When you need to know exactly what size wire to pull for a specific breaker, the definitive answer lives in NEC Table 310.16 (formerly 310.15(B)(16) in older code cycles). This amperage wire size chart dictates the allowable ampacity for insulated conductors rated up to 2000 volts. However, simply finding your AWG size and reading across the row is a fast track to a failed inspection or a melted terminal lug. The chart assumes specific baseline conditions: an ambient temperature of 30°C (86°F), no more than three current-carrying conductors in a raceway, and specific terminal temperature ratings.
Below is the complete reference data for copper and aluminum conductors, followed by the critical jobsite rules for selecting the correct temperature column and adjusting for real-world derating.
The Master Amperage Wire Size Chart (NEC Table 310.16)
How to read this table: The ampacity values are split by conductor material (Copper vs. Aluminum/Copper-Clad) and by the insulation's maximum temperature rating (60°C, 75°C, and 90°C). To use this chart correctly, you must first identify your wire type. Standard NM-B (Romex) is restricted to the 60°C column. THHN/THWN-2 and XHHW-2 in conduit are 90°C wires, but their final allowable ampacity is usually capped by the 75°C column due to breaker and lug terminal ratings.
| AWG / kcmil | Copper 60°C (NM-B) | Copper 75°C (THHN Terminals) | Copper 90°C (THHN Wire) | Aluminum 75°C | Aluminum 90°C |
|---|---|---|---|---|---|
| 14 | 15A* | 20A* | 25A* | — | — |
| 12 | 20A* | 25A* | 30A* | — | — |
| 10 | 30A* | 35A* | 40A* | — | — |
| 8 | 40A | 50A | 55A | 40A | 45A |
| 6 | 55A | 65A | 75A | 50A | 60A |
| 4 | 70A | 85A | 95A | 65A | 75A |
| 3 | 85A | 100A | 110A | 75A | 85A |
| 2 | 95A | 115A | 130A | 90A | 100A |
| 1 | 110A | 130A | 145A | 100A | 115A |
| 1/0 | 125A | 150A | 170A | 120A | 135A |
| 2/0 | 145A | 175A | 195A | 135A | 150A |
| 3/0 | 165A | 200A | 225A | 155A | 175A |
| 4/0 | 195A | 230A | 260A | 180A | 205A |
*Note: Per NEC 240.4(D), standard overcurrent protection for 14, 12, and 10 AWG copper is strictly limited to 15A, 20A, and 30A respectively, regardless of the higher 75°C/90°C column values, unless specific motor or welding exceptions apply.
Which Temperature Column Actually Applies to Your Install?
The most common mistake DIYers and junior apprentices make is looking at a spool of THHN wire, seeing it rated for 90°C, and sizing the breaker using the 90°C column. If you do this, you will overheat your breaker terminals. To determine the correct column, you must apply the weakest link rule found in NEC 110.14(C).
The allowable ampacity of your circuit is limited by the lowest temperature rating of any connected component, including the wire insulation, the breaker lugs, the receptacle terminals, and the splice connectors.
- The 60°C Column: Use this exclusively for Nonmetallic-Sheathed Cable (NM-B / Romex). Even though the individual conductors inside modern NM-B might technically have 90°C insulation, the NEC mandates that the overall assembly is rated for 60°C. It also applies to older homes with legacy breakers and receptacles not explicitly marked with a temperature rating.
- The 75°C Column: This is your default for almost all modern THHN/THWN-2 wire in conduit. While the wire itself is 90°C, standard UL-listed circuit breakers, panel lugs, and commercial receptacles are tested and rated for 75°C terminations. Therefore, your final ampacity is capped at the 75°C value.
- The 90°C Column: You only use this column for one specific purpose: derating. If your wire is 90°C rated, you start your derating math from the 90°C column, but the final derated number must still be compared against the 75°C termination limit.
Derating, Voltage Drop, and What the Chart Cannot Tell You
Table 310.16 assumes a perfect world: 30°C (86°F) ambient air and no more than three current-carrying conductors bundled together. In the real world, wires generate heat, and bundling them traps that heat. When you deviate from the baseline, you must apply adjustment factors from NEC Table 310.15(C)(1).
How Derating Modifies the Base Value
Let’s say you are pulling four separate 120V circuits through a single EMT conduit. That gives you 4 hot wires and 4 neutral wires = 8 current-carrying conductors. Looking at the derating table, 7 to 9 conductors require a 70% adjustment factor.
Here is how the math works using 10 AWG THHN (90°C column = 40A):
40A × 0.70 = 28A.
Your 10 AWG wire is now only legally allowed to carry 28A. Since standard breakers don't come in 28A, you must drop to a 25A breaker, or pull 8 AWG wire to maintain a 30A circuit. Note that equipment grounding conductors (bare or green) do not count as current-carrying conductors for derating purposes.
Ambient Temperature Corrections
If you are routing THHN through an attic in the Southwest US during July, your ambient temperature isn't 30°C; it's likely 50°C (122°F). You must apply the ambient temperature correction factor. For 90°C wire at 50°C ambient, the multiplier is 0.82. A 6 AWG copper wire (90°C column = 75A) drops to 61.5A (75 × 0.82). It can no longer legally sit on a 65A breaker; you must step up to 4 AWG.
What the Amperage Chart Cannot Tell You: Voltage Drop
Table 310.16 only tells you the maximum current the wire can carry before the insulation melts or the breaker terminals overheat. It tells you absolutely nothing about voltage drop.
If you are running a 50A, 240V welder receptacle 150 feet from your subpanel, 6 AWG copper is perfectly legal according to Table 310.16. However, at 150 feet under a full 50A load, 6 AWG copper will experience a voltage drop of roughly 4.5%. Your welder will only see ~229V, which can cause the motor to draw excess current, overheat, and trip its internal thermal overload. For long runs, you must calculate voltage drop (aiming for <3% on branch circuits) and upsized the wire—likely to 4 AWG or 3 AWG—regardless of what the ampacity chart permits. Always verify long-run math using the DC resistance values found in NEC Chapter 9, Table 8.
