Cable Derating Calculator

Calculate derated cable ampacity, correction factors, load margin and required base cable current rating for ambient temperature, grouping and installation conditions.

⚡ Field Rule: Cable current rating from a table is not always usable directly. Ambient temperature, grouping, conduit/tray installation and soil/air conditions can reduce real ampacity. Use this with the Wire Gauge Calculator, Voltage Drop Calculator, Conduit Fill Calculator, Cable Tray Fill Calculator and Breaker Size Calculator.

🌡️ Cable Ampacity Derating — Heat + Grouping + Installation

Base Cable Ampacity

Load Current

Ambient Temperature

Insulation Temperature Rating

Temperature Factor

Cable Grouping Factor

Installation Factor

Installation Factor Value

Additional Factor

Safety Margin

Use correction factors from your applicable code/table when available. This calculator combines them for a practical derating check.

Presets:tray OKconduit OKhot area OK

Load Current

Total Derating Factor

Safety Margin

Available Cable Rating to Check

This tab works backwards: it tells what table/base ampacity you need before derating.

Ambient Temperature

Insulation Rating

Base Ampacity Optional

This gives a simple temperature correction factor estimate. Use actual standard tables for final cable sizing.

📐 Formula Reference

Total Derating Factor

Ftotal = Ftemp × Fgroup × Finstall × Fextra

Derated Ampacity

Iderated = base ampacity × total factor

Usable With Margin

Iusable = Iderated ÷ (1 + margin)

Required Base Rating

Ibase ≥ load current × (1 + margin) ÷ total factor

📋 Quick Reference

Common Derating Causes

High ambientheat

Grouped cablesheat

Conduit/ductless cooling

Check Together

Ampacitythermal

Voltage droplength

Short circuitwithstand

Installation

Free airbetter

Traymedium

Conduitwarmer

📚 Engineering Notes

Derating reduces usable currentA 100A cable table rating may become much lower after temperature and grouping correction factors.

Use actual standard factorsIEC/NEC/local code tables and cable manufacturer data should be used for final design.

Routing affects ampacityUse the Conduit Fill Calculator and Cable Tray Fill Calculator for physical space checks.

Complete the cable checkAlso use the Voltage Drop Calculator, Wire Gauge Calculator and Short Circuit Current Calculator.

What is a Cable Derating Calculator?

A cable derating calculator estimates the usable current capacity of a cable after applying correction factors for ambient temperature, cable grouping, installation method and safety margin.

How cable derating is calculated

The calculator multiplies the base cable ampacity by correction factors such as temperature factor, grouping factor and installation factor. The result is the derated ampacity available for the actual installation condition.

Cable sizing workflow

First choose a cable from current rating tables. Then check derating for installation conditions. After that, check voltage drop with the Voltage Drop Calculator, physical routing with the Conduit Fill Calculator or Cable Tray Fill Calculator, and breaker protection with the Breaker Size Calculator.

Important limitation

This calculator is a planning aid. Final cable sizing must follow local electrical code, manufacturer cable data, installation method, number of loaded conductors, ambient temperature, soil conditions, grouping, protective device coordination and project standards.

❓ Frequently Asked Questions

Cable derating means reducing the usable current capacity of a cable because real installation conditions make the cable run hotter than the base rating condition.

Grouped cables heat each other and have less cooling surface, so each cable may carry less current safely.

No. You should also check voltage drop, short-circuit withstand, breaker coordination, installation method and local code requirements.