The correct American Wire Gauge (AWG) for your circuit is dictated by three hard numbers: the maximum continuous load, the overcurrent protective device (breaker) rating, and the lowest temperature rating of any termination in the circuit. Below is the complete reference data you need to select the right copper wire, followed by the derating math and a final decision matrix to lock in your exact part.
How to Read the NEC Ampacity Table (And Which Column Applies)
The most common mistake DIYers make is looking at a spool of THHN wire, seeing it rated for 90°C, and using the 90°C ampacity column to size their breaker. This is a direct violation of NEC 110.14(C).
Here is how to read the columns and determine which one legally applies to your installation:
- 60°C Column (TW / UF / NM-B): By code, for circuits rated 100A or less, you must use the 60°C column to size your wire and breaker, unless the equipment (breaker, receptacle, lug) is explicitly stamped '75°C'. Standard residential Romex (NM-B) is permanently bound to this column.
- 75°C Column (THWN / XHHW): Used for circuits over 100A, or for circuits 100A and under where every single termination (breaker and device) is definitively rated and marked for 75°C. Most modern THWN/THHN wire in conduit uses this column for termination limits.
- 90°C Column (THHN / THWN-2): You can only use this column for derating purposes (adjusting for ambient heat or wire bundling). You cannot use the 90°C ampacity to determine the base breaker size.
The Master AWG Size Guide: Copper Wire Ampacity Chart
The following data is extracted from NEC Table 310.16 for copper conductors. Bookmark this section for quick jobsite lookups.
| AWG Size | 60°C (NM-B / UF) | 75°C (THWN) | 90°C (THHN) | Max Standard Breaker (NEC 240.4) |
|---|---|---|---|---|
| 14 AWG | 15A | 20A | 25A | 15A (Hard Limit) |
| 12 AWG | 20A | 25A | 30A | 20A (Hard Limit) |
| 10 AWG | 30A | 35A | 40A | 30A (Hard Limit) |
| 8 AWG | 40A | 50A | 55A | 40A (NM-B) / 50A (THHN) |
| 6 AWG | 55A | 65A | 75A | 50A / 60A |
| 4 AWG | 70A | 85A | 95A | 70A / 80A / 90A |
| 3 AWG | 85A | 100A | 110A | 80A / 90A / 100A |
| 2 AWG | 95A | 115A | 130A | 100A / 110A / 125A |
| 1 AWG | 110A | 130A | 145A | 110A / 125A / 150A |
| 1/0 AWG | 125A | 150A | 170A | 125A / 150A |
| 2/0 AWG | 145A | 175A | 195A | 150A / 175A |
| 3/0 AWG | 165A | 200A | 225A | 175A / 200A |
| 4/0 AWG | 195A | 230A | 260A | 200A / 225A / 250A |
Derating: When the Base Chart Lies to You
The ampacities above assume an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a raceway. When you exceed these parameters, you must apply derating factors to the 90°C column (even if you terminate at 75°C) to find your true adjusted ampacity.
1. Ambient Temperature Derating (NEC Table 310.15(B)(1))
If you run THHN through an attic that hits 50°C (122°F) in the summer, you multiply the 90°C base ampacity by 0.82. Example: 8 AWG THHN (55A base) × 0.82 = 45.1A. You must then check if 45.1A satisfies the 75°C termination limit (50A) and the breaker size.
2. Bundling Derating (NEC Table 310.15(C)(1))
If you pull more than three current-carrying conductors through a single conduit, the wires heat each other up. Example: You are wiring a multi-way switch circuit and pull 6 current-carrying 10 AWG THHN wires through one EMT conduit. The bundling derating factor for 4-6 conductors is 80%.
Math: 10 AWG 90°C base is 40A. 40A × 0.80 = 32A adjusted ampacity. Since 32A is still above the 30A breaker limit, 10 AWG passes. If you had used 12 AWG (30A base × 0.80 = 24A), it would fail and become a fire hazard on a 20A breaker.
What This Table Cannot Tell You
Ampacity charts only tell you the thermal limit of the insulation. They completely ignore two critical physical realities:
- Voltage Drop: The NEC recommends a maximum 3% voltage drop on branch circuits and 5% total (feeder + branch). If you run a 20A, 12 AWG circuit 120 feet to a workshop table saw, you will experience roughly a 7.7V drop (over 6%). The wire won't melt, but your saw motor will overheat and stall. Use the Southwire Voltage Drop Calculator for runs over 50 feet. For that 120-foot run, you must step up to 8 AWG to maintain a safe 3% drop.
- Conduit Fill (NEC Chapter 9): You cannot physically stuff twelve 10 AWG wires into a half-inch EMT conduit. The heat cannot dissipate, and you will strip the insulation pulling it. Always check NEC Chapter 9, Table 1 for maximum cross-sectional area fill (typically 40% for 3+ wires).
Wire Sizing Decision Tree: Pick Your Exact AWG
Stop guessing. Follow this decision matrix to terminate your search and pick the exact wire type and gauge for your specific load. Assume standard 120V/240V residential single-phase copper.
| If Your Circuit / Load Is... | And the Run Distance Is... | Then Buy Exactly This: | Breaker Size |
|---|---|---|---|
| Standard 15A Lighting / Receptacles | Under 50 ft | 14 AWG NM-B (Romex) or 14 AWG THHN in conduit | 15A |
| Kitchen/Bath 20A Receptacles, Window AC | Under 50 ft | 12 AWG NM-B or 12 AWG THHN | 20A |
| 30A Dryer, RV Plug, or Water Heater | Under 60 ft | 10 AWG NM-B (for indoor) or 10 AWG THHN (for conduit) | 30A |
| 40A or 50A Electric Range / Hot Tub | Under 50 ft | 6 AWG THHN/THWN-2 in conduit (NM-B is limited to 55A/40A breaker in some jurisdictions for 50A loads; THHN is safer) | 40A or 50A |
| Any 20A Workshop / EV Charger Run | Over 75 ft | 8 AWG THHN (Stepped up from 12 AWG strictly to mitigate voltage drop) | 20A or 30A |
| 100A Subpanel Feeder | Under 75 ft | 3 AWG THHN Copper or 1 AWG Aluminum (XHHW) | 100A |
When in doubt between two sizes, the larger wire (smaller AWG number) always wins on safety and voltage stability. Buy the heavier gauge, use the appropriately rated breaker, and torque your lugs to the manufacturer's inch-pound specifications.
