Derating Wire in Conduit Guide
Wire derating is a heat management step. It helps show how much current a conductor can carry after real installation conditions are considered. A wire in free air cools better than a wire packed inside a conduit. Several current carrying conductors in one raceway create more heat. High ambient temperature also reduces cooling. This calculator joins those two common adjustments in one place.
Why Derating Matters
Conductor insulation has a temperature limit. Terminals also have limits. A wire may have a high insulation rating, yet the equipment terminal may allow a lower ampacity. That lower terminal value can control the final usable rating. This is why the calculator includes a terminal cap. It prevents the result from relying only on insulation temperature.
Input Planning
Start with the correct base ampacity. Use the conductor material, size, insulation, and wiring method from your approved table. Then count only current carrying conductors as required by your project rules. Equipment grounding conductors are often treated differently. Some neutrals may count, and some may not. Local interpretation matters, so document your assumptions.
Heat and Load Review
The ambient temperature field accounts for hotter routing areas. Attics, roofs, equipment rooms, and crowded conduits can raise heat exposure. The calculator converts Fahrenheit to Celsius when needed. It then selects a matching correction factor. If the factor becomes zero, the entered condition is beyond the simplified table range.
Using the Result
The final result compares usable ampacity with the entered load. A spare margin can be added for design comfort. A continuous load multiplier can also be entered. For example, use 125 when you want the load multiplied by 1.25. The output is an estimating aid. It does not replace conduit fill checks, voltage drop design, short circuit review, equipment markings, or adopted code requirements.
Professional Review
Use the CSV and PDF exports to save the calculation record. Keep the project table source with it. Review special conductor rules, rooftop adders, cable tray rules, and local amendments before installation. Final conductor sizing should be approved by a qualified electrical professional.