This calculator provides NEC 2026 ampacity ratings for copper and aluminum conductors based on temperature rating. Use lookup mode to find ampacity for a specific wire gauge or sizing mode to determine the minimum wire gauge needed for a given load. Adjust for derating factors and continuous loads per NEC 310.15(B).
Based on insulation type
Adjust for conduit fill, ambient temp
NEC requires 125% capacity for continuous loads (80% derating)
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Introduction
This Wire Gauge Ampacity is designed for professionals who need accurate and reliable calculations in their daily work. Whether you are planning finances, managing projects, or making critical business decisions, having the right numbers at your fingertips is essential. This tool provides instant results based on proven formulas, saving you time and reducing the risk of manual calculation errors. By using this calculator, you can focus on analysis and decision-making rather than spending time on complex computations. The interface is straightforward and designed for practical use, ensuring that you get the information you need quickly and efficiently.
What This Calculator Does
This wire gauge and ampacity calculator provides NEC 2026 ampacity ratings for copper and aluminum conductors based on insulation temperature rating (60°C, 75°C, or 90°C). Use ampacity lookup mode to find the current-carrying capacity of a specific wire gauge, or wire sizing mode to determine the minimum gauge needed for a given load. The calculator applies derating factors for conduit fill or ambient temperature and accounts for continuous loads requiring 125% capacity per NEC 210.19(A)(1).
The Formula
NEC Table 310.16 (formerly 310.15(B)(16)) lists base ampacity for conductors in raceway or cable at 30°C ambient temperature. For copper, 12 AWG THHN (90°C) is rated 30 amps, while 12 AWG TW (60°C) is only 25 amps. Derating factors reduce ampacity when more than 3 current-carrying conductors are in a conduit (per Table 310.15(B)(3)(a)) or when ambient temperature exceeds 30°C. Continuous loads (3+ hours) require conductors sized to 125% of load current, effectively applying an 80% derating. Terminal temperature ratings (typically 60°C or 75°C) may further limit ampacity regardless of wire insulation rating.
Step-by-Step Example
Identify load and operating conditions
Load: 25 amps continuous. Copper THHN (90°C). 4 current-carrying conductors in conduit (80% derating per NEC Table 310.15(B)(3)(a)). Ambient temp 30°C (no additional derating).
Calculate required base ampacity
Continuous factor: 25A x 1.25 = 31.25A. Apply derating: 31.25A / 0.80 = 39.1A required base ampacity.
Select wire gauge from NEC table
THHN (90°C) ampacity table: 10 AWG = 40A (meets requirement), 12 AWG = 30A (insufficient). Select 10 AWG THHN.
Verify terminal temperature rating
If terminals rated 75°C, verify 10 AWG at 75°C (35A x 0.80 = 28A adjusted). Still meets 25A load after continuous adjustment. If terminals 60°C, may need to upsize.
Real-World Use Cases
Branch Circuit Sizing
Electricians size conductors for lighting, receptacle, and appliance circuits by entering load current, selecting insulation type, applying derating for conduit fill, and adjusting for continuous loads. Ensures code compliance and safety.
Feeder and Service Entrance Conductors
Size large conductors for panel feeders, subpanel feeds, or service entrance cables by calculating total connected load, applying demand factors, and selecting wire gauge based on ampacity tables. Verify voltage drop separately for long runs.
Equipment Grounding Conductor Sizing
While this calculator focuses on current-carrying conductors, electricians reference ampacity tables to ensure equipment grounding conductors are sized per NEC Table 250.122 based on overcurrent device rating.
Common Mistakes to Avoid
Using the insulation temperature rating directly without considering terminal temperature ratings. Even if you use 90°C THHN, if the breaker or device terminals are rated 75°C, you must use the 75°C ampacity column.
Forgetting to apply the 125% factor for continuous loads. A 20-amp continuous load requires a conductor rated for at least 25 amps (20A x 1.25), which is 10 AWG copper at 75°C, not 12 AWG.
Not derating for conduit fill. When you have more than 3 current-carrying conductors in a raceway, NEC Table 310.15(B)(3)(a) requires derating: 4-6 conductors = 80%, 7-9 = 70%, 10-20 = 50%.
Confusing aluminum and copper ampacity. Aluminum requires larger wire sizes for the same ampacity. 12 AWG aluminum is only rated 20A at 75°C, while 12 AWG copper is 25A.
Ignoring ambient temperature correction. If ambient temperature exceeds 30°C (86°F), apply correction factors from NEC Table 310.15(B)(2)(a). In 40°C (104°F) ambient, 90°C wire is derated to 91% of base ampacity.
Frequently Asked Questions
Accuracy and Disclaimer
This calculator provides NEC 2026 ampacity values from Table 310.16 for general reference. Actual wire sizing must account for terminal temperature ratings (typically 60°C or 75°C per NEC 110.14(C)), voltage drop for long runs, ambient temperature correction, conduit fill derating, and specific application requirements. Always consult the full NEC code book and verify with local code amendments. Conductor ampacity is only one component of safe circuit design. Overcurrent protection (breaker or fuse sizing), grounding, and equipment compatibility must also be verified. This tool is for educational purposes and does not constitute professional electrical engineering advice. Consult a licensed electrician or electrical engineer for final wire sizing decisions.
Conclusion
This calculator provides a reliable way to perform essential calculations for your professional needs. The results are based on standard formulas and should be used as estimates for planning and analysis purposes. For critical decisions, especially those involving financial, legal, or medical matters, it is always advisable to verify results with a qualified professional. Use this tool as part of your broader decision-making process, and explore related calculators on this platform to support your comprehensive planning needs. Regular use of accurate calculation tools helps ensure consistency and precision in your professional work.
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