Ampacity Wire Size Chart: NEC Table 310.16 Reference Guide

If you are sizing a branch circuit, subpanel feeder, or solar array, the ampacity wire size chart you need is governed by the National Electrical Code (NEC) Table 310.16. For standard residential copper wiring, 14 AWG is rated for 15A, 12 AWG for 20A, 10 AWG for 30A, and 6 AWG for 55A (at the standard 60°C termination column). However, pulling raw numbers from a chart without understanding temperature ratings, termination limits, and derating factors is the fastest way to fail an inspection or melt a lug.

Below is the complete reference data for copper and aluminum conductors, followed by the exact rules for selecting the correct column and adjusting for real-world installation conditions.

The NEC Table 310.16 Ampacity Wire Size Chart (Copper & Aluminum)

How to read this table: This chart is extracted from NEC Table 310.16 (formerly 310.15(B)(16)). It lists the allowable ampacities for insulated conductors rated up to 2000 volts. The columns are divided by conductor material (Copper vs. Aluminum/Copper-Clad Aluminum) and the temperature rating of the wire insulation (60°C, 75°C, and 90°C). The values assume an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors bundled in a raceway.

Bookmark-friendly quick jumps: Standard residential branch circuits (14-10 AWG), Heavy appliance/dryer (8-6 AWG), Subpanel feeders (4-2/0 AWG).

Which Temperature Column Applies (and How Derating Modifies It)

The most common mistake DIYers and junior apprentices make is looking at a spool of 12 AWG THHN wire, seeing the 90°C rating printed on the jacket, and assuming they can put 30 amps through it. You almost always cannot. To find your actual legal ampacity, you must apply the "weakest link" rule and then apply environmental derating.

1. The Weakest Link Rule (NEC 110.14(C))

The allowable ampacity of a circuit is limited by the lowest temperature rating of any connected device, termination, or conductor in that circuit. Most modern residential breakers, receptacles, and switches are rated for 75°C terminations. However, NEC 240.4(D) imposes a hard, non-negotiable cap on small conductors for standard overcurrent protection:

For wire sizes 8 AWG and larger, you default to the 75°C column because standard residential lugs and breakers are rated for 75°C. You only use the 90°C column for one specific purpose: derating calculations.

2. Ambient Temperature and Bundling Derating (NEC 310.15)

Table 310.16 assumes your wire is sitting in a room that is exactly 86°F (30°C) and that there are no more than three current-carrying conductors in the conduit. If you violate either condition, the wire cannot dissipate heat as efficiently, and you must multiply the base ampacity by a correction factor.

Worked Example: Subpanel Feeder in a Hot Attic
You are running a 100A subpanel feeder using 2 AWG THHN Copper through an attic that reaches 110°F (43°C) in the summer.

1. Base Ampacity: Look at the 90°C column for 2 AWG Copper. The base value is 130A.
2. Temperature Correction: According to the ambient temperature correction factors at the bottom of Table 310.16, a 90°C conductor at 110°F (43°C) gets a multiplier of 0.87.
3. Derated Ampacity: 130A × 0.87 = 113.1A.
4. Final Sizing Check: Your derated ampacity (113.1A) is still greater than your breaker size (100A), and the 75°C column base value (115A) is also greater than 100A. Therefore, 2 AWG THHN Copper is legal and safe for this 100A feeder.

If you had four current-carrying conductors in that same conduit (e.g., two multi-wire branch circuits sharing a neutral), you would apply an additional 80% bundling derating factor to the 90°C base value before checking it against your breaker size.

What the Ampacity Wire Size Chart Cannot Tell You

While the ampacity wire size chart dictates the thermal limits of the wire insulation and prevents fires, it completely ignores three critical real-world engineering constraints. If you size wire solely based on Table 310.16, you may end up with a system that is code-compliant but functionally useless.

Voltage Drop (The Hidden Performance Killer)

Table 310.16 does not account for distance. A 12 AWG copper wire on a 15A breaker is perfectly legal for ampacity whether the run is 10 feet or 400 feet. However, a 400-foot run of 12 AWG carrying 15A will experience a voltage drop of roughly 25 volts (over 20%). Your tools will bog down, LED lights will flicker, and motors will overheat and burn out.

Industry best practice (and NEC informational notes) recommends sizing wire to keep voltage drop under 3% for branch circuits and 5% total from the utility transformer to the furthest outlet. For long runs, you must use a voltage drop calculator and upsizing the wire by 1 to 3 AWG sizes beyond what the ampacity chart demands.

Physical Termination Limits

Ampacity charts assume the wire physically fits into the device. If you calculate that a 250 kcmil aluminum feeder is perfect for your 200A main breaker based on ampacity and derating, but the breaker's mechanical lugs are only rated to accept a maximum of 4/0 AWG, you have a physical incompatibility. Always check the manufacturer's spec sheet for the specific lug wire range (e.g., "#6 - 350 kcmil") before purchasing heavy-gauge feeders.

Short-Circuit Withstand Rating

Ampacity measures the wire's ability to carry continuous, normal operating current without melting the insulation. It does not measure the wire's ability to survive the massive magnetic and thermal forces of a dead short before the breaker trips. In high-fault-current environments (like main service entrances near large utility transformers), the wire must have an adequate short-circuit withstand rating, which often requires verifying the let-through current of the breaker and the cross-sectional mass of the conductor.

Use Table 310.16 as your starting baseline for thermal safety, but always verify voltage drop for long runs, physical lug capacities for heavy feeders, and NEC 240.4(D) limits for small branch circuits before pulling any wire.