What current should I use to size a soft starter?

Start with the motor nameplate full-load current, then apply only documented application and site rating factors.

Does this calculator select an ABB, Siemens or Schneider model?

No. It calculates a vendor-neutral minimum current screen; use the current manufacturer selection tool for the exact model and duty.

Why do start time and starts per hour matter?

They affect semiconductor and bypass-contactor thermal duty, which requires the exact manufacturer's curves or selector.

Is a soft starter the same as a VFD?

No. A soft starter controls starting and stopping voltage or torque; a VFD also controls running frequency and speed.

Soft Starter Sizing Calculator

Calculate a minimum current-rating screen from motor FLA and explicit duty/site factors. This is a planning check before the exact manufacturer thermal and application selection.

About this calculator

Quick Calculator inputs above give an instant result; use Advanced fields (factors, units, or decoding) when your datasheet differs from defaults.

Example: 100 A FLA with application factor 1.15 and ambient derate → screen starter rated current before OEM thermal tables.

Result explanation / How to use: treat values as screening only. Engineering assumption and disclaimer: confirm nameplate limits, OEM selectors, and site conditions before procurement or programming.

Hub: Protection calculators — continue related sizing from this topic cluster.

Soft-starter current screen

Motor nameplate FLA (A)

Application current factor (1.00–2.00)

Ambient rating factor (0–1)

Altitude rating factor (0–1)

Planned current limit (× FLA)

Start duration to verify (s)

Starts per hour to verify

Current-rating result

Formula and boundary:

Do not convert this number directly into a catalog model. Confirm connection method, load type, acceleration time, starts/hour, ambient, altitude, overload class and coordination in the exact current OEM selector.

Motor starter guide → Motor FLA → VFD sizing →

Your Calculation Path

soft starter sizingVerify motor FLA →

Continue Your Calculation

Verify motor FLA and compare whether the application requires a soft starter or VFD.

What this calculation does

The tool divides explicit application current by the product of entered rating factors. It does not invent universal ambient or altitude derating: manufacturer values differ by series and installation.

Primary references: ABB Softstarter Catalog and Schneider Electric Altistart selection guidance.

Frequently Asked Questions

Which current starts the calculation?

Use motor nameplate FLA, not breaker size or locked-rotor current.

Can I use a 1.0 application factor?

Only when the current OEM selector and application classification support it.

Does current limit predict acceleration?

No. Load torque and inertia determine whether the motor accelerates at that limit.

What must I verify next?

Verify thermal start duty, starts per hour, bypass, coordination and installation derating with the manufacturer.