Current Capacity of Wire Chart: NEC 310.16 Ampacity Reference

For standard residential copper wiring, the baseline current capacity is 15 amps for 14 AWG, 20 amps for 12 AWG, and 30 amps for 10 AWG. These values assume you are using NM-B (Romex) or UF-B cable, which are legally restricted to the 60°C column of the National Electrical Code (NEC) ampacity tables, regardless of the actual temperature rating printed on the wire jacket.

If you are pulling individual THHN/THWN conductors through conduit to a subpanel or a heavy appliance, you can often utilize the 75°C column, yielding higher ampacities like 50 amps for 8 AWG and 65 amps for 6 AWG. Below is the complete reference data you need to size your branch circuits and feeders correctly, based on NEC Table 310.16.

How to Read the NEC Ampacity Columns (And Which One Applies to You)

The most common mistake DIYers and junior apprentices make when consulting a current capacity of wire chart is looking exclusively at the highest number on the row. Modern wire insulation like THHN is rated for 90°C, but your circuit's actual ampacity is bottlenecked by the "weakest link" in the system—usually the termination lugs on your breakers, receptacles, or switches.

Here is how to read the columns and determine which applies to your installation:

• 60°C Column (TW, UF, NM-B): You must use this column for any 14, 12, or 10 AWG circuit terminating on standard residential devices (receptacles, switches, basic breakers), per NEC 110.14(C)(1)(a). It also applies to all NM-B and UF-B cables, per NEC 334.80 and 339.10.
• 75°C Column (THHW, THWN-2, XHHW): Use this column for 8 AWG and larger wires terminating on modern breakers and panel lugs (which are almost universally rated 75°C), or for 14-10 AWG wires terminating on equipment explicitly marked "75°C" (like many hardwired water heaters or HVAC disconnects).
• 90°C Column (THHN, XHHN-2): This column is almost never used for final overcurrent protection sizing. It exists primarily as the baseline for calculating derating factors (bundling and ambient temperature), which we will cover below.

Source: Adapted from NFPA 70 National Electrical Code (NEC) Table 310.16. Assumes copper conductors, 30°C ambient temperature, and no more than three current-carrying conductors in a raceway.

Derating: When Base Chart Values Drop

The ampacities listed in the chart above assume ideal conditions: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors bundled together in a conduit or cable. When you exceed these parameters, the heat generated by the wires cannot dissipate efficiently, and you must apply derating factors to the 90°C column base value.

Here is how derating rows modify the base value in practice: You multiply the 90°C ampacity by the adjustment factor, and then compare that result to the ampacity of the termination column (usually 75°C). The final allowable ampacity is the lower of the two numbers.

Real-World Derating Example

You are pulling four 12 AWG THHN current-carrying conductors through a single EMT conduit to feed two separate 20A circuits.

1. Base Value: 12 AWG in the 90°C column is 30A.
2. Adjustment Factor: 4 conductors require an 80% multiplier.
3. Calculation: 30A × 0.80 = 24A.
4. Termination Check: The 75°C column limit for 12 AWG is 25A. Since 24A is lower than 25A, your final derated ampacity is 24A.

Because 24A still exceeds your 20A breaker size, the installation is code-compliant. However, if you had pulled 6 conductors (70% factor), the math would be 30A × 0.70 = 21A, which is dangerously close to the continuous load limits of a 20A breaker.

Source: NEC Table 310.15(C)(1). Note: Equipment grounding conductors and neutral conductors that only carry unbalanced load from the same phase do not count as current-carrying for derating purposes. For comprehensive wiring rules, consult resources like the Copper Development Association.

What the Current Capacity of Wire Chart Cannot Tell You

An ampacity chart is strictly a thermal limit—it tells you the maximum current a wire can carry before its insulation begins to degrade or melt. It does not account for electrical performance, physical constraints, or fault conditions. Relying solely on the chart will lead to failures in the following scenarios:

1. Voltage Drop on Long Runs

The NEC recommends a maximum voltage drop of 3% for branch circuits and 5% for the total feeder-plus-branch system. The ampacity chart does not factor in distance. For example, a 12 AWG copper wire carrying 20 amps on a 120V circuit has a thermal capacity that is perfectly safe. However, if that run is 100 feet long, the resistance of the wire will cause a voltage drop of roughly 7.7 volts (6.4%). Your tools and appliances will receive only 112.3V, causing motors to overheat and lights to dim. To fix this, you must upsize to 10 AWG or 8 AWG to reduce resistance, even though the ampacity chart says 12 AWG is sufficient.

2. Physical Lug Sizing

Ampacity charts might tell you that 4 AWG copper is perfect for an 85-amp continuous load, but they won't tell you that a standard 85A breaker lug physically cannot accept a 4 AWG wire. You must check the manufacturer's datasheet for the specific breaker or disconnect switch to verify the maximum and minimum wire sizes the mechanical lugs can safely torque down on.

3. Short-Circuit Withstand Ratings

If a dead short occurs, thousands of amps can flow through the wire for the milliseconds it takes the breaker's magnetic trip to clear the fault. Smaller wires can vaporize or suffer catastrophic insulation blowout before the breaker trips if the available fault current at the panel is exceptionally high. The ampacity chart only addresses continuous thermal loading, not the extreme electromagnetic and thermal stresses of a short-circuit event.