How do you size a soft starter?

Select a soft starter with current rating at least equal to the motor full load current, then add margin for heavy starting load, starts per hour, ambient temperature, enclosure derating and bypass type.

Can I size a soft starter by motor kW only?

kW can be used for estimation, but nameplate full load current is better because actual current depends on voltage, efficiency and power factor.

What is inside-delta soft starter connection?

Inside-delta connection places the soft starter inside the motor delta winding path, so the starter current is about 58 percent of motor line current when the motor supports six-lead delta connection.

Is a bypass contactor required for a soft starter?

A bypass contactor is not always mandatory, but it reduces heat and power loss after starting. Many soft starters include an internal bypass contactor.

Soft Starter Size Calculator | Motor Soft Starter Selection

Soft Starter Size Calculator

Calculate recommended soft starter current rating for three-phase induction motors using motor FLA, kW/HP, starting load, starts per hour, bypass style, inside-delta wiring and derating.

⚡ Field Rule: Start from the motor nameplate FLA, then increase the soft starter rating for heavy-load starts, frequent starts, high ambient temperature and enclosed panels. Use this with Overload Relay Setting, VFD Size Calculator, Motor Current Calculator and Contactor Size Calculator.

🚀 Soft Starter: controlled voltage ramp for smoother motor starting

Motor Nameplate FLA

Supply Voltage

Starter Connection

Starting Load

Starts per Hour

Design Margin

Panel / Ambient Derating

Best method for real jobs: enter the motor nameplate full-load current, then adjust for load type and site conditions.

Presets:

Motor Power

Supply Voltage

Power Factor

Efficiency

Starting Load

Design Margin

This estimates FLA from kW/HP. Replace the estimate with actual motor nameplate FLA before final ordering.

Power presets:

Motor Line FLA

Connection Mode

Starting Load

Starts per Hour

Design Margin

Derating

Inside-delta sizing is only valid when the motor has six accessible leads and the soft starter supports inside-delta connection.

Presets:

📐 Formula Reference

Base Current Method

Starter A ≥ Motor FLA × load factor × start factor × margin ÷ derating

3-Phase FLA Estimate

I = P(kW) × 1000 ÷ (√3 × V × PF × η)

Inside Delta Current

Starter current ≈ Motor line FLA × 0.58

Bypass / No Bypass

No-bypass continuous SCR duty usually needs more thermal margin

📋 Quick Reference

Load Factor

Pump / fan1.00×

Conveyor1.18×

Crusher / mixer1.35×

Common Checks

Nameplate FLABest input

Starts/hourThermal duty

Inside delta6 leads

Compare Starters

DOLHigh shock

Star-deltaStep change

Soft starterSmooth ramp

📚 Field Notes

Use nameplate FLA firstMotor kW is useful for estimation, but final selection should follow the actual full-load current on the motor nameplate.

Bypass reduces heatingWith a bypass contactor, SCRs conduct mainly during starting and stopping, so continuous heat loss is lower.

Heavy starts need marginConveyors, crushers, mixers and high-inertia loads may need a larger frame, longer ramp time and higher current limit.

Check the manualFinal sizing depends on the manufacturer duty table, ambient temperature, altitude, enclosure ventilation and starting frequency.

What is a Soft Starter Size Calculator?

A soft starter size calculator estimates the recommended soft starter current rating for an induction motor. It uses motor full-load current or estimated motor current, then adds practical correction for starting load, starts per hour, derating, bypass type and connection type.

How to size a soft starter

For most field work, select a soft starter with current rating equal to or higher than the motor nameplate FLA. Then increase the rating for heavy loads, frequent starts, high ambient temperature and non-bypass continuous operation.

Soft starter vs VFD sizing

A soft starter reduces starting current and mechanical shock, but it does not control motor speed during normal running. Use a VFD when you need speed control, energy saving on variable-torque loads or process control.

❓ Frequently Asked Questions

Start with motor nameplate FLA. Multiply by load factor, starts/hour factor, margin and derating. Then select the next available soft starter current rating.

Yes for estimation, but the final selection should use motor nameplate FLA because actual current depends on voltage, power factor, efficiency and motor design.

The soft starter frame current is normally selected from motor FLA and duty class. Current limit and ramp settings must still stay within the soft starter manual.

Yes, inside-delta connection usually reduces starter current to about 58% of line current, but only when the motor has six leads and the starter supports inside-delta wiring.

Not always. Bypass reduces heat and power loss after the motor reaches speed. Many modern soft starters include internal bypass contactors.

Only if you need smooth starting and stopping. It cannot provide normal running speed control like a VFD.

Current limit restricts motor starting current during ramp-up. Too low a limit may stop the motor from accelerating; too high a limit reduces the benefit of soft starting.

Common causes include undersized starter, too short ramp time, too low current limit, heavy mechanical load, voltage drop, overload setting problem, phase loss or high ambient temperature.

Many soft starters include electronic motor protection, but installations may still require correctly set overload protection depending on design, standard and panel wiring.

Yes. Soft starters are commonly used for pumps to reduce starting current, mechanical stress and water hammer during starting and stopping.