Cable Size Calculator
From a homeowner's AC unit to an EPC engineer's motor feeder — enter any load, get the correct conductor size with voltage drop, de-rating, and the right National Cables product.
Enter your load and installation details on the left, then press Calculate to get your recommended cable size.
All calculations per IS 694:2010, IS 3961 (Part 2):2017, and IEC 60364-5-52.
Cable sizing calculator — common questions.
How to use the calculator correctly for the most common installation scenarios.
Which insulation type should I select for a hospital or critical-infrastructure project?
Select HFFR 90°C — IS 17048:2018 Clause 10.3 (halogen-free, higher rating) for hospitals, data centres, metro stations, airports, and any building requiring NBC 2016 Part 4 Cl. 4.14 compliance. HFFR delivers HCl emission < 0.5% (versus < 15% for FR-LSH) — preventing halogen acid gas damage to medical equipment and reducing toxicity during evacuation.
For G+4 high-rise residential and most commercial buildings, select FR-LSH PVC 70°C — IS 694:2010. For low-rise residential (G+3 and below) and back-of-house industrial, standard FR PVC 70°C is sufficient.
Why does the calculator ask for grouping if I am running only one cable?
Grouping de-rating accounts for heat interaction between adjacent cables. Per IEC 60364-5-52 Table B.52.17, the grouping factor Cg ranges from 1.00 (single cable in free air — no reduction) down to 0.50 or lower for tightly packed groups in conduit or tray.
If your cable runs alongside other current-carrying cables in the same conduit, ladder, or trefoil, each cable heats the others, and the tabulated current rating must be reduced. Selecting the correct grouping option ensures the calculator picks a thermally adequate size — ignoring it risks insulation failure.
When does voltage drop become the limiting factor instead of current rating?
Voltage drop typically limits sizing on long runs at moderate load. The calculator first selects the smallest size meeting thermal current (step 5), then verifies voltage drop against the configured limit (default 4% of nominal). If exceeded, it automatically upsizes to the next standard cross-section and re-verifies (step 6).
Submersible pump installations (well depth 60m+), solar DC strings, and long motor feeders commonly require sizing 2–3 standard sizes above the thermal minimum because of voltage drop. The result panel always indicates whether the selected size is thermal-limited or voltage-drop-limited.
What is the difference between the Method B, C, and F installation method options?
The installation method defines how the cable is routed, which directly affects how heat dissipates and therefore the tabulated current rating per IS 3961 (Part 2):2017:
- Method B (in conduit — embedded or surface): restricted air circulation, lowest tabulated rating
- Method C (clipped direct to wall or ceiling): moderate air circulation
- Method F (open cable tray): best air circulation, highest tabulated rating
- Method D (underground in duct): rating depends on soil thermal resistivity
Selecting the method that matches your actual installation is essential — using a higher-rated method than the actual installation will undersize the cable.