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
Selecting the wrong wire gauge for a circuit is one of the most common and most dangerous electrical code violations. The 2026 National Electrical Code (NEC), Article 310 establishes minimum ampacity ratings for conductors based on wire size, insulation type, ambient temperature, number of current-carrying conductors in a conduit, and installation method. A 12 AWG copper wire with THHN insulation in free air carries 30 amperes. The same wire in a conduit with five other current-carrying conductors, in an ambient temperature of 40C, carries significantly less due to derating. Missing these corrections is what causes overheated conductors, tripped breakers under load, and, in the worst cases, electrical fires from conductors running hot inside walls. This calculator applies NEC 2026 Table 310.16 ampacity values, the ambient temperature correction factor from Table 310.15(B)(1), and the conductor bundling derating factor from Table 310.15(C)(1) to give you the actual usable ampacity for your specified installation conditions.
What This Calculator Does
This calculator determines the correct wire gauge for a circuit or verifies that a selected gauge is adequate for the load. Input the conductor material (copper or aluminum), insulation type (THHN, THWN-2, XHHW-2, etc.), installation method (conduit, cable, free air), ambient temperature, and number of current-carrying conductors. The calculator returns the base ampacity from NEC Table 310.16 and the derated ampacity after temperature and bundling corrections.
The Formula
Base ampacity comes from NEC Table 310.16 based on wire size, conductor material, and insulation temperature rating. Temperature correction factor is applied when ambient temperature exceeds the 30C reference temperature in the table. For THHN (90C rated) in 40C ambient: correction factor = 0.91. Bundling derating applies when four or more current-carrying conductors share a raceway. Four to six conductors: multiply by 0.80. Seven to nine: multiply by 0.70. Ten to twenty: multiply by 0.50. Both factors multiply against the base ampacity to produce the usable ampacity for that specific installation.
Step-by-Step Example
Identify the load current requirement
Calculate the load in amperes. A 240V, 7,200W electric range: 7,200W / 240V = 30A continuous load. For continuous loads, NEC 210.19(A) requires the conductor to carry 125% of the load: 30A x 1.25 = 37.5A minimum ampacity required.
Select base conductor from NEC Table 310.16
For 37.5A minimum, copper THHN in conduit: 8 AWG has 50A base ampacity, 10 AWG has 35A (insufficient). Minimum code-compliant conductor: 8 AWG copper THHN.
Apply temperature correction if ambient exceeds 30C
Conduit runs through an unconditioned attic at 45C ambient. THHN (90C rated) temperature correction factor at 45C: 0.87. Derated ampacity: 50A x 0.87 = 43.5A. Still above the 37.5A requirement. 8 AWG remains adequate.
Apply bundling derating if applicable
The conduit contains 6 current-carrying conductors (three circuits sharing a conduit). Bundling factor for 4-6 conductors: 0.80. Final derated ampacity: 50A x 0.87 x 0.80 = 34.8A. Now below the 37.5A requirement. Upsize to 6 AWG THHN (65A base): 65A x 0.87 x 0.80 = 45.2A. 6 AWG is now the correct selection.
Real-World Use Cases
Commercial Kitchen Equipment Circuit
A 208V, 11 kW commercial convection oven on a dedicated circuit: 11,000W / 208V = 52.9A load. Continuous load x 1.25 = 66.1A required. In a 50C ambient kitchen environment with THHN/THWN-2: temperature correction at 50C = 0.82. 4 AWG copper (85A base): 85 x 0.82 = 69.7A. Single conductor in conduit, no bundling. 4 AWG copper THHN is the minimum compliant conductor.
Rooftop HVAC Disconnect Wiring
A 20-ton commercial rooftop unit (RTU) with 208V/3-phase power draws 62A full load amps. Rooftop conduit in Phoenix, AZ can reach 60C ambient. THWN-2 (90C rated) temperature correction at 60C: 0.71. Required ampacity: 62A x 1.25 = 77.5A. 4 AWG (85A base): 85 x 0.71 = 60.4A (insufficient). 3 AWG (100A base): 100 x 0.71 = 71A (still insufficient). 2 AWG (115A base): 115 x 0.71 = 81.65A. Minimum conductor: 2 AWG copper THWN-2.
Residential EV Charger Installation
A 240V, 48A Level 2 EV charger (continuous load). Required ampacity: 48A x 1.25 = 60A. In a garage with 30C ambient, single conductor in conduit: 6 AWG copper THHN provides 65A base ampacity. No temperature or bundling derating needed. Branch circuit breaker: 60A. Wire: 6 AWG copper THHN with 6 AWG ground. NEC 210.12 also requires AFCI on 120V branch circuits in garages for dwellings post-2023.
Comparison
| AWG (Copper) | Base Ampacity (THHN, 75C col) | Base Ampacity (THHN, 90C col) | Common Application |
|---|---|---|---|
| 14 AWG | 15A | 20A | General lighting/outlets (15A circuits) |
| 12 AWG | 20A | 25A | Kitchen/bath outlets (20A circuits) |
| 10 AWG | 30A | 35A | Dryers, A/C units, EV chargers (30A) |
| 8 AWG | 50A | 55A | Ranges, water heaters, 50A circuits |
| 6 AWG | 65A | 75A | Subpanels, large HVAC, EV (50-60A) |
| 4 AWG | 85A | 95A | Commercial HVAC, large subpanels |
| 2 AWG | 115A | 130A | Service entrances, large feeders |
Common Mistakes to Avoid
Using the 90C column ampacity without considering the terminal rating limitation. NEC 110.14(C) requires that conductors be sized based on the lowest temperature rating in the circuit, typically the termination point. Most breakers and devices are rated 60C or 75C. You can use 90C-rated wire but must derate to the 75C column ampacity if the terminals are 75C rated.
Counting all conductors in a conduit when calculating bundling derating. Only current-carrying conductors are counted. Equipment grounding conductors are never counted. Neutral conductors in 3-phase balanced wye systems are not counted. A conduit with three 120V circuits plus a neutral: only the three phase conductors count unless the system is not balanced.
Ignoring ambient temperature derating for attic and rooftop runs. In southern U.S. climates, unconditioned attics regularly reach 55 to 70C in summer. THHN ampacity in a 70C attic is reduced by a factor of 0.58. Conductors sized without this correction will run hot under load and shorten insulation life.
Applying voltage drop sizing as if it replaces NEC minimum ampacity. A voltage drop calculation may indicate 10 AWG for a long 20A circuit run. That result is separate from the minimum ampacity requirement. Both requirements must be met, and the larger resulting conductor size governs.
Frequently Asked Questions
Accuracy and Disclaimer
This calculator provides wire sizing reference based on NEC 2026 Table 310.16 values and standard derating factors. Actual conductor sizing must be verified by a licensed electrician or electrical engineer against the complete NEC installation requirements, local amendments, and site-specific conditions. This tool does not replace a qualified electrical design review or AHJ (Authority Having Jurisdiction) approval.
Conclusion
Wire gauge selection tied to corrected NEC ampacity values is the foundation of code-compliant electrical installations. Once you have confirmed your conductor sizing, use the Conduit Fill Calculator to verify your selected conduit size accommodates the conductors without exceeding NEC Article 358/362 fill limits, and the Electrical Bid Estimator to price the complete installation.
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