The standard gauge of wire chart used by electricians and inspectors in the United States is based on NEC Table 310.16. For standard 15A and 20A residential branch circuits using NM-B (Romex) cable, you must use 14 AWG and 12 AWG copper wire, respectively, referencing the 60°C temperature column. For individual THHN/THWN-2 conductors in conduit, you reference the 75°C or 90°C columns depending on your termination ratings and derating requirements.
Choosing the wrong column or ignoring adjustment factors can result in melted insulation, tripped breakers, or a failed inspection. Below is the complete reference data, followed by the specific rules for applying it to real-world installations.
How to Read the NEC Ampacity Table
Before looking at the numbers, you must understand how the National Electrical Code (NEC) structures ampacity. The chart is divided into three primary temperature columns for copper wire: 60°C, 75°C, and 90°C. These columns represent the maximum heat the wire's insulation can safely dissipate without degrading.
• 60°C Column: Applies to all NM-B (Romex) and UF-B cables, regardless of the fact that modern NM-B wire actually has 90°C rated insulation. Per NEC 334.80, the ampacity is strictly limited to the 60°C column.
• 75°C Column: Applies to most standard residential terminations (breakers, lugs, receptacles) and THHN wire connected to them, per NEC 110.14(C).
• 90°C Column: Used almost exclusively for derating calculations (adjusting for bundled wires or high ambient temperatures) and specific high-temp equipment terminations.
Bookmark-Friendly Quick Jumps
Click the common residential sizes to jump directly to their rows in the master table below:
- 14 AWG (15A Circuits / Lighting)
- 12 AWG (20A Circuits / Standard Receptacles)
- 10 AWG (30A Circuits / Dryers, Water Heaters)
- 8 AWG (40A-50A Circuits / Ranges, EV Chargers)
- 6 AWG (55A-65A Circuits / Subpanels, Hot Tubs)
- 4 AWG (70A-85A Circuits / Large Subpanels)
- 2 AWG (95A-115A Circuits / Service Feeders)
Complete Copper Wire Gauge and Ampacity Chart
The following data is extracted from NFPA 70 (National Electrical Code) Table 310.16 for copper conductors in an ambient temperature of 30°C (86°F). Aluminum values are omitted here as residential branch circuits almost exclusively use copper.
| AWG / kcmil | 60°C (NM-B / Romex) | 75°C (THHN Terminations) | 90°C (THHN Conduit / Derating) |
|---|---|---|---|
| 14 AWG | 15A | 20A | 25A |
| 12 AWG | 20A | 25A | 30A |
| 10 AWG | 30A | 35A | 40A |
| 8 AWG | 40A | 50A | 55A |
| 6 AWG | 55A | 65A | 75A |
| 4 AWG | 70A | 85A | 95A |
| 3 AWG | 85A | 100A | 110A |
| 2 AWG | 95A | 115A | 130A |
| 1 AWG | 110A | 130A | 145A |
| 1/0 AWG | 125A | 150A | 170A |
| 2/0 AWG | 145A | 175A | 195A |
| 3/0 AWG | 165A | 200A | 225A |
| 4/0 AWG | 195A | 230A | 260A |
Source: NFPA 70 National Electrical Code, Table 310.16. Cross-referenced with physical property data from the Engineering Toolbox.
When the Base Chart Fails: Derating and Edge Cases
A raw ampacity chart assumes ideal conditions: an ambient temperature of 86°F (30°C) and no more than three current-carrying conductors bundled together. When real-world conditions deviate, you must apply adjustment factors.
How Derating Modifies the Base Value
When you pull more than three current-carrying conductors through a single conduit, the wires heat each other up. NEC Article 310.15(C)(1) requires you to multiply the base ampacity by a derating factor. Crucially, you always use the 90°C column for this math.
You are pulling 5 current-carrying conductors (e.g., two 120V circuits sharing a neutral, plus a ground) through a single conduit for a 30A circuit.
1. Base ampacity for 10 AWG THHN in the 90°C column = 40A.
2. NEC adjustment factor for 4-6 conductors = 80% (0.80).
3. Derated ampacity: 40A × 0.80 = 32A.
Because 32A is still greater than your 30A breaker, 10 AWG THHN is legally compliant. If you had 7-9 conductors (70% factor), the math would be 40A × 0.70 = 28A, forcing you to upsize to 8 AWG wire.
What the Table Cannot Tell You
The gauge of wire chart is strictly a thermal limit chart. It tells you the maximum current the wire can carry before the insulation melts or degrades. It does not account for:
- Voltage Drop: If you are running a 12 AWG wire 150 feet to a detached garage for a 15A load, the wire will not overheat, but the voltage at the receptacle will drop below 114V, potentially damaging motor-driven tools. For runs over 100 feet, you must calculate voltage drop and typically upsize the wire by one or two gauges.
- Continuous Loads: If a load runs for 3 hours or more (like an EV charger or hardwired lighting), NEC 210.20 requires the branch circuit to be rated at 125% of the continuous load. A 32A continuous EV charger requires a 40A breaker and wire sized for 40A, not 32A.
- Aluminum Wire: The table above is for copper. If you are using aluminum SER cable for a subpanel feeder, you must use the separate aluminum columns in NEC 310.16, which require significantly larger wire diameters for the same ampacity.
Wire Gauge Chart FAQ
What gauge of wire chart column applies to standard residential Romex?
You must always use the 60°C column for NM-B (Romex) cable, even though the individual wires inside the jacket are technically rated for 90°C. NEC 334.80 explicitly limits NM-B ampacity to the 60°C column. Therefore, 14 AWG is capped at 15A, 12 AWG at 20A, and 10 AWG at 30A.
Can I use the 90°C column to size my breaker for THHN wire in conduit?
No. While you use the 90°C column for derating math, the final breaker size and termination ampacity are limited by the lowest-rated component in the circuit. Standard residential breakers and receptacles are rated for 75°C. Per NEC 110.14(C), you cannot exceed the 75°C column ampacity for your final overcurrent protection sizing.
Does the gauge of wire chart account for voltage drop on long runs?
No. Ampacity charts only address heat dissipation and fire safety. Voltage drop is a function of wire length, material resistivity, and current draw. As a rule of thumb, keep voltage drop under 3% for branch circuits and 5% total from the utility transformer to the furthest outlet. Use a dedicated voltage drop calculator for runs exceeding 100 feet.
How does bundling wires in a conduit change the wire gauge I need?
Bundling traps heat. If you have 4 to 6 current-carrying conductors in a conduit, you must multiply the wire's 90°C ampacity by 80%. For 7 to 9 conductors, multiply by 70%. If the resulting derated number is lower than your breaker size, you must increase the wire gauge (move to a thicker wire) until the derated ampacity exceeds the breaker rating.
Why is there no 14 AWG listed in the 75°C and 90°C columns in some charts?
NEC 240.4(D) places a hard, absolute limit on small conductors to prevent fire hazards from high-fault currents. Regardless of the insulation temperature rating, 14 AWG copper is strictly limited to a 15A breaker, 12 AWG to 20A, and 10 AWG to 30A. You cannot use the higher temperature columns to bypass these hard limits for standard overcurrent protection.
