Motor Starter & VFD Selection Calculator
Enter your motor rating and supply to get the approximate full-load current and a matching overload relay, contactor, MPCB and VFD. Free to use, no sign-up. For the final selection, always use the current on the motor nameplate.
| Component | Suggested | Why |
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Typical currents are for 4-pole induction motors at 400–415 V (multiplied by 1.74 for 230 V). Real motors vary by make, poles and efficiency class, so the nameplate always wins. Check the device manual for short-circuit coordination and cable sizing. Products link to live prices and stock.
How the calculator chooses each part
- Full-load current: typical values for 4-pole motors at 400–415 V (for example 3.7 kW ≈ 7.8 A, 7.5 kW ≈ 15.5 A), or the nameplate current if you enter it.
- Overload relay: the Schneider LRE (for LC1E) or LRD (for LC1D) relay whose adjustable range includes the motor current. See the overload relay price list and overload relay sizing guide.
- Contactor: the smallest LC1E and LC1D whose AC-3 rating covers the current, with 25% margin for heavy starting. Compare them in LC1E vs LC1D.
- MPCB: the Schneider GV2ME range that includes the current (up to 25 A).
- VFD: the smallest Delta EL and MS300 drive for your supply whose rated current (or HP) covers the motor. Full list in the VFD price list.
Frequently asked questions
How do I calculate the full-load current of a 3-phase motor?
Use I = P × 1000 ÷ (√3 × V × power factor × efficiency). For a 7.5 kW, 415 V motor with typical power factor 0.85 and efficiency 0.89, that gives about 13.8 A by formula; nameplates of common 4-pole motors show about 14–15.5 A. Always use the current printed on the motor nameplate when you have it.
What size overload relay do I need?
Choose a thermal overload relay whose adjustable range includes the motor full-load current, ideally near the middle of the range, and set the dial to the nameplate current. For example, a 7.5 A motor needs a 5.5–8 A relay (LRE12 or LRD12).
What size contactor do I need for a motor?
For direct-on-line starting, pick a contactor whose AC-3 rating at your voltage is at least the motor full-load current. Many panel builders keep 15–25% margin for frequent starting or high ambient temperature.
MPCB or overload relay?
An MPCB combines adjustable overload protection and short-circuit protection in one device. A contactor with an overload relay needs separate short-circuit protection (MCB or fuses). Both still need a contactor for remote start and stop.
How do I size a VFD?
Match the supply (single-phase 230 V or three-phase 415 V input) and choose a drive whose rated output current is at least the motor full-load current. For heavy starting loads such as conveyors or crushers, go one size up.
Link to this calculator
Teaching motor control, writing a guide or answering on a forum? You are welcome to link to this free calculator. Copy this HTML:
<a href="https://voltkart.in/pages/motor-starter-calculator">Motor starter & VFD selection calculator (Voltkart)</a>
Questions about a specific motor? WhatsApp the nameplate photo to 8800754881 (Mon–Sat, 12:30–7 pm). Reviewed by Amit, Voltkart · 4 October 2026.