The allowable wire ampacity rating for standard copper conductors is dictated by the wire gauge (AWG), the insulation type (e.g., THHN, NM-B), and the temperature rating of the equipment terminations. For the most common residential branch circuits, 14 AWG copper is rated for 15 amps, 12 AWG for 20 amps, and 10 AWG for 30 amps. However, these baseline numbers are governed by specific National Electrical Code (NEC) rules that override the raw thermal limits of the wire itself.
This reference guide provides the complete data table sourced directly from NFPA 70 (NEC) Table 310.16, along with the exact derating math and termination rules you need to size conductors safely and legally.
How to Read the NEC Wire Ampacity Rating Table
Before looking at the numbers, you must understand how to read the table columns. The ampacity of a wire is not a single fixed number; it changes based on the thermal limits of the weakest link in your circuit. The NEC requires you to use the column that matches the lowest temperature rating of any connected device, termination, or conductor in the circuit.
- 60°C Column: Use this column for non-metallic sheathed cable (NM-B / Romex), older breakers, and devices explicitly marked for 60°C. It is also mandatory for circuits rated 100A or less using smaller wire sizes (14, 12, 10 AWG) per NEC 110.14(C)(1)(a).
- 75°C Column: Use this column for most modern commercial and residential equipment. Standard THHW wire, SER cable, and modern breakers/lugs (like Square D QO or Homeline) are rated for 75°C. This column governs feeders and large branch circuits over 100A.
- 90°C Column: Use this column only as the starting baseline for derating calculations (adjusting for ambient heat or bundling). Almost no standard residential breakers or receptacles have 90°C rated terminations, so you cannot use this column to directly size your overcurrent protective device (breaker).
Complete Copper Wire Ampacity Chart (NEC Table 310.16)
The following table details the allowable ampacities for insulated copper conductors rated up to 2000 volts, assuming an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors in a raceway. Source standard: NEC Table 310.16 (via Cerrowire / NFPA).
| AWG / kcmil | 60°C (140°F) NM-B, TW, UF |
75°C (167°F) RHW, THHW, XHHW |
90°C (194°F) THHN, THWN-2 |
|---|---|---|---|
| 14 AWG | 15* | 20* | 25* |
| 12 AWG | 20* | 25* | 30* |
| 10 AWG | 30* | 35* | 40* |
| 8 AWG | 40 | 50 | 55 |
| 6 AWG | 55 | 65 | 75 |
| 4 AWG | 70 | 85 | 95 |
| 3 AWG | 85 | 100 | 110 |
| 2 AWG | 95 | 115 | 130 |
| 1 AWG | 110 | 130 | 145 |
| 1/0 AWG | 125 | 150 | 170 |
| 2/0 AWG | 145 | 175 | 195 |
| 3/0 AWG | 165 | 200 | 225 |
| 4/0 AWG | 195 | 230 | 260 |
Bookmark-Friendly Quick-Jump Rows
- 12 AWG (20A): Standard 120V receptacle circuits, kitchen countertop small-appliance branches, and bathroom GFCI circuits.
- 10 AWG (30A): Electric dryers (120/240V), standard RV shore power hookups, and window AC units requiring dedicated 30A feeds.
- 6 AWG (55A/65A): Commonly used for 50A hot tubs, EV Level 2 chargers (hardwired), and electric ranges. (Note: 6 AWG copper in the 60°C column is 55A, so for a strict 60A breaker, you often need 4 AWG copper unless specific 75°C equipment allowances apply).
- 2 AWG (95A/115A): The standard feeder wire for 100A residential subpanels (using the 75°C column for 115A allows protection at 100A).
Derating Factors and What the Table Cannot Tell You
The baseline numbers in Table 310.16 assume you are working in a 30°C (86°F) ambient environment with no more than three current-carrying conductors bundled together. When real-world jobsite conditions deviate from this, you must apply derating factors.
How Derating Modifies the Base Value
When you pull more than three current-carrying conductors through a single conduit, the trapped heat reduces the wire's ability to dissipate thermal energy. You must multiply the base ampacity by a correction factor found in NEC Table 310.15(C)(1).
Worked Example: You are pulling four current-carrying conductors (two 240V circuits sharing a neutral, or just 4 distinct hot wires) through an EMT conduit in a 40°C (104°F) attic. You want to use 10 AWG THHN for a 30A circuit.
- Start at the 90°C column for THHN: 10 AWG = 40A.
- Apply bundling derating (4-6 conductors = 80%): 40A × 0.80 = 32A.
- Apply ambient temperature derating (40°C = 91% correction for 90°C wire): 32A × 0.91 = 29.12A.
- Result: 29.12A is less than the required 30A breaker size. You must upsize to 8 AWG THHN to pass inspection.
For a deeper dive into the mathematical framework behind NEC Article 310 conductor rules, consult your local AHJ or a licensed master electrician, as local amendments can alter these baseline tables.
What the Ampacity Table Cannot Tell You
Relying solely on Table 310.16 will lead to installation failures if you ignore these three physical realities:
- Voltage Drop: The NEC ampacity tables do not account for distance. If you are running a 50A EV charger 150 feet from the panel, 6 AWG copper will safely handle the heat, but the voltage drop will exceed the recommended 3% threshold, causing the charger to fault or operate inefficiently. You must upsize to 4 AWG or 3 AWG purely for voltage drop mitigation.
- Physical Lug Fitment: A 100A breaker lug is physically designed to accept up to #1 AWG or 1/0 AWG wire. If your derating math forces you to use 2/0 AWG wire, it will not physically fit into the breaker terminal. You must use a rated terminal lug reducer or a larger breaker frame.
- Short-Circuit Let-Through Energy: Ampacity measures continuous thermal load. It does not tell you how the wire will survive a 10,000-amp short circuit event before the breaker's magnetic trip clears the fault. Proper breaker coordination is required for that.
Wire Ampacity Rating FAQ
Which wire ampacity rating column applies to my residential installation?
For almost all standard indoor residential branch circuits using Romex (NM-B cable) and wires 14, 12, or 10 AWG, you must use the 60°C column per NEC 110.14(C)(1)(a). Even though the THHN wires inside some modern cables might have 90°C printed on the jacket, the NM-B assembly as a whole, and the terminations on standard 15A/20A receptacles, are limited to 60°C. For larger feeder wires (like 2 AWG for a subpanel) and equipment rated over 100A, you can typically use the 75°C column, provided the breaker lugs are explicitly marked 75°C.
Can I use the 90°C ampacity column to size my breaker?
No. You cannot use the 90°C column to determine your final breaker size because standard residential breakers, busbars, and receptacles do not have 90°C rated terminations. The 90°C column is strictly a mathematical starting point. You use the 90°C value to apply your derating factors (for heat and bundling), and then you verify that the final derated number is still equal to or greater than the required ampacity in the 60°C or 75°C column.
How does ambient temperature in an attic change the wire ampacity rating?
Table 310.16 assumes an ambient temperature of 30°C (86°F). If you run NM-B cable or THHN conductors across attic joists where summer temperatures routinely exceed 104°F (40°C) or 122°F (50°C), the wire's ability to shed heat drops. You must apply the temperature correction factors in NEC Table 310.15(B)(1). For example, NM-B cable in a 50°C (122°F) attic must be derated to 58% of its base 60°C ampacity. A 12 AWG wire (base 20A) drops to 11.6A, meaning it can no longer safely carry a 15A or 20A load without upsizing the wire or adding ventilation.
Does the ground wire count towards derating the ampacity rating?
No. An equipment grounding conductor (EGC), whether it is a bare copper wire or a green insulated wire, does not carry current under normal operating conditions. Therefore, it does not generate heat and is not counted as a current-carrying conductor when applying bundling derating factors. Similarly, a neutral wire that only carries the unbalanced load from a standard 120/240V split-phase residential circuit (like a dryer or range) is not counted as a current-carrying conductor for derating purposes per NEC 310.15(E).
