Cable ampacity sizing equation
This calculator estimates the minimum reference ampacity a conductor should have after sizing and installation derating.
Ireq = Iload × F / (Kd × Km)Select the first conductor where Itable ≥ Ireqwhere:
- Ireq
- Required reference ampacity after correctionA
- Iload
- Load current entered by the userA
- F
- Sizing factor, such as 125% for additional margin
- Kd
- Installation derating factor
- Km
- Material factor for copper or aluminum
- Itable
- Ampacity from the internal reference conductor tableA
Use the selected size as a starting point only; final cable selection depends on insulation, terminal temperature, grouping, ambient temperature, and the local wiring code.
Assumptions
- Reference copper ampacity table for preliminary sizing
- Aluminum is estimated with a reduced ampacity factor
- Local cable standard is not embedded in this first version
Important Warnings
- Final cable size must be checked against IEC, NEC, BS, or the applicable local standard.
- Voltage drop and short-circuit withstand must be verified separately.
FAQ
Why is this called a reference size?
Cable sizing depends on insulation, installation method, ambient temperature, grouping, and local code tables. This tool gives a starting point only.
Should I also run the voltage drop calculator?
Yes. A cable can pass ampacity but fail voltage-drop requirements on a long run.