Electrical Wire Size Charts: NEC Ampacity Table & Quick Sizing Guide

For standard residential branch circuits using copper wire, the baseline rule is simple: use 14 AWG for 15A, 12 AWG for 20A, 10 AWG for 30A, 8 AWG for 40A, and 6 AWG for 50A/60A. However, picking the right wire isn't just about matching the breaker size. If you ignore temperature ratings, conduit fill, and run length, you risk tripped breakers, melted insulation, or a failed inspection.

This guide provides the exact electrical wire size charts sourced directly from the National Electrical Code (NEC), explains how to read the temperature columns, and gives you a hard decision tree to pick your exact wire and breaker for any standard project.

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

The master reference for wire sizing in the US is NFPA 70 (NEC) Table 310.16. When you look at this table, you will see three distinct temperature columns for copper: 60°C, 75°C, and 90°C. Choosing the wrong column is the most common mistake DIYers make.

Here is how to determine which column applies to your installation:

• Use the 60°C Column if: You are using NM-B (Romex) cable, UF-B underground cable, or working with older equipment rated only for 60°C. Furthermore, NEC 240.4(D) strictly mandates that small conductors (14, 12, and 10 AWG) must be protected at their 60°C ampacity limits (15A, 20A, and 30A respectively), regardless of the insulation's actual 90°C rating.
• Use the 75°C Column if: You are pulling individual THHN/THWN-2 wires through conduit (EMT, PVC) and terminating on modern 75°C-rated breakers and lugs for sizes 8 AWG and larger.
• Use the 90°C Column if: You are calculating derating factors (explained below). You start with the 90°C base ampacity, apply your derating math, and then verify the final number doesn't exceed the 60°C or 75°C termination limits.

The Master Electrical Wire Size Chart (NEC Table 310.16)

Below is the complete ampacity chart for copper conductors rated 0-2000V, based on an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors in a raceway. Source: NFPA 70 (NEC) Table 310.16.

Note: For standard residential breakers, you must use the next standard size up if your calculated load falls between standard breaker sizes (NEC 240.4(B)), provided the wire ampacity is not exceeded. For example, a 6 AWG wire at 75°C (65A) can be protected by a 70A breaker.

Derating: When the Chart Lies to You

The chart above assumes two things: your attic or garage is exactly 86°F (30°C), and you have no more than three current-carrying wires bundled together in a conduit. In the real world, both assumptions frequently fail. When they do, you must apply derating factors to the 90°C column.

Example: Bundling in Conduit
Imagine you are pulling four current-carrying THHN conductors (two hots, a neutral, and a traveler for a 3-way switch setup) through a single 1/2-inch EMT conduit. According to NEC Chapter 9, Note 10 to Table 310.15(C)(1), 4 to 6 current-carrying conductors require an 80% derating factor.

1. Base Ampacity: You are using 10 AWG THHN. The 90°C column lists 40A.
2. Apply Derating: 40A × 0.80 = 32A.
3. Check Terminations: Your 10 AWG wire terminates on a standard receptacle and breaker. NEC 110.14(C) and 240.4(D) force you to cap this at the 60°C column limit, which is 30A.
4. Final Verdict: Your 10 AWG wire is now legally limited to 30A. If your calculated continuous load is 28A, you are safe. If it's 32A, you must upsize to 8 AWG.

Decision Tree: Picking Your Exact Wire and Breaker

Stop guessing. Follow this decision path to lock in your exact materials for the most common residential and workshop circuits.

Default Recommendation: If your project does not perfectly match these rows, calculate your continuous load (multiply by 1.25), find the resulting amperage in the 75°C column (for conduit) or 60°C column (for NM-B), and select the next standard breaker size up.

What This Chart Cannot Tell You (Voltage Drop)

The NEC ampacity table tells you what size wire will prevent a fire. It does not tell you what size wire will actually deliver usable voltage to your device at the end of a long run. If you run a 50A circuit to a detached garage 150 feet away using 6 AWG wire, the wire won't melt, but your power tools will bog down and your EV charger may fault out due to low voltage.

The industry standard (NEC Informational Note 210.19(A)(1)) recommends keeping voltage drop under 3% for branch circuits and 5% overall from the utility transformer to the furthest outlet.

The Voltage Drop Reality Check:
Suppose you are wiring a 120V, 20A receptacle at the end of a 150-foot trench to a shed.

• If you use 12 AWG (the code minimum for 20A), the voltage drop will be roughly 11.8V (nearly a 10% drop). Your 120V outlet will only deliver 108V under full load.
• If you upsize to 8 AWG, the drop falls to 4.6V (3.8%). Still too high for sensitive electronics.
• If you upsize to 6 AWG, the drop is 2.9V (2.4%). This is under the 3% threshold and is the correct pick for this specific run.

Whenever your one-way wire run exceeds 75 feet on a 120V circuit, or 150 feet on a 240V circuit, stop looking at the ampacity chart and start calculating voltage drop. The chart gets you in the door; voltage drop math gets the job done right.