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Cable Size Calculator: Find the Correct Wire Size for Your Circuit

Picking a cable size just by “load current” is not enough – the real safe size depends on ambient temperature, how many cables are grouped together, the installation method and the run length (which causes voltage drop). Cable Size Calculator handles all of this in one pass: it first finds the smallest cable that can safely carry your derated current, then automatically checks if that size also keeps voltage drop within limit (upgrading the size if needed) and optionally checks short-circuit withstand too. This tool is built for diploma/B.Tech Electrical students learning cable sizing fundamentals and for site engineers and electricians who need a fast, standards-based (IS 3961 / IEC 60364-5-52) reference before finalizing a wiring design.

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How to Use

1. Enter your load either as Current (A) directly or switch to Power (kW/HP) mode and enter power, unit and motor/load efficiency.
2. Select your System (single-phase or three-phase), Voltage and Power Factor.
3. Choose the Conductor material (Copper or Aluminium), Insulation type (PVC 70°C or XLPE 90°C) and Installation method (conduit, free air, buried or tray).
4. Set the Ambient Temperature, number of Grouped Circuits and the Run Length in metres.
5. Pick your Voltage Drop Limit (5% for power circuits, 3% for lighting, per IS 732 – or enter a custom limit).
6. (Optional) Tick Short-circuit withstand check and enter fault current (kA) and clearing time (seconds).
7. Click Calculate Cable Size to see the recommended size and which factor (ampacity or voltage drop) governed the result.

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Key Features

• Reference ampacity tables for 4 installation methods (conduit, free air, buried, tray)
• Automatic derating for ambient temperature, grouping, insulation type and conductor material
• Auto-upgrade logic – if the current-based size fails the voltage drop check, the tool automatically selects the next larger size
• Clearly shows which factor (ampacity or voltage drop) governed the final size
• Optional short-circuit thermal withstand rough-check (S ≥ I√t / k)
• Supports both Copper and Aluminium conductors, PVC and XLPE insulation
• Power-to-current conversion built in (kW or HP input, with efficiency)
• Print/PDF export and copy result option

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Formula / Logic Used

Step 1: Derated Ampacity

Derated Capacity = Base Ampacity × k_material​ × k_insulation​ × k_temp​ × k_group​

Where:

• $Base\ Ampacity$ = reference current rating for the chosen size and installation method
• $k_{material}$​ = 1.0 for Copper, 0.78 for Aluminium
• $k_{insulation}$ = 1.0 for PVC, 1.22 for XLPE (XLPE handles higher temperature, so it carries more current)
• $k_{temp}$​ = correction factor for ambient temperature (drops as temperature rises)
• $k_{group}$​ = correction factor for number of circuits grouped together (drops as more cables are bunched)

The tool picks the smallest cable size whose derated capacity is greater than or equal to your load current.

Step 2: Voltage Drop Cross-Check

Where:

• $I$ = load current, $L$ = cable run length in metres (one-way)
• $r$ = conductor resistance (ohm/km), based on material and cable size
• $x$ = conductor reactance (ohm/km), taken as a standard approximate value
• $\cos\phi$ = power factor, $\sin\phi$ = derived from power factor

If the voltage drop percentage exceeds your selected limit (5% or 3%), the tool automatically checks the next larger cable size until it passes.

Step 3: Short-Circuit Withstand (optional)$$Required\ Size \geq \frac{I_{fault} \times \sqrt{t}}{k}$$

Where:

• $I_{fault}$​ = prospective fault current in Amperes
• $t$ = fault clearing time in seconds
• $k$ = a constant depending on conductor and insulation type (accounts for how much heat the cable can absorb before damage)

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Frequently Asked Questions (FAQs)

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Related Tools

• Voltage Drop Calculator – for a focused, standalone voltage drop check on any circuit
• Home Load Calculator – find your load current first, then use this tool to size the cable
• Transformer Calculator – useful for sizing cables on the secondary side of a transformer

Read More>>>