Current Capacity Calculator
Free Current capacity Calculator for electrical engineering projects. Enter dimensions to get material lists and cost estimates.
Formula
Derated Ampacity = Base Ampacity x Temp Factor x Fill Factor
Start with the base ampacity from NEC Table 310.16 for the wire gauge and insulation temperature rating. Multiply by the ambient temperature correction factor from Table 310.15(B)(1). Then multiply by the conduit fill adjustment factor from 310.15(C)(1) based on the number of current-carrying conductors. The result is the maximum safe current the wire can carry continuously.
Worked Examples
Example 1: Standard Circuit at 75C
Problem: Find the derated ampacity of 10 AWG THHN wire with 6 conductors in conduit at 95F ambient.
Solution: Base ampacity (75C) = 30A\nTemp correction (35C/95F) = 0.82\nFill derating (4-6 conductors) = 0.80\nDerated = 30 x 0.82 x 0.80 = 19.7A\nContinuous (80%) = 15.7A
Result: 19.7A derated, 15.7A continuous capacity
Example 2: Heavy Feeder Run
Problem: Find capacity of 4/0 AWG at 75C with 3 conductors at normal temperature.
Solution: Base ampacity = 230A\nTemp correction (30C) = 1.00\nFill derating (3 conductors) = 1.00\nDerated = 230 x 1.00 x 1.00 = 230A\nContinuous = 184A
Result: 230A full capacity, 184A continuous
Frequently Asked Questions
What is the current capacity (ampacity) of copper wire?
Current capacity, or ampacity, is the maximum amount of electrical current a conductor can carry continuously without exceeding its insulation temperature rating. Per NEC Table 310.16, common copper wire ampacities at 75 degrees C are: 14 AWG at 15 amps, 12 AWG at 20 amps, 10 AWG at 30 amps, 8 AWG at 45 amps, 6 AWG at 65 amps, 4 AWG at 85 amps, 2 AWG at 115 amps, and 1/0 at 150 amps. These ratings assume no more than 3 current-carrying conductors in a raceway and an ambient temperature of 86 degrees F (30 degrees C). Higher temperatures or more conductors require derating.
What is conduit fill derating for current capacity?
When multiple current-carrying conductors share a conduit, each conductor generates heat that affects the others, requiring ampacity reduction per NEC 310.15(C)(1). For 4 to 6 conductors, ampacity is reduced to 80 percent of the table value. For 7 to 9 conductors, it drops to 70 percent. For 10 to 20 conductors, the factor is 50 percent. For 21 to 30 conductors, it is 45 percent. Neutral conductors carrying only unbalanced current are typically not counted, nor are equipment grounding conductors. This derating stacks with temperature correction, so a wire in a hot conduit with many conductors can have significantly reduced capacity.
Can I use Current Capacity Calculator on a mobile device?
Yes. All calculators on NovaCalculator are fully responsive and work on smartphones, tablets, and desktops. The layout adapts automatically to your screen size.
Is my data stored or sent to a server?
No. All calculations run entirely in your browser using JavaScript. No data you enter is ever transmitted to any server or stored anywhere. Your inputs remain completely private.
How do I interpret the result?
Results are displayed with a label and unit to help you understand the output. Many calculators include a short explanation or classification below the result (for example, a BMI category or risk level). Refer to the worked examples section on this page for real-world context.
Does Current Capacity Calculator work offline?
Once the page is loaded, the calculation logic runs entirely in your browser. If you have already opened the page, most calculators will continue to work even if your internet connection is lost, since no server requests are needed for computation.