If you need the direct answer for standard residential branch circuits: 14 AWG is rated for 15 amps, 12 AWG for 20 amps, and 10 AWG for 30 amps. These baseline values are pulled directly from the 60°C column of the National Electrical Code (NEC) Table 310.16 for copper conductors. However, relying solely on these three numbers will lead to undersized feeders, tripped breakers, or melted insulation when you move beyond basic receptacle circuits.

This reference guide provides the complete wire and amp size chart for copper conductors, explains exactly which temperature column applies to your specific installation, and walks through the derating math that modifies these base values in real-world conditions.

The Master Wire and Amp Size Chart (NEC Table 310.16)

The following data is sourced directly from NFPA 70 (National Electrical Code) Table 310.16. It applies to copper conductors with common insulation types like THHN, THWN-2, and XHHW-2, rated for up to three current-carrying conductors in a raceway or cable, at an ambient temperature of 30°C (86°F).

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Copper Conductor Ampacities (NEC Table 310.16)
AWG / kcmil 60°C (140°F) Column 75°C (167°F) Column 90°C (194°F) Column
14 AWG15A20A25A
12 AWG20A25A30A
10 AWG30A35A40A
8 AWG40A50A55A
6 AWG55A65A75A
4 AWG70A85A95A
3 AWG85A100A110A
2 AWG95A115A130A
1 AWG110A130A145A
1/0 AWG125A150A170A
2/0 AWG145A175A195A
3/0 AWG165A200A225A
4/0 AWG195A230A260A

How to Read This Table (And Which Column Actually Applies)

The most common mistake DIYers and junior apprentices make is looking at the 90°C column because THHN wire is rated for 90°C, and assuming they can use that higher ampacity. You almost never can. The column you must use is dictated by the temperature rating of the terminations (the lugs on your breaker, panel, or receptacle), as enforced by NEC Article 110.14(C).

  • The 60°C Column: You must use this column for circuits rated 100 amps or less, unless the equipment is explicitly marked otherwise. Most standard residential receptacles and older breakers fall into this category. This is why 12 AWG is strictly limited to 20A, even though the wire itself can handle 30A at 90°C.
  • The 75°C Column: Use this for circuits over 100 amps, or for circuits 100A and under if the equipment is specifically stamped with a 75°C rating (common on modern main lugs and larger commercial breakers).
  • The 90°C Column: This column is only used as the starting baseline for derating calculations (explained below) or when terminating to equipment explicitly rated for 90°C, which is exceedingly rare in residential work.

Derating: When the Chart Lies to You

The ampacities in the chart above assume two ideal conditions: an ambient temperature of 86°F (30°C) and no more than three current-carrying conductors in a single raceway or cable. When you violate either condition, the base ampacity drops. This is where the 90°C column finally earns its keep.

According to the Copper Development Association and NEC 310.15, you apply derating factors to the 90°C ampacity, but the final derated value cannot exceed the ampacity of the temperature column required by your terminations (usually the 60°C or 75°C column).

Worked Derating Example: You are pulling two 120V circuits (4 current-carrying conductors total) through a single conduit in an attic where the ambient temperature reaches 104°F (40°C). You want to use 10 AWG THHN for a 30A circuit.

1. Base 90°C Ampacity: 40A.
2. Bundling Derating (4 conductors): 80% factor (40A × 0.80 = 32A).
3. Ambient Temp Correction (104°F): 91% factor (32A × 0.91 = 29.12A).
4. Termination Check: The final derated ampacity is 29.12A. Because your 30A breaker terminations require the 60°C column (which caps 10 AWG at 30A), 29.12A is below the 30A limit. However, because it is below the actual 30A load requirement, 10 AWG is now undersized. You must step up to 8 AWG.

What This Table Cannot Tell You

An ampacity chart only tells you the thermal limit of the wire's insulation before it degrades. It does not account for the following critical installation factors:

  • Voltage Drop: NEC Table 310.16 does not limit voltage drop. A 14 AWG wire can safely carry 15 amps for 500 feet without melting, but the voltage at the end will drop to roughly 85V, destroying motor loads. Always calculate voltage drop for runs over 50 feet, aiming for a maximum 3% drop on branch circuits and 5% on feeders.
  • Physical Lug Fit: You might calculate that 1/0 AWG is perfectly sized for a 125A feeder, but if you are landing it on a residential 125A subpanel main breaker, the lug might only be rated to accept up to 2 AWG. Always check the panel manufacturer's lug sizing specifications.
  • Short-Circuit Withstand: The chart assumes steady-state loading. Under a massive short-circuit fault, the magnetic and thermal forces can snap or melt conductors before the breaker trips. This requires fault-current calculations, not just ampacity lookups.

Decision Path: Pick Your Exact Wire Size in 4 Steps

Stop guessing. Follow this exact sequence to terminate your wire sizing decision for any standard residential or light commercial circuit.

Step Action Rule / Calculation
1. Calculate True Load Find the maximum continuous load and apply the 125% safety multiplier. Example: 16A continuous load × 1.25 = 20A minimum required ampacity.
2. Select Termination Column Identify the temperature rating of your breaker and receptacle lugs. If ≤100A, default to the 60°C column. If >100A, default to the 75°C column.
3. Apply Derating Math Check conduit fill (>3 wires) and ambient heat (>86°F). Multiply the 90°C column value by the derating factors. If the derated 90°C value is lower than your Step 2 column value, the derated value becomes your new maximum ampacity.
4. Verify Voltage Drop Measure the one-way distance from panel to the furthest receptacle. If distance > 50 ft, run a voltage drop calculation. If drop > 3%, upsize the wire by one AWG step and re-verify.

Final Default Recommendation: For a standard 20A, 120V residential branch circuit under 50 feet in length, with no more than 3 current-carrying conductors in a standard basement or wall cavity, buy 12 AWG NM-B (Romex) copper cable and terminate it on a 20A breaker. This satisfies the 60°C termination rule, provides exactly 20A of ampacity, and eliminates the need for complex derating math.