NEC Table 310.16 — Conductor Ampacity Reference

NEC Table 310.16 — Allowable Ampacities of Insulated Conductors

Table 310.16 is the primary ampacity table for conductors rated 0 through 2000 volts. It provides the maximum continuous current a conductor can carry without exceeding its temperature rating, based on not more than three current-carrying conductors in a raceway, cable, or earth, and an ambient temperature of 30 degrees C. The table is organized by conductor size (AWG or kcmil) and provides ampacity values for three temperature ratings of the conductor insulation: 60 degrees C, 75 degrees C, and 90 degrees C.

The temperature rating column used for sizing must correspond to the lowest temperature rating of any component in the circuit. This includes the conductor insulation, the terminals of the overcurrent device (breaker), and the terminals of the equipment being served. For example, if a breaker has 75 degrees C rated terminals and the wire is rated 90 degrees C, you must use the 75 degrees C column for sizing.

Copper Conductors

Aluminum or Copper-Clad Aluminum Conductors

How to Use NEC Table 310.16

Using Table 310.16 correctly requires understanding several key concepts. First, identify the conductor material (copper or aluminum) and the temperature rating of the circuit's terminations. The temperature rating is typically printed on the circuit breaker and on the equipment terminals. Most residential breakers manufactured since 2000 are rated for 75 degrees C, but many older breakers and small devices are rated for 60 degrees C only.

Second, locate the wire gauge in the left column and read across to the appropriate temperature column. The value shown is the maximum continuous ampacity for that conductor under standard conditions. For example, 10 AWG copper at 75 degrees C has an ampacity of 35 amps. This means a 10 AWG copper conductor can safely carry up to 35 amps of continuous current when installed with not more than three current-carrying conductors in a raceway at an ambient temperature of 30 degrees C.

Third, apply any required adjustment factors. If more than three current-carrying conductors share a raceway, the ampacity must be reduced per NEC Table 310.15(C)(1). For 4-6 conductors, multiply the table value by 0.80; for 7-9 conductors, multiply by 0.70; for 10-20 conductors, multiply by 0.50. If the ambient temperature exceeds 30 degrees C, apply the correction factors from NEC Table 310.15(B)(1). When both conditions exist (bundled conductors and high ambient temperature), both factors must be applied.

Fourth, for continuous loads (those expected to last 3 hours or more), the conductor must be sized at 125% of the continuous load current. This means if your continuous load is 32 amps, you need a conductor rated for at least 40 amps (32 × 1.25 = 40). The overcurrent protection device must also be rated at 125% of the continuous load unless it is specifically listed for 100% continuous rating.

NEC Chapter 9, Table 8 — Conductor DC Resistance

Table 8 in NEC Chapter 9 provides the direct-current resistance of conductors at 75 degrees C. These resistance values are used in the standard voltage drop formula to determine the voltage loss over a given conductor length. The table provides values for both uncoated and coated (tin or nickel) copper conductors, as well as aluminum conductors. The values used in our calculator are for uncoated copper conductors, which are the most common type in building wiring.

The resistance values are expressed in ohms per 1000 feet of conductor length. To calculate the voltage drop for a specific installation, multiply the resistance per 1000 feet by the one-way conductor length (in feet) and the current (in amps), then divide by 1000. For single-phase circuits, multiply by 2 to account for the return path. For three-phase circuits, multiply by 1.732 (the square root of 3).

Voltage Drop Formulas

The standard formulas for calculating voltage drop using the resistance values from Chapter 9, Table 8 are shown below. These formulas assume a unity power factor and use DC resistance values, which provide sufficiently accurate results for most residential and commercial applications. For large conductors (over 250 kcmil) or circuits with significant reactive loads, the AC impedance values from Chapter 9, Table 9 should be used instead.

Important Notes and Limitations

The values presented on this page are extracted from the NEC for educational and reference purposes. Several important limitations and considerations apply when using these tables for actual electrical installations:

• Ambient Temperature: Table 310.16 values assume an ambient temperature of 30 degrees C (86 degrees F). For installations in attics, enclosed ceilings, or hot environments, ambient temperature correction factors from Table 310.15(B)(1) must be applied.
• Conductor Bundling: When more than three current-carrying conductors share a raceway or cable, the ampacity must be reduced per Table 310.15(C)(1) to account for reduced heat dissipation.
• Continuous Loads: For loads expected to operate for 3 hours or more, conductors and overcurrent protection devices must be sized at 125% of the continuous load current, unless the device is specifically listed for 100% continuous loading.
• Voltage Drop: The 3% and 5% voltage drop recommendations are Informational Notes in the NEC, not mandatory requirements. However, they represent accepted industry practice and most inspectors enforce them. Some jurisdictions have adopted local amendments making these limits mandatory.
• NEC Edition: The values shown are representative of the 2020 NEC. Your jurisdiction may have adopted a different edition or may have local amendments. Always verify against the locally adopted code edition.
• AC vs DC Resistance: The resistance values from Table 8 are DC values. For larger conductors (250 kcmil and above), the AC impedance from Table 9 may produce more accurate voltage drop results due to skin effect and reactance.