Cable Width (Cross-Section) Calculator
Estimate required cable cross-sectional area based on current, length, voltage drop, conductor material, and installation margin.
Note: This is an engineering estimate. Final cable selection must follow local electrical code, insulation type, ambient correction factors, conduit fill, and protective device rules.
What this calculator means by “cable width”
In electrical work, cable “width” usually means the conductor cross-sectional area (mm²), not the outer jacket diameter. Cross-sectional area determines how much current a cable can safely carry and how much voltage drop you get over distance.
This tool calculates the minimum area needed from two constraints:
- Voltage drop limit (to keep equipment performance stable)
- Ampacity estimate (to avoid overheating under load)
Then it applies a design margin and rounds up to the next common standard cable size.
How the cable width calculation works
1) Voltage drop method
For a given current and length, required area is solved from the standard resistance-based drop formula:
- Single-phase/DC: A = (2 × I × ρ × L) / Vdrop
- Three-phase: A = (√3 × I × ρ × L) / Vdrop
Where:
- I = load current (A)
- ρ = resistivity (Ω·mm²/m), adjusted for temperature
- L = one-way length (m)
- Vdrop = allowed voltage drop in volts
2) Ampacity rule-of-thumb check
The calculator also checks a simplified current-density estimate:
- Copper: around 6 A/mm²
- Aluminum: around 4 A/mm²
The larger value between voltage-drop size and ampacity size is used before applying margin.
3) Practical rounding
Real cables are sold in standard areas (1.5, 2.5, 4, 6, 10, 16 mm², etc.). The calculator rounds up to the next available standard size so your design remains practical.
Input tips for better accuracy
- Length: Enter one-way distance from source to load.
- Voltage drop: Common design targets are 3% for branch circuits and up to 5% total feeder + branch (depending on code and system).
- Temperature: Higher conductor temperature increases resistance and raises required cable size.
- Margin: 10–25% margin is commonly used for growth, startup current, and real-world uncertainty.
Copper vs aluminum cable width
Aluminum has higher resistivity than copper, so aluminum conductors usually need a larger cross-section for the same load and voltage drop. It can still be cost-effective on large feeders, but lugs, anti-oxidation handling, and terminations must be selected correctly.
From area to approximate conductor diameter
The calculator also shows an equivalent solid-conductor diameter using:
d = √(4A/π)
That gives a useful reference, but stranded cables and insulation layers make actual overall cable diameter larger.
Important design checks beyond this calculator
- Local code compliance (NEC, IEC, BS, AS/NZS, or your jurisdiction)
- Installation method (tray, conduit, buried, free air)
- Ambient temperature correction
- Grouping / bundling derating
- Short-circuit withstand and protective coordination
- Motor start current and harmonic effects
Quick FAQ
Is lower voltage always worse for cable sizing?
Usually yes. At lower system voltage, a fixed voltage-drop percentage means fewer allowable volts drop, which increases required conductor area.
Why does long distance increase cable width?
Longer cable means higher resistance, causing more voltage drop and heat. Increasing conductor area reduces resistance.
Can I use this for solar DC runs?
Yes, for preliminary sizing. Still confirm with DC-specific protection rules, temperature corrections, and manufacturer data.