The Ultimate NEC Wire Size Chart for Copper THHN and NM-B

When sizing conductors for residential and commercial branch circuits, the direct answer for standard 15A and 20A receptacle circuits is 14 AWG and 12 AWG NM-B (Romex), respectively, referencing the 60°C ampacity column. For individual THHN wires pulled in conduit, you use the 75°C column up to the breaker's termination rating. This guide provides the exact ampacity values, derating math, and a decision path to lock in your wire and breaker size without guessing.

How to Read This Wire Size Chart (The Columns That Matter)

The most common mistake DIYers and junior electricians make is looking at the highest ampacity number on a wire's insulation jacket and using that to size the breaker. The NFPA 70 National Electrical Code (NEC) strictly forbids this. Under NEC 110.14(C), your circuit's ampacity is limited by the lowest temperature rating of any component in the circuit, including the wire insulation, the breaker lugs, and the device terminals.

Here is how to read the three primary temperature columns in the master chart below:

• 60°C Column (NM-B / Romex): Even though the individual conductors inside NM-B cable are rated for 90°C, NEC 334.80 mandates that the ampacity of NM-B cable must be determined using the 60°C column. If you are running Romex through your walls, this is your column.
• 75°C Column (THHN in Conduit): Most modern breakers and panel lugs are rated for 75°C. When you pull individual THHN/THWN-2 wires through EMT or PVC conduit and terminate them on 75°C rated lugs, you use this column for your final breaker sizing.
• 90°C Column (Derating Base): You are almost never allowed to use this column for final breaker sizing. Its sole purpose is to serve as the starting baseline when you must apply ambient temperature or conduit bundling derating factors.

The Master Wire Size Chart (NEC Table 310.16)

The following data is extracted from NEC Table 310.16 for copper conductors with an ambient temperature of 30°C (86°F). Bookmark this section for quick reference on the most queried residential and light-commercial sizes.

Derating: When the Chart Lies to You

The ampacity values in the table above assume you have no more than three current-carrying conductors in a raceway and an ambient temperature of 30°C (86°F). If you exceed these conditions, the chart's base values are no longer valid. You must apply adjustment factors from NEC Table 310.15(C)(1).

This is where the 90°C column finally earns its keep. You apply derating multipliers to the 90°C baseline, not the 75°C or 60°C columns.

Worked Numeric Example:
You are pulling four current-carrying conductors (two hots, two neutrals for two multi-wire branch circuits) through a single 3/4-inch EMT conduit using 12 AWG THHN. You want to protect them with a 20A breaker.

1. Identify Baseline: Look at the 90°C column for 12 AWG. The baseline is 30A.
2. Apply Adjustment Factor: NEC Table 310.15(C)(1) states that 4 to 6 current-carrying conductors require an 80% derating factor.
3. Calculate Derated Ampacity: 30A × 0.80 = 24A.
4. Verify Termination Limits: 24A is greater than the 20A breaker size, and it does not exceed the 75°C termination limit of 25A. The installation is code-compliant.

If you had added two more wires (totaling 6), the derating factor would drop to 80%. If you had 7 to 9 wires, the factor drops to 70% (30A × 0.70 = 21A), which still barely passes for a 20A breaker. However, at 10 wires (50% factor), 30A × 0.50 = 15A, and you would be forced to upsize to 10 AWG THHN to maintain a 20A circuit.

Decision Path: Picking Your Exact Wire and Breaker

Stop guessing and follow this decision tree to lock in your materials list. This path assumes standard residential copper wiring and typical 75°C rated breakers.

What This Chart Cannot Tell You (Voltage Drop & Exceptions)

While NEC Table 310.16 is the bible for thermal ampacity, it is blind to distance and specific appliance rules. Before you cut your wires, check these three critical blind spots.

1. Voltage Drop Over Distance

Ampacity tells you what the wire can handle without melting; it does not tell you if the voltage will drop too low at the load. The NEC recommends a maximum 3% voltage drop on branch circuits and 5% on feeders. If you are running a 12 AWG circuit 150 feet to a shed to power a 15A table saw, the wire won't melt, but the saw will stall and burn out its motor due to voltage drop. Rule of thumb: For every 100 feet of run beyond 50 feet, upsize your wire by one AWG step (e.g., use 10 AWG instead of 12 AWG for a 20A circuit at 120 feet).

2. NEC 240.4(D) Small Conductor Limits

3. Specific Appliance Code Requirements

The chart assumes generic loads. Specific appliances have their own NEC articles that dictate minimum wire sizes and maximum breaker ratings. For example, NEC 440 dictates that air conditioning disconnects and branch circuits must be sized based on the manufacturer's Minimum Circuit Ampacity (MCA) and Maximum Overcurrent Protection (MOCP) listed on the unit's data plate, which often allows a smaller wire on a larger breaker (e.g., 10 AWG on a 40A breaker for a specific HVAC condenser). Always defer to the equipment data plate and the ECMWeb NEC 110.14(C) Terminations Guide when dealing with hardwired appliances.

Disclaimer: This wire size chart reflects NEC-style guidance for standard copper conductors. Aluminum conductors require entirely different ampacity tables (NEC Table 310.16, right side). Your local Authority Having Jurisdiction (AHJ) has final authority on all code compliance determinations.