When sizing conductors for residential or commercial branch circuits, the definitive wire size chart ampacity reference in the United States is NEC Table 310.16 (formerly 310.15(B)(16)). For standard copper branch circuits, the baseline rules are simple: 14 AWG is rated for 15 amps, 12 AWG for 20 amps, and 10 AWG for 30 amps. However, simply memorizing these three numbers will eventually lead to a failed inspection or a melted terminal lug. True ampacity depends on insulation type, termination temperature ratings, and conduit bundling.
This reference guide breaks down the exact values from the National Electrical Code, explains how to select the correct temperature column, and details the derating math that trips up even experienced DIYers.
How to Read the NEC Wire Size Chart Ampacity Table
Before pulling wire, you must understand how the table is structured. Table 310.16 is divided by conductor material (Copper vs. Aluminum) and then by temperature rating (60°C, 75°C, and 90°C). The values below represent the maximum allowable ampacities for not more than three current-carrying conductors in a raceway, at an ambient temperature of 30°C (86°F).
| AWG / kcmil | 60°C (140°F) TW, UF |
75°C (167°F) RHW, THHW, THW |
90°C (194°F) THHN, THWN-2, XHHW |
|---|---|---|---|
| 14 | 15 A | 20 A | 25 A |
| 12 | 20 A | 25 A | 30 A |
| 10 | 30 A | 35 A | 40 A |
| 8 | 40 A | 50 A | 55 A |
| 6 | 55 A | 65 A | 75 A |
| 4 | 70 A | 85 A | 95 A |
| 3 | 85 A | 100 A | 115 A |
| 2 | 95 A | 115 A | 130 A |
| 1 | 110 A | 130 A | 145 A |
| 1/0 | 125 A | 150 A | 170 A |
Notice that modern THHN/THWN-2 wire (the standard slick-jacketed wire sold at big-box stores) falls into the 90°C column. However, you rarely get to use that 90°C ampacity for final breaker sizing. That brings us to the most misunderstood rule in the codebook.
Which Temperature Column Applies to Your Installation
The golden rule of wire sizing is the weakest link principle, codified in NEC 110.14(C). You must size the wire based on the temperature rating of the lowest-rated component in the circuit, which is almost always the terminal lugs on your breakers, receptacles, or switches.
- Circuits 100 Amps or Less (14 AWG through 1 AWG): Standard residential breakers and receptacles are typically rated for 60°C or 75°C, but the NEC mandates that for conductors 14 through 1 AWG, you must use the 60°C column to determine the base ampacity for overcurrent protection, unless the equipment is explicitly marked otherwise.
- Circuits Over 100 Amps (1/0 AWG and larger): You default to the 75°C column for sizing and termination limits.
Real-world example: You are wiring a 30-amp dryer outlet using 10 AWG THHN wire. The 90°C column says 40A, and the 75°C column says 35A. But because the circuit is under 100A and uses standard 10 AWG wire, NEC 110.14(C)(1)(a) forces you to look at the 60°C column. The 60°C column lists 10 AWG at exactly 30A. Therefore, a 30A breaker is the maximum allowed. You cannot put it on a 35A breaker just because the wire insulation is rated for 90°C.
How Derating Factors Modify the Base Ampacity
The baseline table assumes three current-carrying conductors in a conduit at 86°F (30°C). Real jobsites rarely match this. When you bundle more than three current-carrying conductors in a single raceway, or when the ambient temperature exceeds 86°F, you must apply derating factors. This is where the 90°C column finally becomes useful.
Per NEC guidelines published by EC&M, you apply derating percentages to the 90°C column, regardless of your termination temperature. However, the final derated ampacity cannot exceed the base ampacity of your termination column (60°C or 75°C).
| Number of Current-Carrying Conductors | Percent of Base Ampacity (Apply to 90°C Column) |
|---|---|
| 1 - 3 | 100% |
| 4 - 6 | 80% |
| 7 - 9 | 70% |
| 10 - 20 | 50% |
| 21 - 30 | 45% |
Worked Numeric Example: You are pulling a multi-wire branch circuit through a single conduit. You have 6 current-carrying conductors (two 120V circuits sharing a neutral, plus two more circuits). You are using 10 AWG THHN (90°C) wire.
- Base 90°C ampacity for 10 AWG = 40A.
- Derating factor for 6 conductors = 80%.
- 40A × 0.80 = 32A derated ampacity.
- Now check the termination limit: Because it is under 100A, we use the 60°C column, which is 30A.
- Since the derated value (32A) is higher than the termination limit (30A), the wire is still valid for a 30A breaker.
What the Ampacity Table Cannot Tell You
Table 310.16 is strictly a thermal limit chart for steady-state current. It does not account for several critical engineering factors that can cause a circuit to fail or perform poorly.
1. Voltage Drop
Ampacity tables do not care if your wire is 10 feet long or 500 feet long. A 10 AWG wire carrying 24 amps for 300 feet will not melt, but the voltage drop will be roughly 15 volts (over 12% on a 120V circuit). Your tools will bog down, and LED drivers will flicker. For runs exceeding 50 feet, always calculate voltage drop and upsize the wire accordingly, even if the ampacity table says the smaller wire is legal.
2. Conduit Fill Capacity
The ampacity table tells you how many wires can share a conduit thermally (derating), but it does not tell you if they will physically fit. For physical jamming and heat dissipation limits, you must cross-reference NEC Chapter 9, Table 1, which limits conduit fill to 40% for three or more wires. Pulling six 10 AWG THHN wires through a 1/2-inch EMT conduit is thermally legal with derating, but physically impossible without destroying the insulation.
3. Short-Circuit Withstand (Let-Through Current)
Ampacity is about continuous heat. It does not indicate whether a wire can survive the magnetic and thermal shock of a dead short before the breaker trips. For standard residential branch circuits, the breaker clears the fault fast enough to protect the wire. But in high-available-fault-current environments (like main service feeders near a utility transformer), you must verify the wire's short-circuit withstand rating against the let-through current of the specific breaker or fuse model.
Always treat the wire size chart ampacity table as the starting point of your design, not the finish line. Verify your termination temperatures, apply your bundling derating, check your voltage drop, and confirm your conduit fill before buying your spools.
