Motor current & cable size in seconds
Enter the motor rating and cable length. We estimate full-load and starting current and the smallest copper cable that meets current capacity and voltage drop.
- Full-load and starting current
- Cable size with voltage drop
- Starter-method comparison
Your inputs
Cable assumptions
Full-load vs starting current
Indicative estimate only. Real results depend on your equipment, site conditions and applicable standards. This is not a design or an offer.
Want this checked by an engineer?
Send us your result and we’ll review it and prepare a recommendation and quotation.
Chat on WhatsApp insteadMethod & assumptions
- Full-load currentkW × 1000 ÷ (√3 × V × power factor × efficiency).
- Starting multiplesDOL 6.5×, star-delta 2.2×, soft starter 3×, VFD 1.2× of full-load (typical).
- Cable sizeSmallest copper multicore (PVC, installed on tray / clipped) whose derated rating covers 1.25 × FLC, within the allowed voltage drop.
- Voltage drop√3 × I × L × (R cosφ + X sinφ) with resistance at 70°C and reactance 0.08 Ω/km.
- ImportantThis is a sizing aid. Final cable selection must follow IS 3961 / IEC 60364, the cable maker’s tables, short-circuit withstand and installation conditions.
About this calculator
Is the starting current exact?
No. It varies with motor design (typically 5-8× for DOL). Check the motor datasheet.
Why does the cable get larger than the current needs?
Long runs cause voltage drop; the calculator upsizes the cable to keep the drop within your limit.
Does this cover aluminium or armoured cables?
It assumes PVC copper multicore. Aluminium, XLPE and armoured cables have different ratings; ask us for a design check.
What about star-delta or soft-start cable sizing?
The running cable is sized for full-load current; starter wiring has its own rules.