For standard residential copper branch circuits, the baseline ampacities are: 14 AWG is 15A, 12 AWG is 20A, 10 AWG is 30A, 8 AWG is 40A, and 6 AWG is 55A. These values apply when using standard NM-B (Romex) cable, which requires you to read from the 60°C column of the National Electrical Code (NEC) ampacity tables. If you are pulling individual THHN conductors in conduit to a 75°C-rated breaker panel, you can utilize the higher 75°C column values for wires 1/0 AWG and larger, but the 60°C rule still governs most small-wire terminations.
How to Read the NEC Wire Size Chart Amps Table
The master reference for conductor ampacity in the US is NEC Table 310.16 (formerly Table 310.15(B)(16) in older code cycles). This table dictates the maximum continuous current a wire can carry before its insulation begins to degrade.
The table is divided into three temperature columns: 60°C, 75°C, and 90°C. These do not represent the ambient room temperature; they represent the maximum temperature the wire's insulation can safely withstand at the termination point. The fundamental rule of wire sizing is the 'weakest link' principle (NEC 110.14(C)): your allowable ampacity is limited by the lowest temperature rating of any connected component, including the wire insulation, the breaker lug, the receptacle, or the splice connector.
Master Copper Ampacity Table (NEC Table 310.16)
The following data is extracted directly from the NFPA 70 National Electrical Code, Table 310.16, for copper conductors with an ambient temperature of 30°C (86°F).
| AWG / kcmil Size | 60°C (140°F) NM-B, TW, UF |
75°C (167°F) THWN, RHW, Panel Lugs |
90°C (194°F) THHN, XHHW-2 (Derating Base) |
|---|---|---|---|
| 14 AWG | 15A | 20A | 25A |
| 12 AWG | 20A | 25A | 30A |
| 10 AWG | 30A | 35A | 40A |
| 8 AWG | 40A | 50A | 55A |
| 6 AWG | 55A | 65A | 75A |
| 4 AWG | 70A | 85A | 95A |
| 3 AWG | 85A | 100A | 115A |
| 2 AWG | 95A | 115A | 130A |
| 1 AWG | 110A | 130A | 145A |
| 1/0 AWG | 125A | 150A | 170A |
| 2/0 AWG | 145A | 175A | 195A |
| 3/0 AWG | 165A | 200A | 225A |
| 4/0 AWG | 195A | 230A | 260A |
Source: NFPA 70 (NEC) Table 310.16. Ampacities shown are for not more than three current-carrying conductors in a raceway or cable, at an ambient temperature of 30°C.
Which Temperature Column Applies to Your Installation?
Choosing the wrong column is the most common mistake DIYers and junior apprentices make. Here is the exact decision logic based on NEC 110.14(C):
- Use the 60°C Column if: You are using NM-B (Romex), UF-B, or TW cable. You must also use this column for any circuit rated 100A or less that uses 1 AWG or smaller wire, unless the equipment (breaker, receptacle) is explicitly tested and listed for 75°C terminations. Because standard 15A and 20A receptacles are typically only rated for 60°C, the 60°C column governs almost all standard branch circuits.
- Use the 75°C Column if: You are using THWN or THWN-2 wire in conduit, and the circuit is over 100A, OR the circuit is 100A or less but uses wires larger than 1 AWG, AND all termination lugs are marked 75°C (most modern square D and Eaton breaker panels are).
- Use the 90°C Column ONLY for Derating: You cannot use the 90°C ampacity as your final allowable current for breaker sizing. The 90°C column is strictly used as the starting baseline before applying ambient temperature or bundling adjustment factors (NEC 310.15). Once derated, the final ampacity must still be compared to the 60°C or 75°C termination limits.
Derating Factors: When Base Ampacity Drops
The ampacities in the chart above assume you have no more than three current-carrying conductors (CCCs) in a conduit and an ambient temperature below 86°F (30°C). If you violate either condition, you must derate the wire using the 90°C column as your starting point, per DOE and NEC wire sizing guidelines.
1. Base 90°C ampacity for 12 AWG = 30A.
2. NEC Table 310.15(C)(1) dictates an 80% adjustment factor for 4-6 CCCs.
3. 30A × 0.80 = 24A derated ampacity.
4. Because 24A is still greater than the 20A breaker protecting the circuit, this installation is code-compliant. If you added a 5th wire (derating to 70%, or 21A), you would be forced to upsize to 10 AWG.
Sizing Decision Tree: Pick Your Exact Wire and Breaker
Use this decision matrix to terminate your sizing process with a concrete material pick for standard 120V/240V residential applications.
| Target Load / Breaker | Application Scenario | Concrete Wire Pick (Copper) | Governing Column |
|---|---|---|---|
| 15 Amp | General lighting, bedroom receptacles | 14 AWG NM-B (Romex) | 60°C (15A) |
| 20 Amp | Kitchen small appliance, bathroom GFCI, garage | 12 AWG NM-B (Romex) | 60°C (20A) |
| 30 Amp | Electric water heater, window AC units, RV outlet | 10 AWG NM-B or THHN | 60°C (30A) |
| 40 Amp | Standard electric range (older), large EV charger | 8 AWG NM-B (or 8 AWG THHN in conduit) | 60°C (40A) |
| 50 Amp | Modern electric range, 50A subpanel feeder (short run) | 6 AWG NM-B (or 8 AWG THHN if 75°C lugs) | 60°C (55A) or 75°C (50A) |
| 60 Amp | Subpanel feeder, heat pump emergency heat | 6 AWG THHN in conduit (NM-B 6 AWG is only 55A) | 75°C (65A) |
| 100 Amp | Main subpanel feeder (under 100 ft) | 3 AWG THHN/THWN-2 in conduit | 75°C (100A) |
What This Chart Cannot Tell You (and the Default Rule)
Ampacity tables only solve for thermal limits under ideal conditions. They do not account for the following real-world constraints:
- Voltage Drop: NEC 310.16 does not calculate voltage drop. If your run exceeds 100 feet, a 12 AWG wire on a 20A breaker will technically not overheat, but the voltage at the receptacle may drop below 114V, causing motors to overheat and electronics to brown out. For runs over 100 feet, upsize your wire by one AWG step for every additional 50 feet.
- Conduit Fill Capacity: You might derate perfectly, but if you try to pull four 6 AWG THHN wires through a 1/2-inch EMT conduit, you will violate NEC Chapter 9 fill tables and physically jam the wires, stripping the insulation.
- Short-Circuit Interrupting Ratings: The chart assumes standard thermal tripping. It does not verify that the wire can withstand the magnetic forces of a 10,000A short circuit before the breaker clears the fault.
The Concrete Default: When sizing standard residential branch circuits under 100 feet in length, always default to the 60°C column for NM-B cable and match your breaker exactly to that value (e.g., 12 AWG = 20A breaker). When pulling individual THHN conductors in conduit for feeders or high-load appliances, use the 75°C column provided your panel lugs are rated for it, and always perform a voltage drop calculation if the distance from the panel exceeds 80 feet.
