Choosing the right electrical cable size is one of the most important steps in any electrical job. If the cable is too small, it can overheat, trip protection devices, fail short-circuit checks, or cause voltage drop problems. If the cable is too large, the installation becomes more expensive than necessary.
A Cable Sizing Calculator removes the guesswork. It checks the installation method, grouping, distance, voltage, load rating, conductor type, soil conditions, and more. It also verifies short-circuit performance, loop impedance, and protective device requirements.
Most calculators today—including this one—follow the methods of AS/NZS 3008, the main cable selection standard used across Australia and New Zealand. Using a calculator that follows AS/NZS 3008 gives you results that match standard engineering practice and align with inspection expectations.
This guide explains how the Cable Sizing Calculator works, why AS/NZS 3008 matters, how to use all the inputs, what happens behind the scenes, and how to read the results confidently.
Table of Contents
What the Cable Sizing Calculator Does
The calculator analyzes your installation and selects the smallest cable size that satisfies all electrical and thermal requirements. It performs three main checks:
1. Current Carrying Capacity
This check ensures the cable can safely carry the load current without overheating.
The calculator considers:
- Cable material (copper or aluminium)
- Insulation type (PVC or XLPE)
- Installation method (air, tray, conduit, buried, trefoil, duct bank, etc.)
- Grouping (number of circuits)
- Cables per enclosure
- Ambient or soil temperature
- Soil resistivity and burial depth
- Flexible vs. non-flexible cable
- Parallel cables
All these conditions affect heat dissipation and therefore allowable current.
2. Voltage Drop
Every cable has resistance and reactance. Over long distances, this causes voltage loss.
The calculator checks:
- Load current
- Length
- PF mode (worst case or specified PF)
- Resistive and reactive components
- AC or DC system
- Temperature-corrected resistance
If the voltage drop exceeds your limit, the cable is rejected.
3. Protection and Fault-Related Checks
This calculator also verifies:
- Short-circuit thermal withstand (1-second rating)
- Protective device breaking capacity
- Loop impedance for instant tripping (B, C, D curves; fuses; MCCB)
- Source impedance (measured, estimated, or calculated from fault level)
A cable must pass all checks to be recommended.
Why AS/NZS 3008 Is Important
AS/NZS 3008 defines the correct way to size cables in Australia and New Zealand. It gives:
- Base ampacity tables
- Voltage drop values
- Temperature derating
- Grouping derating
- Installation correction factors
- Soil correction factors
- Rules for buried cables
A calculator based on AS/NZS 3008 ensures:
- No undersizing
- No unnecessary oversizing
- Safety margins
- Compliance with common engineering practice
- Repeatable and consistent results
Without AS/NZS 3008, cable choices often rely on assumptions or outdated rules of thumb.
Inputs Used by the Cable Sizing Calculator
Every field in the calculator’s interface has a purpose. Below is a clear explanation of every input.
A. Standard Settings
AS/NZS 3008 Part
- Part 1: Australia (default ambient air 40°C, soil 25°C)
- Part 2: New Zealand (air 30°C, soil 15°C)
Installation Medium
- Air
- Soil (enables soil resistivity and burial depth fields)
Installation Method
Examples:
- Air: Unenclosed (spaced/touching), trays, conduit
- Soil: Direct buried, conduit, trefoil, duct bank, concrete duct
Each has a different derating factor.
Ambient Temperature Mode
- Default: Uses standard values
- Custom: Allows your own temperatures
Different for air or soil.
B. Load Settings
Phase
- Single-phase
- Three-phase
- DC
Voltage
Preset or custom.
Rating Type
- A
- kW
- kVA
- hp
The calculator converts these into line current.
Power Factor (PF)
Required for:
- kW
- hp
- Specified PF mode in voltage drop settings
C. Voltage Drop Settings
- Maximum allowed voltage drop (%)
- Distance (m)
- Conductor temperature mode (calculated or maximum)
- PF mode (worst case or specified)
D. Cable Settings
Material
- Copper
- Aluminium
Insulation
- PVC V-90
- XLPE X-90
Cable Type
- 2C+E (multi-core)
- 2×1C+E (single cores)
Flexible Cable
Yes/No.
Grouping
Number of circuits installed together (required).
Cables per Enclosure
Used when several circuits share the same tray or conduit.
Parallel Cables
Option to size cables in parallel per phase.
Soil Resistivity (ρ)
Used for buried cables.
Burial Depth
Affects heat transfer and derating.
E. Protection & Short-Circuit Checks
This section adds safety and compliance checks beyond basic sizing.
Short Circuit Rating
Ensures the cable can withstand fault currents thermally.
Loop Impedance
Checks that protection will trip instantly.
Protection Type
- MCB
- MCCB
- Fuse
MCB Curve
- B
- C
- D
Determines instantaneous trip multiples.
Device Rating
Auto or manual selection.
Breaking Capacity (kA)
Ensures the device can interrupt the prospective fault current.
Earthing System
- TN-S
- TN-C
- TT
Determines loop impedance path.
Earth Core Area
Used in loop impedance calculations.
Source Impedance Mode
- Measured
- Estimated
- Calculated from fault level
Step-by-Step: How to Use the Cable Sizing Calculator
- Choose the standard part (Australia or New Zealand).
- Select Air or Soil installation.
- Choose the installation method from the list.
- Set phase and voltage.
- Enter load rating using A, kW, kVA, or hp.
- Enter your PF if required.
- Set your allowed voltage drop and the cable length.
- Choose cable material, insulation, and type.
- Enter grouping and any soil or parallel cable details.
- Configure protection settings if you want short-circuit or loop checks.
- Click Calculate.
The calculator will show:
- Recommended size
- Ampacity margin
- Voltage drop
- Short-circuit checks
- Loop impedance checks
- A comparison summary
- Option to download a TXT report
Example Job
Example:
- 10 kW motor
- 3-phase, 400 V
- PF = 0.8
- 50 m run
- Max drop = 5%
- Copper XLPE cable
- On perforated tray
- 2 circuits grouped
The calculator will:
- Convert 10 kW @ PF 0.8 into line current.
- Apply installation derating.
- Apply grouping derating.
- Apply temperature factors.
- Calculate voltage drop.
- Check short-circuit thermal limit.
- Check loop impedance.
- Recommend a cable size and show a summary.
What Happens Behind the Scenes
The Cable Sizing Calculator performs many engineering steps:
Current Conversion
Handles A, kW, kVA, hp for any phase system.
Ampacity Calculation
Base rating × derating factors:
- Temperature
- Installation
- Grouping
- Soil resistivity
- Burial depth
- Cable type
- Flexibility
- Parallel cables
Voltage Drop
Uses:
- Resistance (R) at operating temperature
- Reactance (X)
- PF mode
- Distance
- AC or DC formulas
Fault and Protection Checks
- Instantaneous trip using curve type
- Device breaking capacity
- Short-circuit thermal withstand (kA × s)
- Total loop impedance = source Z + cable Z
If any check fails, the tool alerts you.
Understanding the Results
The results include:
- Recommended cable size
- Line current
- Base capacity
- Adjusted (derated) capacity
- Capacity margin
- Voltage drop (%) and volts
- Maximum allowable distance
- Impedance (R, X, Z)
- Loop impedance result
- Short-circuit withstand result
- Summary banner
- TXT export file
These values let you justify your design clearly.
Common Questions
Do I need power factor?
Only for kW or hp loads, or if you choose to use it for voltage drop.
Can I enter custom voltage?
Yes, the Cable Sizing Calculator allow any voltage from 1 to 1000 V.
Does grouping matter?
Yes. More circuits mean more heat, which reduces the allowable capacity.
Is this enough for all jobs?
It follows the same ideas used in AS/NZS 3008, but for critical work, it’s always good to double-check with project requirements and manufacturer data.
Try the Cable Sizing Calculator
Standard
Load
Voltage Drop
Cable
Short Circuit Protection
Result
Conclusion
A Cable Sizing Calculator based on AS/NZS 3008 is one of the most helpful tools for anyone working with electrical cables. It takes the important ideas from the standard and applies them automatically. By entering accurate information, you get a cable size that is safe, reliable, and suitable for your installation.
This saves time, reduces errors, and makes electrical design much easier. Whether you work on homes, workshops, commercial buildings, or industrial systems, using a Cable Sizing Calculator gives you confidence that your cable choices are correct and well-justified.