Ampacity DeratingCalculator
Apply ambient temperature correction and current-carrying conductor adjustment before final wire sizing.
Estimate the combined effect of ambient temperature correction and current-carrying conductor adjustment before final wire sizing. The calculation follows the common NEC workflow: start with a base table ampacity, apply temperature correction, apply conductor-count adjustment, then compare the result with load current and any terminal temperature limitation.
TL;DR
Multiply the base conductor ampacity by the ambient-temperature correction and current-carrying-conductor adjustment factors. Apply any terminal ampacity limit, then compare the result with the load; voltage drop and equipment requirements need separate checks.
Calculator Inputs
Base ampacity, ambient, conductors, and load
Results
Corrected and adjusted ampacity estimate
Margin after derating: -5.4 A.
Hot locations reduce conductor ampacity. Rooftops, attics, equipment rooms, and grouped raceways often need more conservative ambient assumptions than a simple 30 C design value.
This tool estimates correction and adjustment factors only. It does not handle every NEC exception, cable assembly rule, rooftop adder, continuous-load sizing rule, or equipment listing requirement.
How To Use This Tool
Start with the conductor ampacity from the table column that is permitted for correction and adjustment. Enter the number of current-carrying conductors in the raceway or cable, the expected ambient temperature, and the load current. Use the result as a planning check before final conductor selection, voltage-drop review, and NEC compliance documentation.
Use the correct conductor material, insulation, and installation table before applying any factors.
Ambient correction and conductor-count adjustment multiply together, so either factor can control the result.
After ampacity passes, verify conductor size, voltage drop, termination limits, and equipment requirements.
Related Tools
Continue from ampacity derating into full voltage-drop, sizing, reference, and standards pages.
Wire Size Calculator
Use the derated ampacity as a practical input for final conductor sizing.
AC Voltage Drop Calculator
Check voltage drop after ampacity, conductor count, and ambient conditions are acceptable.
DC Voltage Drop Calculator
Apply the same ampacity discipline to battery, PV, and controls conductors.
Wire Tables
Look up base conductor ampacity and resistance values before derating.
NEC Standards
Review code context for conductor ampacity, correction, adjustment, and voltage drop notes.
Frequently Asked Questions
How do I derate conductor ampacity under NEC?
Start with the base ampacity for the conductor material, insulation and installation method. Multiply by the applicable ambient-temperature and conductor-count factors, apply the terminal limit if entered, and compare the result with the load current.
What inputs does the wire derating calculator need?
Enter base ampacity, insulation temperature rating, ambient temperature, the number of current-carrying conductors, load current and any terminal ampacity limit. Use the correct base table before applying correction factors.
Do temperature and conductor-count derating factors both apply?
The tool multiplies both factors together. Either may reduce the final ampacity; a terminal limit can further cap the result.
Does a passing conductor derating result also verify voltage drop?
No. After the ampacity check passes, separately verify conductor size, voltage drop, termination limits and equipment requirements.
Sources and references
- NFPA 70 — National Electrical Code
The code reference for conductor ampacity, adjustment factors and termination limitations used in this workflow.
Hommer Zhao
Founder & General Manager
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