The allowable ampacity of an AWG copper cable is determined by its gauge, insulation temperature rating, and installation conditions as defined in NEC Table 310.16. For standard residential branch circuits using common NM-B (Romex) cable, the baseline ampacities are 15A for 14 AWG, 20A for 12 AWG, and 30A for 10 AWG. However, if you are pulling individual THHN conductors in conduit, the baseline numbers change, and derating factors apply.

This reference guide provides the exact data-dense table you need on the bench, explains how to select the correct temperature column, and details the derating math that inspectors actually check.

The NEC 310.16 AWG Cable Ampacity Reference Chart

How to read this table: This chart applies to copper conductors with up to three current-carrying conductors in a raceway or cable, at an ambient temperature of 30°C (86°F). The columns represent the temperature rating of the wire insulation (60°C, 75°C, and 90°C). The rows represent the American Wire Gauge (AWG) size. Bookmark this section for quick lookups of the most common residential and light commercial sizes (highlighted in bold).

Table 310.16 Allowable Ampacities of Insulated Copper Conductors (Source: NFPA 70 / NEC 2023 Edition)
AWG Size 60°C (140°F)
NM-B, TW, UF		 75°C (167°F)
THHW, THW, XHHW		 90°C (194°F)
THHN, XHHW-2
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 110A
2 AWG 95A 115A 130A
1 AWG 110A 130A 150A
1/0 AWG 125A 150A 170A
2/0 AWG 145A 175A 195A
3/0 AWG 165A 200A 225A
4/0 AWG 195A 230A 260A
Bench Tip: Notice that 8 AWG jumps from 40A (60°C) to 50A (75°C). This is a critical threshold for 40A circuits (like EV chargers or heavy welders). You cannot use 8 AWG NM-B (60°C column) for a 40A breaker; you must pull individual 8 AWG THHN in conduit to utilize the 75°C column.

Which Column Applies to Your Installation?

The most common mistake DIYers and junior electricians make is looking at the 90°C column for THHN wire and assuming they can push that full current through the circuit. In almost all residential and commercial terminations, you cannot use the 90°C column for your final breaker sizing.

The 110.14(C) Terminal Temperature Rule

According to NEC 110.14(C), the ampacity of a circuit is limited by the lowest temperature rating of any connected device, termination, or conductor. This is the 'weakest link' rule.

  • 100A and below: Standard breakers, receptacles, and lugs are typically rated for 60°C or 75°C. Unless the equipment is explicitly marked otherwise, you must use the 60°C column for circuits rated 100A or less (or sized 14 AWG through 1 AWG).
  • Over 100A: Equipment rated over 100A is generally assumed to be rated for 75°C. You may use the 75°C column.

So why buy 90°C THHN? The 90°C column is exclusively used as the starting baseline for derating calculations before you apply the terminal limit cap.

How Derating Modifies the Base Value

Table 310.16 assumes an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors (CCCs) in a raceway. If you exceed these conditions, you must derate the wire using the 90°C column as your starting point.

Worked Example: Bundling in Conduit
Imagine you are pulling four current-carrying conductors (two 240V circuits sharing a neutral, or just 4 hot wires) through a single conduit. Per NEC Table 310.15(C)(1), 4 to 6 CCCs require an 80% derating factor.

  1. Start with 10 AWG THHN in the 90°C column: 40A.
  2. Apply the 80% bundling derating factor: 40A × 0.80 = 32A.
  3. Apply the 110.14(C) terminal limit: Your breaker lugs are rated 75°C (35A for 10 AWG) or 60°C (30A for 10 AWG).
  4. Final Result: The derated ampacity (32A) is higher than the 60°C terminal limit (30A), so the wire is still legally permitted to be protected by a standard 30A breaker. If you had pulled 6 CCCs (derated to 20A), you would be forced to upsize to 8 AWG wire.
Ambient Temperature Warning: If your conduit runs across an attic space where temperatures regularly hit 110°F (43°C), you must apply the ambient temperature correction factor from the bottom of Table 310.16. At 41-45°C, the 90°C column is multiplied by 0.87. Failing to calculate this is a primary cause of melted wire insulation in attic fan circuits.

What the Ampacity Table Cannot Tell You

While NEC Table 310.16 dictates the thermal limits of the wire insulation, it does not account for the physics of voltage drop or the physical constraints of your raceways. Relying solely on ampacity charts will result in undersized feeders for long runs.

Voltage Drop Limitations

Ampacity tells you the wire won't melt; it doesn't tell you if your equipment will actually run. Electrical Contractor Magazine frequently highlights that while the NEC treats voltage drop as an 'Informational Note' rather than a strict mandate in most articles, poor voltage delivery destroys motor windings and causes LED drivers to flicker.

  • Branch Circuits: Keep voltage drop under 3% (e.g., max 3.6V drop on a 120V circuit).
  • Feeders and Branches Combined: Keep total drop under 5%.

Practical Impact: If you are wiring a detached garage subpanel 150 feet away with a 60A load, 6 AWG copper is perfectly legal for ampacity (65A at 75°C). However, 6 AWG copper over 150 feet at 60A will yield a ~4.5% voltage drop on the feeder alone. You must upsize to 4 AWG or 3 AWG copper strictly to maintain voltage, even though the ampacity table says 6 AWG is sufficient to prevent a fire.

Conduit Fill and Physical Pulling Limits

Table 310.16 assumes the wire fits in the conduit. Chapter 9, Table 1 limits conduit fill to 40% for three or more wires. If you are pulling multiple circuits, you may find that the physical cross-sectional area of the wires exceeds the conduit capacity long before you hit the thermal ampacity limit. Always cross-reference your AWG size with Chapter 9, Table 5 (Dimensions of Insulated Conductors) to ensure you aren't jamming wires, which damages insulation and creates hidden hot spots that ampacity charts cannot predict.