Ampacity Calculator
Determine wire ampacity based on NEC Table 310.16 with temperature and conduit fill correction factors.
Results
How to Use This Calculator
Select the conductor material, either copper or aluminum. Choose the insulation temperature rating: 60C, 75C, or 90C. Common insulation types include THHN (90C), THWN (75C), and TW (60C). Enter the wire gauge from 14 AWG up to 2000 kcmil. Specify the ambient temperature if it differs from the standard 30 degrees Celsius. Enter the number of current carrying conductors in the raceway. The calculator provides the base ampacity from NEC Table 310.16, applies the ambient temperature correction factor from NEC 310.15(B)(1), and applies the conduit fill adjustment factor from NEC 310.15(C)(1). For example, 6 AWG THHN copper in a raceway with six current carrying conductors at 40C ambient has a base ampacity of 75A, corrected to 75 x 0.91 x 0.80 = 54.6A.
Understanding the Concept
Ampacity is the maximum current a conductor can carry continuously without exceeding its insulation temperature rating. NEC Table 310.16 is the primary reference for conductor ampacity under standard conditions, which assume an ambient temperature of 30 degrees Celsius and no more than three current carrying conductors in a raceway. Real world conditions often differ from these assumptions. Higher ambient temperatures reduce the conductor's ability to dissipate heat, so correction factors from NEC 310.15(B)(1) must be applied. Similarly, bundling multiple conductors in a single raceway reduces each conductor's ability to shed heat, requiring adjustment factors from NEC 310.15(C)(1). Understanding ampacity is essential because undersized conductors overheat, degrade insulation, and create fire hazards. The ampacity of a conductor determines the maximum overcurrent protection device (breaker or fuse) that can protect the circuit per NEC 240.4.
The Formula Explained
The adjusted ampacity formula is Adjusted Ampacity = Base Ampacity x Temperature Correction Factor x Conduit Fill Adjustment Factor. The base ampacity comes directly from NEC Table 310.16 for the specific conductor material, gauge, and insulation temperature rating. The temperature correction factor from NEC Table 310.15(B)(1) adjusts for ambient temperatures other than 30C. For example, at 40C ambient with 75C rated insulation, the factor is 0.88. The conduit fill adjustment factor from NEC Table 310.15(C)(1) accounts for conductor bundling: 1.0 for one to three conductors, 0.80 for four to six, 0.70 for seven to nine, and so on. Neutral conductors that carry only unbalanced current are generally not counted as current carrying conductors per NEC 310.15(E). The final adjusted ampacity determines the maximum breaker size per NEC 240.4.
Frequently Asked Questions
What is the ampacity of 12 AWG copper wire?
Per NEC Table 310.16, 12 AWG copper wire has an ampacity of 20A at 60C, 25A at 75C, and 30A at 90C. However, per NEC 240.4(D), 12 AWG copper conductors must be protected by overcurrent devices rated at no more than 20A, regardless of the insulation temperature rating. The higher ampacity values at 75C and 90C are only usable when applying temperature correction or conduit fill adjustment factors.
When do I need to apply ampacity correction factors?
You must apply the ambient temperature correction factor from NEC 310.15(B)(1) whenever the ambient temperature around the conductor exceeds 30 degrees Celsius. This commonly occurs in attics, rooftops, boiler rooms, and outdoor installations in hot climates. You must apply the conduit fill adjustment factor from NEC 310.15(C)(1) whenever more than three current carrying conductors are installed in a single raceway or cable. Both factors are multiplied together when both conditions exist.
Does the neutral wire count as a current carrying conductor?
In most cases, the neutral conductor in a single phase, line to neutral circuit is not counted as a current carrying conductor for conduit fill adjustment purposes per NEC 310.15(E). However, if the neutral carries harmonic currents, as is common in circuits supplying nonlinear loads like computers and LED drivers, the neutral must be counted. In a three phase, four wire wye system with nonlinear loads, the neutral can carry significant harmonic currents and must be counted.
What is the difference between ampacity and breaker size?
Ampacity is the maximum continuous current a conductor can safely carry based on its physical properties. Breaker size is the rating of the overcurrent protection device. NEC 240.4 requires that conductors be protected by overcurrent devices that do not exceed the conductor ampacity. For standard wire sizes 14, 12, and 10 AWG, NEC 240.4(D) limits breaker sizes to 15A, 20A, and 30A respectively, even if the conductor ampacity at higher temperature ratings would allow more. For larger conductors, the next standard breaker size above the conductor ampacity is permitted per NEC 240.4(B).
How does ambient temperature affect wire ampacity?
Higher ambient temperatures reduce wire ampacity because the conductor starts closer to its maximum insulation temperature, leaving less room for heat generated by current flow. At 40C ambient, a 75C rated conductor retains 88% of its base ampacity. At 50C ambient, it drops to 75%. At 60C ambient, only 58% remains. This is critical for wiring in hot environments such as attics in summer, which can reach 50C or higher. Use the correction factors from NEC Table 310.15(B)(1) for precise derating.