For standard residential branch circuits, the direct answer for copper wire sizing is based on the 60°C column of the National Electrical Code (NEC): 14 AWG is rated for 15 amps, 12 AWG for 20 amps, and 10 AWG for 30 amps. However, looking at a raw wire size amperage chart without understanding temperature columns and derating factors is the most common way DIYers and junior electricians end up with undersized feeders or tripped breakers. This guide provides the exact data from NEC Table 310.16, explains which column legally applies to your specific terminals, and details the math required when you pull more than three wires through a single conduit.
The Master Wire Size Amperage Chart (NEC Table 310.16)
Before scrolling to the data, you need to know how to read this table. This chart applies strictly to copper conductors with an ambient air temperature of 30°C (86°F) and assumes you have no more than three current-carrying conductors bundled in a raceway or cable. The three temperature columns (60°C, 75°C, and 90°C) correspond to the thermal rating of the wire's insulation, not the copper itself. Common insulation types are listed in the column headers.
Bookmark Quick-Jumps for Most Queried Breaker Sizes:
- 15A Breaker: Requires minimum 14 AWG (60°C column)
- 20A Breaker: Requires minimum 12 AWG (60°C column)
- 30A Breaker: Requires minimum 10 AWG (60°C column)
- 50A Breaker: Requires minimum 6 AWG (75°C column, assuming 75°C terminals)
- 100A Feeder: Requires minimum 3 AWG (75°C column, assuming 75°C terminals)
| AWG / kcmil | 60°C (140°F) Types: TW, UF, NM-B |
75°C (167°F) Types: THWN, XHHW, RHW |
90°C (194°F) Types: THHN, THWN-2 |
Standard Max Breaker |
|---|---|---|---|---|
| 14 | 15A | 20A | 25A | 15A |
| 12 | 20A | 25A | 30A | 20A |
| 10 | 30A | 35A | 40A | 30A |
| 8 | 40A | 50A | 55A | 40A / 50A* |
| 6 | 55A | 65A | 75A | 60A |
| 4 | 70A | 85A | 95A | 80A |
| 3 | 85A | 100A | 110A | 100A |
| 2 | 95A | 115A | 130A | 110A / 125A |
| 1 | 110A | 130A | 145A | 125A |
| 1/0 | 125A | 150A | 170A | 150A |
*Note: 8 AWG copper is rated 40A in the 60°C column, but standard 40A and 50A breakers typically feature 75°C rated terminals, allowing 8 AWG to be used on a 50A breaker (like a standard electric range or spa circuit) per NEC 110.14(C).
Decoding the Columns: Which Temperature Rating Applies to You?
The most frequent mistake made when using a wire size amperage chart is blindly reading the 90°C column because modern THHN/THWN-2 wire is cheap, ubiquitous, and printed with that rating. You cannot size your breaker based on the 90°C column unless every single termination point in the circuit is also rated for 90°C.
Under NEC Article 110.14(C), the ampacity of a circuit is limited by the lowest temperature rating of any connected component. This is known as the 'weakest link' rule.
Almost all standard residential circuit breakers (up to 100A) and standard 15A/20A duplex receptacles are rated for 75°C. Older equipment, specific small breakers, and some light fixtures are rated for 60°C. Very few residential terminals are rated for 90°C.
Scenario A: Romex (NM-B) Cable
If you are running standard yellow 12/2 NM-B Romex through wall cavities, NEC 334.80 explicitly mandates that you must use the 60°C column for ampacity, regardless of the fact that the individual wires inside the sheath might technically have 90°C insulation. Therefore, 12 AWG NM-B is strictly limited to 20 amps.
Scenario B: THHN in Conduit
If you pull 12 AWG THHN (90°C rated) through EMT conduit to a 75°C rated breaker and a 75°C rated receptacle, your final allowable ampacity is dictated by the 75°C column (25A). Because 25A is not a standard breaker size (NEC 240.6), you round down to the next standard size, which is a 20A breaker. You do not get to use the 30A rating from the 90°C column.
So why buy 90°C wire at all? The 90°C column is legally used as the starting baseline for derating calculations, which we cover next.
Derating Factors and What This Chart Cannot Tell You
The wire size amperage chart above assumes ideal conditions: a cool 86°F day and wires spaced out with plenty of breathing room. Real jobsites rarely cooperate. When conditions change, you must apply correction and adjustment factors.
How Derating Modifies the Base Value
When you bundle more than three current-carrying conductors in a single conduit, the wires heat each other up. NEC Table 310.15(C)(1) requires you to multiply the wire's base ampacity by a derating percentage. You always start this math using the 90°C column.
Worked Jobsite Example:
You are pulling four separate 20A circuits (8 current-carrying conductors total: 4 hots, 4 neutrals) through a single 3/4-inch EMT conduit using 12 AWG THHN.
- Base Ampacity: 12 AWG in the 90°C column is 30A.
- Derating Factor: 7 to 9 conductors requires a 70% multiplier.
- Adjusted Ampacity: 30A × 0.70 = 21A.
- Terminal Check: 21A is greater than the 20A breaker protecting the circuit, and the termination limits (75°C = 25A) are not exceeded. 12 AWG is legally permitted.
If you added a fifth circuit (10 conductors total), the derating factor drops to 50%. 30A × 0.50 = 15A. Because 15A is less than your 20A breaker, you would be forced to upsize to 10 AWG wire to pass inspection.
What the Ampacity Chart Cannot Tell You
Relying solely on Table 310.16 leaves three critical engineering blind spots that can cause voltage sag, overheating, or failed inspections:
- Voltage Drop: The NEC ampacity chart does not account for distance. A 6 AWG copper wire is rated for 55A (60°C column) and will safely handle a 50A load without melting. However, if that 50A load (like a subpanel or EV charger) is 150 feet away on a 240V circuit, the voltage drop will be approximately 3.2%. If the run extends to 250 feet, the drop hits 5.4%, exceeding the NEC's recommended 5% maximum (NEC Informational Note 310.14). For a 250-foot 50A run, you must upsize to 4 AWG or 3 AWG purely to maintain voltage stability, even though 6 AWG satisfies the thermal ampacity chart.
- Conduit Fill Capacity: Just because the math allows you to pull nine 10 AWG wires through a 1/2-inch conduit based on derating doesn't mean they will physically fit. You must cross-reference Chapter 9, Table 1 of the NEC to ensure the cross-sectional area of the wires does not exceed 40% of the conduit's internal area. Jamming wires into an undersized conduit destroys insulation during the pull.
- Aluminum vs. Copper: This chart is exclusively for copper. Aluminum wire (commonly used for heavy feeders like 2/0 or 4/0 to service panels) has a lower thermal conductivity and higher resistance. Aluminum requires entirely different columns and generally requires upsizing by one or two AWG steps compared to copper to achieve the exact same ampacity.
Keep this chart bookmarked for quick bench reference, but always verify your terminal temperatures, count your current-carrying conductors, and run a voltage drop calculator for any circuit exceeding 75 feet.
