When you look up a wire chart size, you are referencing the ampacity tables defined in the National Electrical Code (NEC), specifically NFPA 70 Table 310.16 (formerly 310.15(B)(16)). For standard residential copper branch circuits, the baseline rule of thumb is: 14 AWG handles 15 amps, 12 AWG handles 20 amps, 10 AWG handles 30 amps, 8 AWG handles 40 amps, and 6 AWG handles 55 amps. However, pulling a single number from a chart without understanding temperature columns and derating factors is how breakers trip and terminals melt. This reference guide gives you the exact numbers, the physics behind the columns, and the edge cases that the base table hides.
How to Read the NEC Wire Size Chart
The most common mistake DIYers and junior apprentices make is reading the 90°C column because the wire insulation (like THHN) is rated for it. Do not do this. To use the wire chart size table correctly, you must understand the three temperature columns and the termination rules outlined in NEC 110.14(C).
- 60°C Column (TW, UF): This is your default column for almost all residential branch circuits rated 100A or less. Even if you use 90°C THHN wire, the terminals on standard receptacles, breakers, and switches are only tested and rated for 60°C. Therefore, the ampacity is limited by the weakest link: the terminal.
- 75°C Column (THWN, XHHW): Used for circuits over 100A, or when equipment is explicitly marked for 75°C terminations (common on larger subpanels, ranges, and HVAC disconnects).
- 90°C Column (THHN, THWN-2): You can only use this column for derating calculations (adjusting for heat or bundling). The final derated ampacity must still be high enough to support the overcurrent device, but the termination limit still applies.
Complete Copper Wire Chart Size Table (NEC 310.16)
Below is the complete ampacity chart for copper conductors based on an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors in a raceway. We have added anchor IDs to the most queried residential sizes so you can bookmark them for quick jobsite lookups.
| AWG / kcmil | 60°C (140°F) Default for ≤100A |
75°C (167°F) Default for >100A |
90°C (194°F) Derating Only |
|---|---|---|---|
| 14 AWG | 15 A | 20 A | 25 A |
| 12 AWG | 20 A | 25 A | 30 A |
| 10 AWG | 30 A | 35 A | 40 A |
| 8 AWG | 40 A | 50 A | 55 A |
| 6 AWG | 55 A | 65 A | 75 A |
| 4 AWG | 70 A | 85 A | 95 A |
| 3 AWG | 85 A | 100 A | 110 A |
| 2 AWG | 95 A | 115 A | 130 A |
| 1 AWG | 110 A | 130 A | 145 A |
| 1/0 AWG | 125 A | 150 A | 170 A |
| 2/0 AWG | 145 A | 175 A | 195 A |
| 3/0 AWG | 165 A | 200 A | 225 A |
| 4/0 AWG | 195 A | 230 A | 260 A |
Source: NFPA 70 (NEC) Table 310.16. Values assume copper conductors, 30°C ambient, max 3 current-carrying conductors.
Derating Factors: What the Base Chart Cannot Tell You
The table above assumes perfect conditions: a cool 86°F day and wires spaced out with plenty of breathing room. In the real world, you must apply derating factors. Here is how derating rows modify the base value, and what the table completely fails to warn you about.
1. Bundling (More Than 3 Current-Carrying Conductors)
When you pull four or more current-carrying conductors through a single conduit, they heat each other up. You must multiply the 90°C column value by a derating percentage (NEC Table 310.15(C)(1)).
Worked Example: You are pulling four 12 AWG THHN wires in a conduit for a multi-way switch loop.
1. Base 90°C ampacity for 12 AWG = 30A.
2. Four conductors require an 80% derating factor.
3. 30A × 0.80 = 24A derated ampacity.
4. However, because your breaker and switch terminals are rated for 60°C, your final circuit limit remains 20A. The 90°C column simply gave you the thermal headroom to survive the bundling without violating code.
2. Ambient Temperature Corrections
If your conduit runs through a 110°F attic in Texas, the 30°C baseline is invalid. You must apply an ambient temperature correction factor to the 90°C column. If the derated value drops below the rating of your overcurrent protective device (breaker), you must jump up to the next wire chart size.
3. What the Table Hides: Voltage Drop and Conduit Fill
The ampacity chart only tells you the point at which the insulation will melt. It tells you absolutely nothing about voltage drop. If you are running a 12 AWG wire to a shed 150 feet away on a 20A breaker, the wire won't catch fire, but the voltage at the shed will drop below 110V, causing motors to overheat and lights to dim. For runs over 100 feet, always calculate voltage drop (aim for <3%) and size up accordingly. Furthermore, the chart ignores physical conduit fill limits (NEC Chapter 9, Table 1); you might be allowed 30 amps of 10 AWG, but you physically might not be able to fit four 10 AWG wires into a 1/2-inch EMT pipe.
Wire Chart Size FAQ
What wire chart size do I need for a 50-amp breaker?
For a 50-amp breaker, you need 6 AWG copper wire (or 4 AWG aluminum). Looking at the 60°C column, 6 AWG is rated for 55 amps, which safely covers the 50-amp requirement. If you are wiring a hot tub or an EV charger outdoors, ensure you are using wet-location rated insulation like THWN-2, and remember that if the equipment lugs are rated 75°C, 8 AWG (rated 50A at 75°C) is technically permissible by code, but 6 AWG is the universal standard practice to account for voltage drop and future-proofing.
Can I use the 90°C column to size my breaker for home wiring?
No. NEC 110.14(C)(1)(a) strictly limits the ampacity of conductors terminating in equipment rated 100A or less to the 60°C column, unless the equipment is specifically listed and identified for use with 75°C or 90°C conductors. Standard residential breakers, receptacles, and light switches are not. You use the 90°C column strictly as a starting point for derating math, not for final breaker sizing.
How does aluminum wire change the chart size?
Aluminum has higher resistance and expands more under heat than copper, so it requires a larger physical diameter to carry the same current. As a general rule, you must jump up two AWG sizes when switching from copper to aluminum. For example, a 100A subpanel feeder requires 3 AWG copper, but requires 1 AWG aluminum (or 2 AWG if using the 75°C column for specific XHHW-2 setups). Always consult the aluminum section of NEC Table 310.16 and ensure your lugs are rated for aluminum (CO/ALR) to prevent galvanic corrosion and arcing.
Does the ground wire count toward wire chart size derating?
No. When calculating bundling derating factors for more than three conductors in a raceway, equipment grounding conductors (bare copper or green) do not count as current-carrying conductors (NEC 310.15(C)(1)). They only carry current during a fault condition. Therefore, if you pull three 12 AWG THHN hot/neutral wires plus one 12 AWG ground wire through a conduit, you only have three current-carrying conductors, and no bundling derating is required.
