Electrical Wire Amperage Chart: NEC Table 310.16 Sizing Guide

If you are sizing a branch circuit, feeder, or service entrance, the single most important reference in the National Electrical Code (NEC) is the electrical wire amperage chart found in Table 310.16 (formerly 310.15(B)(16)). This chart dictates the maximum allowable ampacity for copper and aluminum conductors before the insulation melts or the breaker fails to protect the wire.

However, simply reading the highest number on the chart is a fast track to a failed inspection or a melted terminal lug. The values in the chart assume specific baseline conditions: copper conductors, an ambient temperature of 30°C (86°F), and no more than three current-carrying conductors bundled in a raceway. Deviate from any of those, and the base numbers change.

The Master Electrical Wire Amperage Chart (NEC Table 310.16)

Before looking at the numbers, you need to know how to read the columns. The NEC separates ampacity by insulation temperature rating: 60°C, 75°C, and 90°C. The table below focuses on copper conductors, which are standard for 99% of residential and light commercial branch circuits.

How to read this table: Locate your wire gauge (AWG or kcmil) on the left. Read across to the column that matches your installation's terminal temperature rating (more on this below). The intersection is your base ampacity before any derating factors are applied.

Bookmark Quick-Jump: Most Queried Residential Sizes

• 14 AWG: 15A (60°C column limit)
• 12 AWG: 20A (60°C column limit)
• 10 AWG: 30A (60°C column limit)
• 8 AWG: 40A (60°C) / 50A (75°C)
• 6 AWG: 55A (60°C) / 65A (75°C)
• 4 AWG: 70A (60°C) / 85A (75°C)

Which Temperature Column Actually Applies to Your Install?

The most common mistake DIYers and apprentice electricians make is using the 90°C column because modern THHN/THWN-2 wire is rated for 90°C. In almost all residential scenarios, you cannot use the 90°C column for your final ampacity.

This restriction comes from NEC Section 110.14(C), which governs terminal temperature limitations. The rule states that the ampacity of the wire cannot exceed the temperature rating of the equipment terminals it connects to.

• The 60°C Column: Mandatory for circuits rated 100A or less, or for wire sizes 14 AWG through 1 AWG, unless the equipment is specifically marked otherwise. This is why 12 AWG wire is strictly limited to 20A, even though the 90°C column says 30A.
• The 75°C Column: Used for circuits over 100A, or wire sizes larger than 1 AWG. Most modern breakers, lugs, and disconnects are rated for 75°C, allowing you to use this column for larger feeders (e.g., sizing a 6 AWG copper feeder at 65A instead of 55A).
• The 90°C Column: Used almost exclusively as a starting point for derating calculations (explained below) or for specific high-temp industrial terminations.

Derating Factors: When the Chart Lies to You

The base values in the electrical wire amperage chart assume ideal conditions. When real-world conditions deviate, you must apply derating factors that reduce the wire's allowable ampacity. There are two primary derating triggers:

1. Ambient Temperature Correction

If your wire runs through an environment hotter than 30°C (86°F)—such as an uninsulated attic in the summer or near a boiler—the wire cannot dissipate heat as effectively. You must multiply the base ampacity by a correction factor found in NEC Table 310.15(B)(1).

Example: You are running 10 AWG THHN (90°C rated, base 40A) through an attic that reaches 50°C (122°F). The correction factor for 90°C wire at 50°C ambient is 0.82.
Calculation: 40A × 0.82 = 32.8A. The wire is now only good for 32.8 amps.

2. Bundling (More Than 3 Current-Carrying Conductors)

When you pull more than three current-carrying conductors through a single conduit, the wires heat each other up. NEC Table 310.15(C)(1) requires you to reduce the ampacity based on the number of wires. Note that grounded (neutral) conductors carrying only unbalanced current do not count, but neutrals in 3-phase or multi-wire branch circuits feeding non-linear loads often do.

• 4-6 conductors: Multiply base ampacity by 80%
• 7-9 conductors: Multiply base ampacity by 70%
• 10-20 conductors: Multiply base ampacity by 50%

What This Amperage Chart Cannot Tell You

While NEC Table 310.16 is the bible for wire heating limits, it does not account for system performance or specific load rules. Relying solely on this chart will leave blind spots in your design.

Voltage Drop Over Distance

A 12 AWG wire might be legally allowed to carry 20A over a 200-foot run according to the amperage chart, but at 120V, that run will experience a severe voltage drop (over 6%). The NEC recommends (via Informational Notes in NFPA 70 Article 310.15(B)) a maximum 3% voltage drop on branch circuits and 5% total for feeder and branch combined to ensure equipment operates efficiently. For long runs, you must size up the wire gauge purely for voltage drop, even if the ampacity chart says the smaller wire is safe from a thermal perspective.

Continuous Load Requirements

The chart tells you the wire's thermal limit, but it doesn't know what you are plugging in. Under NEC Article 210.20(A), if a load is expected to run for 3 hours or more (like a commercial EV charger, a hardwired heater, or continuous lighting), the circuit must be rated at 125% of the continuous load. If your continuous load is 16A, you cannot use a 20A breaker and 12 AWG wire; you must multiply 16A × 1.25 = 20A, meaning you need a 25A or 30A circuit (requiring 10 AWG wire) to remain code-compliant.

Local AHJ Overrides and Specific Insulation Types

Finally, the chart assumes you are using standard building wire (THHN, XHHW). It does not apply to specialized cables like NM-B (Romex), which is strictly limited to the 60°C column regardless of the internal wire's actual insulation rating, per NEC 334.80. Always verify the exact cable type and consult your local inspector, as regional amendments frequently override baseline NEC tables.