Cable Sizing Calculator Formula
Understand the math behind the cable sizing calculator. Each variable explained with a worked example.
Formulas Used
Minimum Conductor Area
min_area = current / max_current_densityVariables
| Variable | Description | Default |
|---|---|---|
current | Design Current (I)(A) | 40 |
max_current_density | Max Current Density(A/mm^2) | 4 |
How It Works
Cable Sizing by Current Density
The cable must carry the required current without exceeding safe temperature limits.
Simplified Method
A = I / J
where A is the conductor cross-sectional area, I is the design current, and J is the allowable current density. Typical values: enclosed cables 3-4 A/mm^2, open-air cables 5-6 A/mm^2. Always verify against manufacturer ampacity tables and installation derating factors.
Worked Example
40 A load with 4 A/mm^2 allowable current density.
- 01A = 40 / 4 = 10 mm^2
- 02Select the next standard size: 10 mm^2 or 16 mm^2 depending on available sizes.
- 03Also verify voltage drop is within limits for the cable run length.
Frequently Asked Questions
What are standard cable sizes?
Common metric sizes: 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240 mm^2. Always round up to the next standard size above the calculated minimum.
What affects cable ampacity?
Installation method (open air, conduit, buried), ambient temperature, number of cables grouped together, insulation type (PVC, XLPE), and conductor material (copper vs aluminium) all affect the maximum safe current.
Should I check voltage drop after sizing?
Yes. Cable sizing must satisfy both thermal (ampacity) and voltage drop criteria. For long runs, voltage drop often requires a larger cable than the thermal calculation alone.
Learn More
Guide
Understanding Electrical Circuits: Fundamentals for Engineers
Learn the fundamentals of electrical circuits including Ohm's law, Kirchhoff's laws, series and parallel circuits, AC vs DC, power calculations, and three-phase systems.
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