AWG Wire Rating Chart: Ampacity, Sizing, and NEC Derating Rules

The AWG wire rating (ampacity) for standard copper conductors in residential and commercial wiring is dictated by the National Electrical Code (NEC) Table 310.16. For typical branch circuits using NM-B or THHN wire terminated on standard devices, the baseline ratings are: 14 AWG for 15 amps, 12 AWG for 20 amps, 10 AWG for 30 amps, and 8 AWG for 40 amps.

Bookmark-Friendly Quick-Jump: Most Queried AWG Sizes

• 15A Circuit (Lighting/General Receptacles): 14 AWG (60°C column)
• 20A Circuit (Kitchen/Bath/Garage Receptacles): 12 AWG (60°C column)
• 30A Circuit (Dryers/Water Heaters): 10 AWG (60°C column)
• 40A Circuit (Ranges/HVAC Condensers): 8 AWG (60°C column)
• 50A Circuit (Subpanels/Welders/Hot Tubs): 6 AWG (75°C column)

How to Read the NEC AWG Wire Rating Table

The ampacity table below is sourced directly from NFPA 70 (NEC) Table 310.16. Before looking at the numbers, you must understand the baseline assumptions baked into the chart. These values assume:

1. Conductor Material: Copper (aluminum has a completely different, lower ampacity chart).
2. Ambient Temperature: 30°C (86°F). If your attic or conduit run exceeds this, the base rating drops.
3. Conductor Count: No more than three current-carrying conductors bundled together in a single raceway or cable.

The table is divided into three temperature columns: 60°C, 75°C, and 90°C. These do not represent the temperature of the room; they represent the thermal rating of the wire's insulation (e.g., TW, THW, THHN) and the termination points (lugs and breakers) it connects to.

Complete Copper AWG Wire Rating Chart (NEC 310.16)

Source: NFPA 70 (National Electrical Code), Table 310.16. Values are for copper conductors, 30°C ambient, up to 3 current-carrying conductors.

Which Temperature Column Applies to Your Installation

The most common mistake DIYers and junior apprentices make is looking at a spool of 90°C THHN wire, reading the 90°C column, and assuming they can push that wire to its absolute thermal limit. You cannot. NEC Article 110.14(C) enforces the "weakest link" rule for terminations.

Your wire's final allowable ampacity is capped by the lowest temperature rating of any connected component. Standard residential breakers, receptacles, and switches are almost universally rated for 75°C, and for circuits 100A or less, the NEC mandates you use the 60°C column unless the equipment is explicitly marked otherwise.

Therefore, if you pull 12 AWG THHN (rated 90°C) through a conduit to a standard 20A breaker and a 15A/20A duplex receptacle, you must size your overcurrent protection using the 60°C column. The 60°C rating for 12 AWG is 20A. The 90°C rating is irrelevant for the final breaker size.

Derating Rules: How Bundling and Ambient Heat Modify Ampacity

The base numbers in the chart above fall apart the moment you change the environment. If you run wires through a hot attic or bundle multiple circuits in a single conduit, the wires cannot dissipate heat effectively. This is where the 90°C column actually becomes useful: you use the 90°C column to calculate derating, then cap the final result at the 60°C or 75°C termination limit.

If your ambient temperature exceeds 30°C (86°F), you must apply temperature correction factors from NEC Table 310.15(C)(1) before applying the bundling derating multiplier. The math stacks multiplicatively, which is why running NM-B cable (which is already limited to the 60°C column) through hot, insulated attics often requires upsizing the wire gauge entirely.

Decision Tree: Picking the Exact Wire Size for Your Circuit

Use this decision matrix to lock in your wire gauge and breaker size for standard 120V/240V single-phase residential loads. Always round up your continuous loads (loads running for 3+ hours) by 125% before consulting this table.

What This Table Cannot Tell You

Ampacity charts only solve for thermal melting and insulation degradation. They do not account for power quality or physical installation limits. When planning a run, you must also verify:

• Voltage Drop: NEC Chapter 9, Table 8 provides DC resistance per 1,000 feet. For long runs (typically over 50 feet for 120V circuits or 100 feet for 240V circuits), the wire might be thermally safe but still suffer from excessive voltage drop. The NEC recommends keeping voltage drop under 3% for branch circuits and 5% total from the service entrance. If your run is 150 feet to a detached garage on a 20A circuit, 12 AWG will overheat the equipment via low voltage; you must upsize to 10 AWG or 8 AWG purely for voltage drop mitigation.
• Conduit Fill Capacity: You cannot physically stuff an infinite number of 10 AWG wires into a half-inch EMT conduit. NEC Chapter 9, Table 1 limits conduit fill to 40% for three or more wires. Always check the physical cross-sectional area of the wire insulation before buying your conduit.
• Short-Circuit Withstand: Ampacity assumes normal operating loads. Under a dead short, the breaker must trip fast enough to prevent the wire from vaporizing. This is generally handled by standard breaker trip curves, but if you are working near high-fault-current service entrances, the wire must be rated for the available fault current.

For comprehensive wiring design methods and termination rules, refer to the EC&M guide on ampacity and wire sizing. Always treat NEC tables as baseline guidance; your local Authority Having Jurisdiction (AHJ) or municipal inspector has the final say on code compliance and acceptable installation practices in your specific region.