The copper wire size chart dictates the maximum safe current (ampacity) a conductor can carry before its insulation degrades. For standard residential branch circuits, the baseline rules are simple: 15A circuits require 14 AWG, 20A circuits require 12 AWG, and 30A circuits require 10 AWG. However, once you move beyond basic plug-and-play branch circuits into subpanel feeders, HVAC disconnects, or bundled conduit runs, simply matching wire size to breaker size is no longer sufficient. You must navigate the temperature columns and derating factors defined by the National Electrical Code (NEC).
How to Read the NEC Copper Wire Size Chart
The foundational reference for wire sizing in the United States is NEC Table 310.16 (formerly 310.15(B)(16)). This table assumes an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a single raceway or cable.
To read the table correctly, locate your wire gauge (AWG or kcmil) in the far-left column, then read across to the appropriate temperature rating column (60°C, 75°C, or 90°C). The intersection is your maximum allowable ampacity before any correction factors are applied.
• 15A Receptacles: Look at 14 AWG (60°C column = 15A)
• 20A Kitchen/Bath: Look at 12 AWG (60°C column = 20A)
• 30A Dryer/RV: Look at 10 AWG (60°C column = 30A)
• 40A Range/EVSE: Look at 8 AWG (60°C column = 40A)
• 50A Hot Tub/Subpanel: Look at 6 AWG (60°C column = 55A Note: 6 AWG is rated 55A, but NEC 240.4(B) allows rounding up to the next standard 60A breaker if specific conditions are met, though 4 AWG is preferred for continuous 50A loads)
• 100A Subpanel Feeder: Look at 4 AWG (75°C column = 85A) or 3 AWG (75°C column = 100A)
| AWG / kcmil | 60°C (140°F) | 75°C (167°F) | 90°C (194°F) |
|---|---|---|---|
| 14 | 15 | 20 | 25 |
| 12 | 20 | 25 | 30 |
| 10 | 30 | 35 | 40 |
| 8 | 40 | 50 | 55 |
| 6 | 55 | 65 | 75 |
| 4 | 70 | 85 | 95 |
| 3 | 85 | 100 | 115 |
| 2 | 95 | 115 | 130 |
| 1 | 110 | 130 | 145 |
| 1/0 | 125 | 150 | 170 |
Source: Adapted from the NFPA 70 (National Electrical Code) Table 310.16. Always consult the latest adopted code cycle in your jurisdiction.
Which Temperature Column Applies to Your Installation?
The most common mistake DIYers and junior electricians make is looking at the 90°C column because the wire jacket (like THHN) is printed with '90°C'. This is almost always incorrect for determining final breaker size. The governing rule is NEC 110.14(C), often called the 'Weakest Link' rule. Your final allowable ampacity is limited by the lowest temperature rating of any connected component, termination, or conductor in the circuit.
The 60°C Column (The Residential Default)
You must use the 60°C column if you are using NM-B (Romex) or UF-B cable, regardless of the fact that the individual wires inside modern NM-B are technically rated for 90°C. The NEC restricts the overall cable assembly to 60°C ampacities. Furthermore, most standard residential receptacles, light switches, and breakers under 100A are only tested and listed for 60°C terminations. If your wire connects to a standard 15A or 20A duplex receptacle, the 60°C column governs.
The 75°C Column (Feeders and Panels)
You can use the 75°C column when using individual conductors in conduit (like THHN/THWN-2) or specific cable assemblies like SER or USE-2, provided the terminations are rated for 75°C. Most modern panelboard lugs, main breakers, and breakers rated 100A and above carry a 75°C termination rating. This is why a 100A subpanel feeder can legally be wired with 4 AWG copper (75°C column = 85A) or 3 AWG copper (75°C column = 100A), rather than the thicker wire the 60°C column would demand.
The 90°C Column (Derating Only)
The 90°C column is virtually never used to determine the final breaker size. Instead, it serves as the starting baseline for derating calculations. Because 90°C insulation can withstand more heat, the code allows you to use this higher number when applying penalty factors for high ambient temperatures or bundling multiple wires in a single conduit, before capping the final result at your termination limit.
Derating, Voltage Drop, and What the Chart Cannot Tell You
NEC Table 310.16 is a baseline. It assumes perfect conditions: a cool 30°C (86°F) environment and no more than three current-carrying conductors sharing a conduit. Real-world jobsites rarely match these assumptions.
How Derating Modifies the Base Value
When you pull more than three current-carrying conductors through a single raceway, they heat each other up. NEC Table 310.15(C)(1) requires you to multiply the base ampacity by a correction factor.
Worked Example: You are pulling six current-carrying conductors (two 120V circuits and one 240V circuit) through a single EMT conduit to a detached garage. You are using 10 AWG THHN copper.
- Start at 90°C: The 90°C column lists 10 AWG at 40A.
- Apply Derating: 6 conductors require an 80% derating factor. 40A × 0.80 = 32A.
- Check Termination Limit: Your breakers and lugs are rated 75°C. The 75°C column lists 10 AWG at 35A.
- Final Ampacity: Compare the derated value (32A) to the termination limit (35A). The lower number wins. Your wire is now legally rated for 32A.
- Breaker Sizing: You must protect this circuit with a standard 30A breaker.
If you had blindly used the 75°C column baseline (35A) and applied the 80% factor, you would have calculated 28A, unnecessarily forcing you to upsize to 8 AWG wire. Starting at 90°C saves material and money while remaining entirely code-compliant.
What the Chart Cannot Tell You
Ampacity charts only address thermal limits (preventing the insulation from melting). They do not address electrical efficiency or physical fit.
| Blind Spot | Why It Matters | How to Solve It |
|---|---|---|
| Voltage Drop | Over long distances, wire resistance causes voltage to sag. A 120V circuit dropping to 108V will cause motors to overheat and draw excess current, bypassing the breaker's thermal trip curve. | Keep branch circuit drop under 3%. Use the Southwire Voltage Drop Calculator to size up for runs over 50 feet. |
| Conduit Fill | You cannot physically jam too many wires into a pipe. Overfilling prevents heat dissipation and makes pulling impossible without damaging insulation. | Consult NEC Chapter 9, Table 1. Generally, conduit cross-section cannot exceed 40% fill for 3 or more wires. |
| Ambient Heat | Running wire through an uninsulated attic in Arizona where ambient temps hit 50°C (122°F) severely reduces ampacity. | Apply NEC Table 310.15(B)(1) ambient temperature correction factors to the 90°C column before checking termination limits. |
Always treat the copper wire size chart as the starting line, not the finish line. Verify your termination ratings, calculate your derating factors for bundled wires, and run a voltage drop calculation for any circuit exceeding 50 feet in length. When in doubt, upsizing the wire by one AWG step costs marginally more in copper but provides a massive margin of safety and efficiency.
