The Ultimate Electrical Cable Amperage Chart: NEC 310.16 Sizing Guide

The most common wire sizes for standard residential branch circuits are 14 AWG for 15A, 12 AWG for 20A, and 10 AWG for 30A loads. These values are derived directly from the 60°C column of NEC Table 310.16 for copper conductors. If you are wiring a standard 120V/240V home circuit, start with these three sizes. For larger feeders, EV chargers, or high-temperature environments, you must consult the full chart and apply derating factors.

How to Read the Electrical Cable Amperage Chart

Before jumping to the numbers, you need to understand which column applies to your specific installation. The National Electrical Code (NEC) categorizes ampacity by the temperature rating of the wire insulation and the terminations (lugs, breakers, receptacles) it connects to.

• 60°C (140°F) Column: Use this for NM-B cable (commonly known as Romex) and any circuit rated 100A or less where the termination temperature is unknown. Most standard residential breakers and receptacles are rated for 60°C or 75°C, but NEC 110.14(C) forces you to use the 60°C column for smaller circuits as a conservative baseline.
• 75°C (167°F) Column: Use this for THHN/THWN wires in conduit when the equipment terminations are explicitly marked 75°C. This is standard for most commercial panels, subpanels, and breakers over 100A.
• 90°C (194°F) Column: Never use this column to size your final breaker. The 90°C column is used exclusively as the starting baseline for derating calculations (adjusting for high ambient heat or bundling multiple wires in a conduit). Your final adjusted ampacity cannot exceed the 60°C or 75°C termination limits.

The table below assumes copper conductors, an ambient temperature of 30°C (86°F), and no more than three current-carrying conductors in a single raceway. Aluminum requires larger gauges for the same current and is generally avoided in DIY residential branch circuits due to termination oxidation risks.

NEC Table 310.16: Copper Ampacities (Bookmark This)

This table reflects the allowable ampacities for insulated copper conductors. The most queried residential and light-commercial sizes are highlighted.

Source: Adapted from NEC 2023 Table 310.16 and the Southwire Ampacity Chart for copper conductors.

Derating Factors: When the Base Chart Fails You

The base chart assumes ideal conditions: 30°C (86°F) ambient air and a maximum of three current-carrying conductors (e.g., one hot, one neutral, one ground—where the ground doesn't count). When conditions change, the wire's ability to shed heat drops, and you must derate the ampacity.

Worked Example: 12 AWG THHN in a Hot Attic
You are running a 20A kitchen circuit through an attic that reaches 110°F (43°C). You are using 12 AWG THHN (90°C rated insulation).

1. Find Baseline: 12 AWG in the 90°C column is 30A.
2. Apply Temperature Correction: According to NEC Table 310.15(B)(1), the correction factor for 105°F–113°F is 0.87.
3. Calculate Adjusted Ampacity: 30A × 0.87 = 26.1A.
4. Apply Termination Limit: Your breaker and receptacles are rated 60°C/75°C. The 60°C limit for 12 AWG is 20A.
5. Final Verdict: 26.1A is greater than 20A, so the wire is thermally safe. However, because the termination limit is 20A, your breaker must remain 20A. You cannot upsize the breaker to 25A.

If you were bundling 6 current-carrying conductors in that same hot attic, you would apply an additional 80% bundling derating factor (26.1A × 0.80 = 20.88A). It still passes for a 20A breaker, but you are approaching the margin of safety.

Decision Path: Pick Your Exact Wire Size

Use this decision tree to select the correct copper wire gauge for standard single-phase AC applications. Follow the row that matches your load and installation type.

Concrete Default Recommendation: If you are wiring general-purpose 120V receptacles and want to future-proof your work while minimizing voltage drop, default to 12 AWG copper on a 20A breaker. The material cost difference between 14 AWG and 12 AWG is roughly $15 per 250ft roll, but the 33% increase in current capacity and reduced heat generation is well worth the investment.

What This Chart Cannot Tell You

Ampacity charts dictate thermal limits—how much current a wire can carry before its insulation melts or degrades. They do not account for the following critical engineering factors:

• Voltage Drop: NEC 310.16 does not enforce voltage drop limits (NEC 210.19 Informational Note recommends a maximum 3% drop for branch circuits). If you are running a 12 AWG wire 150 feet to a shed drawing 15A, the wire won't melt (it's under the 20A ampacity), but the voltage at the shed will drop to ~112V, which can damage compressor motors. For runs over 100 feet, always calculate voltage drop and upsize the wire by one or two gauges.
• Conduit Fill Capacity: The chart tells you the wire size, but NEC Chapter 9 dictates how many of those wires physically fit inside a conduit. For example, you can only fit three 10 AWG THHN wires in a 1/2-inch EMT conduit. If you need a neutral and ground, you must upsize to 3/4-inch conduit.
• Short-Circuit Withstand Rating: Ampacity is for continuous, steady-state load. During a dead short, thousands of amps flow for milliseconds. The breaker must trip before the magnetic forces and extreme heat vaporize the conductor. Ensure your breaker's AIC (Ampere Interrupting Capacity) rating matches your panel's available fault current.

Always treat the electrical cable amperage chart as the starting point for thermal safety, not the final word on system design. Verify your termination temperatures, calculate voltage drop for long runs, and consult your local Authority Having Jurisdiction (AHJ) for regional amendments to the NEC.