated 40A in the 60°C column) and use a 40A breaker. The ampacity chart gives you the wire's limit, but the continuous load rule dictates the operational ceiling.