Cable ampacity sizing equation
This calculator estimates the minimum reference ampacity a conductor should have after sizing and installation derating.
Select the first conductor where Itable ≥ Ireqwhere:
- Ireq
- Required reference ampacity after correction[A]
- Iload
- Load current entered by the user[A]
- 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 table[A]
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.
How to calculate cable size
Cable sizing starts with the design current and then applies the project sizing factor, conductor material, and installation derating. The selected cross-section must have a reference ampacity at least equal to the corrected current.
- Determine the normal load current.
- Apply the required continuous-load or design margin.
- Select copper or aluminum.
- Apply installation, ambient, and grouping correction factors.
- Verify voltage drop, short-circuit withstand, and protective-device coordination.
Cable sizing example
A 32 A copper load with a 125% sizing factor requires a 40 A reference ampacity before any additional installation derating. In the calculator's simplified table, the first listed conductor that reaches this reference is 10 mm².
Ireq = 32 A × 1.25 = 40 A
| Input or result | Value |
|---|---|
| Load current | 32 A |
| Sizing factor | 125% |
| Installation | Normal reference |
| Required reference ampacity | 40 A |
| Preliminary conductor | 10 mm² copper |
Cable ampacity vs. cable cross-section
Cross-sectional area alone does not define ampacity. Insulation temperature rating, number of loaded conductors, installation method, ambient temperature, grouping, soil conditions, terminal ratings, and local tables can change the allowable current for the same conductor size.
Why aluminum cable usually needs a larger size
Aluminum has higher electrical resistivity than copper, so an aluminum conductor normally needs a larger cross-section for a comparable current and voltage-drop target. Terminals, joint preparation, mechanical properties, and approved conductor material must also be checked.
Checks required after cable sizing
Treat this result as a starting size. A final design must verify the adopted ampacity table, voltage drop at normal and starting conditions, fault-loop performance, short-circuit thermal withstand, neutral loading, harmonics, and compatibility with the selected protective device.
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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.