How to Size a VFD for Your Motor (kW, Amps and Load Type)

Size a VFD by current, not just kW: choose a drive for your supply whose rated output current is at least the motor's nameplate full-load current, and go one size up (about 10–15% margin) for heavy-starting loads. For example, a 5 HP, 415 V motor drawing about 7.8 A fits a 9 A Delta MS300 (₹19,900).

By Team Voltkart · Reviewed by Amit, Voltkart · Last updated 6 October 2026 · Prices checked 6 October 2026

Step 1: match the supply and the motor voltage

Start with the supply you have, because it decides the drive family. A three-phase 415 V supply needs a three-phase input drive (Delta EL43 or MS43 models) and runs a standard 415 V motor. A single-phase 230 V supply needs a single-phase input drive (EL21, EL21W or MS21 models); these output three-phase 230 V, so the motor must be connected for 230 V, which on a dual-voltage 230/400 V motor usually means delta. A single-phase input drive does not step the voltage up to 415 V, so it cannot run a 415 V-only motor at full power. Check the motor nameplate before ordering: it shows the voltage for each connection and the full-load current for each. If the motor is marked 415 V delta only, you need a three-phase supply. In practice, single-phase input drives cover small motors (up to 3 HP in our range), and larger motors need a three-phase supply and drive.

Step 2: size by motor full-load current, not only kW

The current on the motor nameplate is the figure that matters, because two motors with the same kW can draw different currents depending on poles, efficiency class and power factor. Choose a drive whose rated continuous output current is at least that full-load current. Delta's MS300 model numbers carry the rating directly: VFD9A0MS43ANSAA is a 9.0 A drive, VFD17AMS43ANSAA is 17 A. For EL drives, check the datasheet output current for the kW rating. The kW or HP rating on a drive assumes a typical four-pole motor at normal duty, so use it only as a first guide. If you do not have the nameplate, typical 415 V currents are about 1.9 A for 0.75 kW, 5 A for 2.2 kW, 7.8 A for 3.7 kW and 15.5 A for 7.5 kW; the free motor starter & VFD calculator uses these values and suggests the matching drive.

Step 3: allow for the load type

Pumps and fans are variable-torque loads: they need little torque at low speed and start easily, so a drive matched to the motor current is normally enough. Conveyors, crushers, mixers, positive-displacement pumps, hoists and compressors starting under load are constant-torque or heavy-start loads: they need full torque from standstill and may need an overload above rated current for several seconds while starting. For these, choose the next larger drive (about 10–15% current margin or more), check the drive's overload rating in the datasheet, and lengthen the acceleration time. Very high inertia loads, or loads that must stop quickly, may also need a braking resistor; check the Delta manual for the braking option on your model. If you are unsure whether your load is light or heavy starting, send us the application details.

Step 4: check the installation conditions

Drive ratings assume a normal ambient temperature, altitude and mounting. Hot panels, high altitude, or several drives mounted close together reduce the current a drive can deliver, and the manual gives the derating. Long motor cables between drive and motor can also need attention; follow the manual for the maximum length and any output reactor or filter. Feed the drive through the input protection the manual specifies, usually an MCB or fuses rated for the drive's input current (see MCB vs MCCB), and do not switch a contactor between the drive output and the motor for routine starting and stopping. The drive provides its own motor overload protection once the motor current is set in its parameters.

Worked examples

• 1 HP pump on a single-phase supply: 0.75 kW, 230 V three-phase motor (delta). Choose a single-phase input drive such as the VFD007EL21W-1 (₹8,180).
• 5 HP centrifugal pump, 415 V: about 7.8 A full-load current, light start. A 9 A VFD9A0MS43ANSAA (₹19,900) or the 5 HP VFD037EL43A (₹21,900) fits.
• 5 HP conveyor, 415 V, heavy start: 7.8 A × 1.15 ≈ 9 A, which puts a 9 A drive right at its limit; choose the 7.5 HP, 13 A VFD13AMS43ANSAA (₹27,500) for margin.
• 10 HP fan, 415 V: about 15.5 A; the 17 A VFD17AMS43ANSAA (₹28,500) fits.

Delta VFD ratings and prices (incl. GST)

Rated output current is shown where Delta puts it in the model number (MS300). See the VFD price list for the full comparison of EL and MS300, and the AC drives range for stock. Choosing between a drive and other starters? Read motor starting methods and VFD vs soft starter.

Why buy from Voltkart

• Genuine Delta drives and genuine Schneider contactors, overload relays and MPCBs from stock, with a GST invoice on every order.
• Authorised distributor of Selec and EAPL (star-delta timers) and Minilec (single-phase preventors).
• Pan-India dispatch; typically 2–3 days to Delhi NCR, UP and Haryana and 5–6 days elsewhere.
• Send the motor nameplate on WhatsApp +91 8800754881 (Mon–Sat, 12:30–7 pm) and we will suggest the starter or drive.

Frequently asked questions

How do I select a VFD for a motor?

Match the drive input to your supply, then choose a drive whose rated output current is at least the motor nameplate full-load current. Go one size up for heavy-starting loads such as conveyors or crushers.

What size VFD for a 5 HP motor?

A 5 HP (3.7 kW) motor at 415 V draws about 7.8 A, so a 5 HP or 9 A drive fits for pumps and fans, such as the Delta VFD9A0MS43ANSAA (₹19,900). For a heavy start, choose the next size up.

Can a single-phase VFD run a 415 V motor?

No. Single-phase 230 V input drives output three-phase 230 V, so the motor must be connected for 230 V (usually delta on a 230/400 V motor). A 415 V-only motor needs a three-phase drive.

Should I size a VFD by kW or by amps?

By amps. The motor full-load current decides the drive, because motors with the same kW can draw different currents. The drive kW rating is only a guide for a typical motor.

Do I need an overload relay with a VFD?

Normally no. The drive provides electronic motor overload protection once the motor current is set in its parameters. It still needs the input protection specified in its manual.