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.
Reviewed by Daniel Agrici, Founder & Lead Developer
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.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy