MPCB Sizing for Star-Delta and Soft Starting

How do you size an MPCB for star-delta and soft starting? The answer depends less on the starting method itself and more on where the MPCB sits in the current path, per IEC 60947-4-1: on the incoming line, where it always reads full motor full-load current (FLC), or wired in series with the winding, where delta-connected phase current is only about 58% of line current. Get that placement wrong and the thermal dial ends up set to a number that either trips a correctly loaded motor early or lets a genuine overload run past the trip curve unnoticed. This article covers line-side versus winding-side thermal setting, the 58% rule and its formula, magnetic trip behavior through the star-to-delta transition, sizing an MPCB ahead of a soft starter, coordination tables for both starting methods, and the sizing mistakes we see most in panel shops.

Two Starting Methods, One Sizing Question the MPCB Doesn't Answer by Itself

A direct-on-line (DOL) start pulls 6-8x FLC for roughly half a second to a few seconds, and the MPCB's fixed magnetic trip — typically around 12-13x the selected current range — is built to ride through that without opening. Star-delta starting cuts the line current at the moment of connection to roughly a third of the DOL start value, because each winding sees line voltage divided by √3 during the star phase. Soft starters do something different again: an SCR ramp holds starting current to a set multiple of FLC, commonly 2-4x, stretched over several seconds instead of a near-instantaneous DOL spike. None of these profiles come close to tripping a correctly selected magnetic element; the margin built into 12-13x In exists for exactly this range of starting currents. What changes the sizing math is not the starting current magnitude; it's where in the circuit the protective device is connected and what current it actually measures there.

Line-Side vs Winding-Side Placement in a Star-Delta Starter

A star-delta starter for a 6-lead motor uses three contactors: main, star, and delta. Two wiring conventions put the protective device in different places. Placed upstream of all three contactors, in the supply line, the MPCB or overload sees full line current continuously once the motor reaches delta run — the same current a DOL starter's device would see. Placed in series with the main and delta contactors only (commonly called "inside-delta" or winding-connected), the device carries only the winding's phase current, which never equals line current in a delta-connected run.

This distinction matters because manufacturer selection tables for combination starters often carry two current columns for the same frame: one for direct-on-line duty, one for star-delta "inside-delta" duty, precisely because the current the device experiences is different even though the motor and its FLC haven't changed. Pull the DOL column when the device is winding-connected and the setting will be wrong before you even touch the dial.

Setting the Thermal Element — the 58% Rule

In a delta-connected winding, line current equals winding (phase) current times √3. Rearranged, phase current is line current divided by √3, or about 0.577 times line current — call it 58%. If the thermal element is winding-connected, set the dial to 58% of the motor's nameplate FLC, not the full FLC value. Set it to full FLC on a winding-connected device and it will never see enough current to trip during a genuine sustained overload, because the device is reading a value that is structurally 42% lower than what a line-connected device would see at the same motor loading. If the thermal element is line-connected, ahead of all three contactors, set it to nameplate FLC exactly as on a DOL starter. No adjustment, no 58% factor.

What we see in the field: a panel builder swaps a failed winding-connected overload for a spare rated in DOL amps, dials it to nameplate FLC out of habit, and the motor runs for months under a real 15% overload the device never catches. Check which side of the delta contactor the device sits on before touching the dial, every time.

Magnetic Trip and Selecting the Current Range Through the Transition

The magnetic element on a combined thermal-magnetic MPCB is fixed, not adjustable — it trips at a set multiple of whatever current range (frame/dial range) the device was selected in. That means the 58% rule doesn't just change the thermal dial reading; it changes which current range you buy the device in, and that range carries its own fixed magnetic pickup. A device selected for winding-connected duty at 58% of FLC has a lower magnetic pickup in absolute amps than the same motor's DOL-rated device, because it's a smaller range. This is correct for faults occurring on the winding side of that contactor pair, since a fault there produces current referred through the same delta relationship, but it means the identical physical MPCB model can't be swapped between line-side and winding-side duty without re-selecting the range from the manufacturer's star-delta column.

Sizing an MPCB Ahead of a Soft Starter

Soft starters carry their own electronic overload protection, usually an I²t thermal-memory model built into the controller, so an MPCB placed ahead of one does not need to duplicate thermal overload sensing. Most panel builders use a magnetic-only MPCB upstream of a soft starter for short-circuit and isolation only, and let the soft starter's own algorithm handle overload. A thermal-magnetic MPCB in that position, calibrated against a bimetal curve built for instantaneous DOL-style inrush, doesn't match a multi-second SCR ramp profile well; the RMS current shape during a soft start isn't the shape the thermal curve assumes, and mismatched calibration produces trips that have nothing to do with an actual overload.

The safer route, and the one most manufacturers document, is to follow the tested combination listed in the soft starter's own coordination table rather than guessing at an MPCB model. Swap either half of that pair and the declared short-circuit rating no longer applies.

Coordination Tables for Star-Delta and Soft-Start Assemblies

Type 1 and Type 2 coordination per IEC 60947-4-1 are declared for a tested assembly: SCPD plus contactor(s), plus overload relay or soft starter, plus starting method, not for a protective device in isolation. A star-delta assembly's fault let-through and clearing behavior depends on which contactor is in the fault path at the moment of the fault (main, star, or delta), and a soft-starter assembly's depends on the SCR bank's own let-through contribution during a fault. This is why star-delta wiring diagrams from the manufacturer specify exact catalog numbers for every component, and why the Type 1 vs Type 2 coordination rating only holds for that documented combination. Substitute a different frame size, even one with a higher individual rating, and the tested rating no longer applies — the panel has to be re-verified or re-tested.

Common Sizing Mistakes

The most frequent error is setting a winding-connected thermal element to full nameplate FLC instead of the 58% value, which under-protects the motor silently for as long as it takes someone to notice the winding running hot. The second is the reverse: applying a 58% derate to a line-connected device by habit, which nuisance-trips a motor that was never overloaded at all. The third is picking an MPCB or overload relay directly ahead of a soft starter and expecting the bimetal curve to catch an overload the way it would on a DOL circuit. It won't, reliably, because the current shape it's calibrated against isn't the shape a soft starter produces. The fourth is reusing an existing star-delta enclosure for a larger motor without re-checking the available fault current against the assembly's declared coordination rating, not just the new motor's FLC.

Conclusion

Star-delta and soft starting don't make MPCB sizing harder in the way most people expect — the magnetic trip margin already covers both starting profiles comfortably. The real work is confirming where the device sits in the circuit, applying the 58% rule only when it's winding-connected, matching the current range to the manufacturer's star-delta or DOL column, and treating a soft starter's coordination table as fixed rather than as a suggestion. Get the placement and the range right, and the rest of the sizing follows the same rules as a standard MPCB selection and setting process built around any motor protection circuit breaker and contactor pairing used in a motor starter assembly. For background on the device itself, see the MPCB engineering guide.