Cable Ampacity Calculator
Determine cable ampacity (current-carrying capacity) based on NEC tables and derating factors. Enter values for instant results with step-by-step formulas.
Formula
Derated Ampacity = Base Ampacity * Temp Factor * Fill Factor
Base ampacity from NEC Table 310.16 is multiplied by the temperature correction factor from Table 310.15(B)(1) and the conduit fill adjustment factor from Table 310.15(C)(1). The result must be greater than or equal to the load current (times 1.25 for continuous loads).
Worked Examples
Example 1: Branch Circuit in Warm Attic
Problem: Size a copper THWN-2 conductor for a 30A continuous load in a conduit with 3 current-carrying conductors. The conduit runs through an attic at 50C ambient temperature.
Solution: Required ampacity = 30A * 1.25 (continuous) = 37.5A\nTemperature correction at 50C = 0.82\nConduit fill (3 conductors) = 1.00\nDerated ampacity needed = 37.5 / (0.82 * 1.00) = 45.7A\nFrom NEC Table: 8 AWG copper THWN-2 = 55A base\nDerated: 55 * 0.82 * 1.00 = 45.1A\nThis is slightly under 45.7A, so step up to 6 AWG = 75A base\nDerated: 75 * 0.82 = 61.5A - passes
Result: Use 6 AWG copper THWN-2 (61.5A derated capacity for 37.5A required)
Example 2: Multi-Conductor Conduit Run
Problem: A conduit contains 8 current-carrying copper THWN-2 conductors at 35C ambient. Each circuit carries 20A. What minimum wire gauge is needed?
Solution: Required ampacity = 20A (assuming non-continuous)\nTemperature correction at 35C = 0.96\nConduit fill (8 conductors) = 0.70\nCombined derating = 0.96 * 0.70 = 0.672\nDerated ampacity needed = 20 / 0.672 = 29.8A\nFrom NEC Table: 10 AWG = 40A base\nDerated: 40 * 0.672 = 26.9A - fails\n12 AWG = 30A, derated: 30 * 0.672 = 20.2A - fails\nStay with 10 AWG: need to verify 26.9 > 20A? No, need 29.8A.\nUse 8 AWG = 55A base, derated: 55 * 0.672 = 37.0A - passes
Result: Use 8 AWG copper THWN-2 (37.0A derated for 29.8A required)
Frequently Asked Questions
What is cable ampacity and why is it important?
Cable ampacity is the maximum continuous current a conductor can safely carry without exceeding its temperature rating. It is one of the most critical parameters in electrical system design because exceeding ampacity causes the conductor insulation to overheat, degrade, and eventually fail, creating fire and electrocution hazards. The National Electrical Code (NEC) establishes ampacity tables and derating rules that are legally enforceable in most jurisdictions. Proper ampacity calculations ensure that wiring is safe, code-compliant, and will pass electrical inspections. Undersized conductors waste energy as heat, reduce voltage at the load, and shorten the lifespan of the wiring system. Always size conductors based on derated ampacity, not the base table values.
How does ambient temperature affect cable ampacity?
Ambient temperature directly reduces cable ampacity because the insulation rating specifies a maximum conductor temperature, not a maximum temperature rise. At higher ambient temperatures, the allowable temperature rise is smaller, so the conductor can carry less current. NEC Table 310.15(B)(1) provides temperature correction factors. For example, a THWN-2 conductor rated at 90 degrees Celsius has full ampacity at 30 degrees ambient but only 82 percent at 50 degrees ambient. In hot environments like attics, boiler rooms, or desert climates where ambient temperatures regularly exceed 40 degrees Celsius, the derating can reduce usable ampacity by 10 to 30 percent. This is why it is critical to consider installation environment when sizing conductors.
How accurate are the results from Cable Ampacity Calculator?
All calculations use established mathematical formulas and are performed with high-precision arithmetic. Results are accurate to the precision shown. For critical decisions in finance, medicine, or engineering, always verify results with a qualified professional.
What formula does Cable Ampacity Calculator use?
The formula used is described in the Formula section on this page. It is based on widely accepted standards in the relevant field. If you need a specific reference or citation, the References section provides links to authoritative sources.
Can I share or bookmark my calculation?
You can bookmark the calculator page in your browser. Many calculators also display a shareable result summary you can copy. The page URL stays the same so returning to it will bring you back to the same tool.
How do I get the most accurate result?
Enter values as precisely as possible using the correct units for each field. Check that you have selected the right unit (e.g. kilograms vs pounds, meters vs feet) before calculating. Rounding inputs early can reduce output precision.