The Ultimate NEC Wire Sizing Chart: Ampacity, AWG, and Breaker Matching

For standard residential branch circuits using NM-B (Romex) cable on 15A to 50A breakers, you must use the 60°C column of NEC Table 310.16. The direct matches are: 14 AWG for 15A, 12 AWG for 20A, 10 AWG for 30A, 8 AWG for 40A, and 6 AWG for 55A (protected by a 50A breaker). If you are pulling individual THHN wires in conduit, you can use the 90°C column for derating calculations, but your final breaker size is still capped by the 75°C termination rating of your panels and devices.

How to Read the NEC Wire Sizing Chart (and Which Column to Use)

The National Electrical Code (NFPA 70) publishes ampacity tables that list the maximum current a conductor can carry before its insulation degrades. The most referenced is NEC Table 310.16. When you look at this chart, you will see three primary temperature columns for copper: 60°C, 75°C, and 90°C. Choosing the right column is where most DIYers and junior apprentices make critical errors.

• 60°C Column: Use this for NM-B (Romex) cable, UF-B cable, and any circuit where the termination points (breakers, receptacles, switches) are not explicitly marked with a temperature rating. NEC 334.80 strictly limits NM-B ampacity to the 60°C column, even if the cable jacket says 90°C.
• 75°C Column: Use this for THHN/THWN wires in conduit when terminating at standard modern breakers and lugs, which are typically rated for 75°C. This is your baseline for most subpanel feeders and individual wire runs.
• 90°C Column: Use this only as a starting point for derating calculations (adjusting for ambient heat or bundling). You almost never use the 90°C ampacity as your final breaker size because standard terminals will melt or degrade at that heat.

The Master Copper Wire Sizing Chart (NEC Table 310.16)

Below is the complete ampacity chart for copper conductors, rated up to 2000 volts, in an ambient temperature of 30°C (86°F). Use the quick-jump links to find the most common residential sizes.

*Note on Breaker Sizing: NEC 240.4(B) allows you to round up to the next standard breaker size if the ampacity does not correspond to a standard breaker (e.g., 6 AWG at 55A in the 60°C column can be protected by a 60A breaker, provided the load is not continuous. For continuous loads, stick to the exact or lower breaker size).

Derating: When the Base Chart Values Change

The chart above assumes two things: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors (CCCs) bundled together. When you exceed these conditions, the wire cannot dissipate heat as efficiently, and you must apply derating factors to the 90°C column (for THHN) before checking against your termination limits.

Real-World Bundling Example

Imagine you are pulling two 20A circuits (4 current-carrying conductors total: 2 hots, 2 neutrals) through a single 3/4-inch EMT conduit using 12 AWG THHN.

1. Find Base Ampacity: 12 AWG in the 90°C column is 30A.
2. Apply Bundling Factor: NEC Table 310.15(C)(1) dictates that 4 to 6 CCCs require an 80% derating factor.
3. Calculate Derated Ampacity: 30A × 0.80 = 24A.
4. Verify Termination Limit: Your 20A breaker terminals are rated 75°C. The 75°C ampacity for 12 AWG is 25A. Since your derated 90°C value (24A) is lower than the termination limit (25A), the 24A value governs.
5. Final Pick: 24A is greater than your 20A load and breaker. 12 AWG THHN is approved for this run.

If you added a third circuit to that conduit (6 CCCs), you would still use the 80% factor. But if you added a fourth circuit (8 CCCs), the derating drops to 70%. 30A × 0.70 = 21A. You would still barely pass for a 20A breaker, but you are running hot. Upsizing to 10 AWG is the professional move here to prevent voltage drop and heat buildup.

Decision Path: Pick Your Exact Wire and Breaker

Use this decision tree to lock in your materials for standard 120V/240V residential branch circuits and feeders. Do not guess; follow the path to the concrete pick.

What This Chart Cannot Tell You

Ampacity charts only tell you what the wire can handle thermally. They do not account for physics over distance or physical space constraints. Before you buy your wire, check these two blind spots:

1. Voltage Drop Over Distance

NEC Table 310.16 does not calculate voltage drop. If you run 12 AWG wire 150 feet to a 15A receptacle at the end of your driveway, the wire won't melt (it's under the 20A thermal limit), but your tools will only see 110V instead of 120V, causing motors to overheat and fail. For any run over 100 feet, use a voltage drop calculator. The Copper Development Association recommends sizing up one AWG for every 100 feet of run to keep voltage drop under the recommended 3% threshold for branch circuits.

2. Conduit Fill Capacity

You might have the thermal ampacity to pull four 6 AWG wires in a 1/2-inch conduit, but NEC Chapter 9, Table 1 limits conduit fill to 40% for three or more wires. If you exceed the physical fill capacity, you will strip the insulation off your THHN while pulling it through the bends, creating a dead short before you even turn the breaker on. Always cross-reference your wire count with NEC Annex C tables for conduit fill limits.