When sizing conductors for residential or commercial branch circuits, guessing is not an option. The wire gauge sizes chart maps American Wire Gauge (AWG) to maximum allowable ampacity based on the National Electrical Code (NEC) Table 310.16. Whether you are pulling THHN through EMT conduit or routing NM-B (Romex) through wall cavities, picking the wrong column or ignoring derating factors can lead to tripped breakers, melted insulation, or a failed inspection.
Below is the complete reference data for copper conductors, followed by the exact rules for reading the temperature columns, applying derating factors, and calculating voltage drop for long runs.
How to Read the NEC Wire Gauge Sizes Chart
The most common mistake DIYers and junior apprentices make is looking at the highest ampacity number on a spec sheet and assuming that is the legal limit. The NEC dictates that your allowable ampacity depends entirely on the weakest link in your circuit's temperature rating chain.
The chart is divided into three primary temperature columns for copper: 60°C (140°F), 75°C (167°F), and 90°C (194°F). Here is how to determine which column applies to your installation:
- The 60°C Column: This is your baseline for almost all standard residential branch circuits using NM-B (Romex) cable. Per NEC 334.80, even though the individual wires inside NM-B are rated for 90°C, the ampacity must be determined using the 60°C column. This also applies to circuits rated 100A or less where the termination equipment (breakers, lugs) is not explicitly marked with a higher temperature rating (NEC 110.14(C)(1)(a)).
- The 75°C Column: Use this when pulling individual THHN/THWN-2 wires in conduit for circuits over 100A, or when the equipment terminations are specifically marked "75°C" or "AL/CU". Most modern commercial breakers and panel lugs are rated 75°C.
- The 90°C Column: You generally cannot use the 90°C ampacity to size your breaker. However, you must use the 90°C column as your starting baseline when calculating derating factors for bundling or high ambient temperatures.
The Master Wire Gauge Sizes Chart (NEC Table 310.16 Extract)
The following table covers the most queried residential and light-commercial copper wire sizes. Data is sourced from NFPA 70 (National Electrical Code) Table 310.16, assuming an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors in a raceway. Bookmark this page and use your browser's find function (Ctrl+F / Cmd+F) to jump to the quick-jump IDs for the most common AWG sizes.
| AWG / kcmil | 60°C Ampacity (NM-B / Romex) | 75°C Ampacity (THHN Terminations) | 90°C Ampacity (THHN Derating Baseline) | Max Standard Breaker (Typical) |
|---|---|---|---|---|
| 14 AWG | 15A | 20A | 25A | 15A (NEC 240.4(D) limits) |
| 12 AWG | 20A | 25A | 30A | 20A (NEC 240.4(D) limits) |
| 10 AWG | 30A | 35A | 40A | 30A (NEC 240.4(D) limits) |
| 8 AWG | 40A | 50A | 55A | 40A (NM-B) / 50A (THHN) |
| 6 AWG | 55A | 65A | 75A | 60A (NM-B) / 70A (THHN) |
| 4 AWG | 70A | 85A | 95A | 70A (NM-B) / 90A (THHN) |
| 3 AWG | 85A | 100A | 110A | 90A (NM-B) / 100A (THHN) |
| 2 AWG | 95A | 115A | 130A | 100A (NM-B) / 125A (THHN) |
| 1 AWG | 110A | 130A | 150A | 125A (THHN) |
| 1/0 AWG | 125A | 150A | 170A | 150A (THHN) |
| 2/0 AWG | 145A | 175A | 195A | 175A (THHN) |
| 3/0 AWG | 165A | 200A | 225A | 200A (THHN) |
| 4/0 AWG | 195A | 230A | 260A | 225A (THHN) |
Note: Standard breaker sizes per NEC 240.6 include 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250A. NEC 240.4(B) allows rounding up to the next standard breaker size if the calculated ampacity does not correspond to a standard size, provided the load is not continuous.
Derating, Edge Cases, and What the Chart Cannot Tell You
The wire gauge sizes chart assumes ideal conditions: an ambient temperature of 86°F (30°C) and no more than three current-carrying conductors bundled together. When you deviate from this, you must apply correction and adjustment factors.
How Derating Modifies the Base Value:
If you pull four to six current-carrying conductors through a single conduit, NEC Table 310.15(C)(1) requires you to derate the ampacity to 80%. You always apply this percentage to the 90°C column, then verify that the resulting number is still higher than your breaker size and termination limits.
Worked Example: You are pulling four 12 AWG THHN wires in a conduit for two 20A circuits. 1. The 90°C ampacity for 12 AWG is 30A. 2. Multiply by the 80% derating factor: 30A × 0.80 = 24A. 3. Because 24A is still greater than your 20A breaker, 12 AWG is perfectly legal. If you had pulled 10 conductors (derated to 50%), the math would be 30A × 0.50 = 15A. You would be forced to upsize to 10 AWG to maintain a 20A circuit.
What the Table Cannot Tell You: Voltage Drop
Ampacity charts only tell you what the wire can handle before the insulation melts or the breaker trips. They do not account for voltage drop over distance. For a 120V circuit, the NEC recommends keeping voltage drop under 3% for branch circuits (3.6V). If you run 12 AWG wire 120 feet to a 15A space heater, the resistance of the wire will cause a ~4.5% drop. The heater will run poorly, and the wire will run warm. For long runs, always use a voltage drop calculator and upsize your wire by one or two gauges, regardless of what the ampacity chart permits.
Frequently Asked Questions
What size wire do I need for a 20 amp breaker?
For a standard 20-amp residential branch circuit, you must use a minimum of 12 AWG copper wire. While 12 AWG THHN in conduit has a 75°C ampacity of 25A, NEC 240.4(D) strictly limits the overcurrent protection for 12 AWG to 20A. If you are running NM-B (Romex), the 60°C column limits 12 AWG to 20A anyway. Never use 14 AWG on a 20A breaker; it is a severe fire hazard and an immediate code violation.
Can I use 14 AWG wire on a 15 amp breaker for a long 100-foot run?
Legally, yes. 14 AWG is rated for 15A and is permitted on a 15A breaker. Practically, you should upsize to 12 AWG. At 100 feet, a 14 AWG wire carrying a 12A load (like a window AC or power tools) will experience roughly a 3.8% voltage drop. This exceeds the NEC's 3% recommended limit for branch circuits, causing motors to run hot and lights to dim. Upsizing to 12 AWG drops the resistance and keeps the voltage drop around 2.4%.
Why does my THHN wire chart show higher ampacity than my NM-B cable?
THHN (Thermoplastic High Heat-resistant Nylon-coated) wire features a thin, slick nylon jacket rated for 90°C in dry locations. NM-B (Non-Metallic Sheathed Cable) bundles multiple insulated wires inside a thick PVC jacket that traps heat. Because of this thermal trapping, NEC 334.80 mandates that the ampacity of NM-B be based on the 60°C column, regardless of the fact that the individual wires inside the sheath are technically rated for 90°C.
How do I adjust the wire gauge sizes chart for high ambient temperatures?
If you are routing wire through an attic in a hot climate where ambient temperatures regularly exceed 86°F (30°C), you must apply temperature correction factors from NEC Table 310.15(B)(1). For example, if your attic reaches 113°F (45°C) and you are using 90°C-rated THHN, you must multiply the base 90°C ampacity by 0.87. If you are using NM-B, the correction factor applies to the 60°C column, severely limiting your allowable current. In high-heat attics, it is often best practice to route circuits through conditioned wall cavities or use conduit with derated THHN.
