In-Line vs Inside-Delta Soft Starter Connection

What is the difference between in-line and inside-delta soft starter connection? In-line (3-wire) connection wires the soft starter's SCR pairs in series with the incoming line conductors, so each pair carries the motor's full line current; inside-delta (6-wire) connection wires the SCR pairs into the three legs of the motor's delta winding instead, so each pair carries only phase current — line current divided by √3, about 58% of the line value. That current split lets a lower-current starter frame run a motor that would otherwise need the next frame size up in-line, at the cost of six motor leads instead of three and a firing sequence that has to match the correct winding leg. Covered here: the current math behind the 58% figure, wiring and lead count, phase pairing risk, which models actually support inside-delta, and when in-line stays the simpler default.

In-Line Connection: The 3-Wire Default

Most soft starters ship set up for in-line connection out of the box. The starter's three power terminals (commonly marked L1/L2/L3 in, T1/T2/T3 out) sit directly between the supply and the motor's three main leads, the same three leads used for a direct-on-line contactor. Each of the three SCR pairs conducts the full line current the motor draws, both during the ramp and, on units without a bypass, continuously while running. Because the wiring matches a standard DOL hookup, in-line is the default on almost every catalog soft starter from PSR-class economy units up through flagship torque-control models, and it is what most panel builders wire without a second thought.

The practical implication is sizing: the starter's line-current rating in the catalog has to cover the motor's full-load current with margin for the start profile and duty cycle (see sizing a soft starter for a motor). There is no current-splitting benefit here — the starter frame sees exactly what the motor sees.

Inside-Delta Connection: Wiring Into the Motor Windings

To wire inside-delta, the motor has to bring out all six winding ends to its terminal box (typically marked 1U1/1U2, 1V1/1V2, 1W1/1W2 per IEC convention), not just the three line leads used for a standard hookup. The starter's three SCR pairs are broken into the winding itself, one pair per phase, with the delta closed inside the starter's connection rather than at the motor terminal box. The line conductors then connect straight to the motor's three main terminals as usual; only the internal delta bridge routes through the starter.

This only works on a delta-wound motor. A star (wye) connected motor has no accessible internal loop for the SCRs to sit in, so inside-delta is not an option there regardless of the starter model.

The Current Math: Why Inside-Delta Carries About 58% of Line Current

Divide by 1.732 and a phase current works out to roughly 58% of line current. A motor drawing 200 A at the line terminals only asks each inside-delta SCR pair for about 116 A. That is the entire commercial case for inside-delta: a starter frame catalog-rated for, say, 125 A inside-delta can control a motor that would need a 200 A-class frame if wired in-line.

What we see in the field: panel builders reach for inside-delta less to save on the starter itself and more to fit a smaller physical frame into an enclosure that was already laid out, or to reuse a starter left over from a decommissioned line on a bigger motor than its in-line rating allows.

Wiring and Lead Count Differences

In-line needs three conductors from the starter's output to the motor, identical to a DOL bucket. Inside-delta needs six conductors run to the motor terminal box, plus space inside the starter's own terminal compartment for the extra connections, and typically a jumper or parameter setting on the starter to tell it inside-delta mode is in use (some units also have a switch or dedicated inside-delta wiring diagram printed inside the enclosure door). Cable length matters here too: unequal lead lengths between the starter and the three winding pairs create a small resistance imbalance across an otherwise balanced load, and on long runs it is worth keeping the six leads matched.

A common retrofit case is a motor already wired six-lead for a star-delta starter (see soft starter vs star-delta starter) that is being upgraded to a soft starter. The six leads and delta bridge points are already in place, so inside-delta connection can be a drop-in reuse of the existing wiring rather than extra work — one of the few cases where inside-delta is the easier install, not just the cheaper starter.

Firing Sequence and Phase Pairing: The Risk Unique to Inside-Delta

In-line wiring has no pairing risk: swap two line leads and the motor just runs the other direction. Inside-delta is less forgiving. Each SCR pair has to fire in sync with the correct winding leg, matched exactly to the starter's own internal phase reference, not just connected to any three of the six terminals in any order. Get a pairing wrong and the fault does not always show up as a simple no-start; it can show up as unbalanced current between the three legs, uneven heating in the winding, or a shorted-SCR-style protection trip after a few starts once the imbalance heats one leg's insulation.

This depends on which terminal marking convention the manufacturer uses, and it is not universal across brands or even across product lines from the same brand, so wire strictly from the model's own inside-delta diagram, never from a generic delta sketch or memory of a different starter. Commissioning technicians who skip this step and wire by feel are the most common source of inside-delta call-backs.

Which Soft Starters Actually Support Inside-Delta

Not every model in a manufacturer's range supports inside-delta, and catalog current tables usually list two separate ratings for the same frame: one for in-line, one for inside-delta. Flagship torque-control units — ABB PSTX, Schneider Altistart ATS480, Siemens SIRIUS 3RW55 — generally support inside-delta connection across most of their frame sizes, since the larger-motor use case is exactly where the current saving matters. Compact economy units built for small motors, such as ABB PSR, are sized for loads small enough that inside-delta rarely changes the frame selection, so it is less commonly offered or relevant on that class. Before assuming inside-delta is available, check the specific model's technical data rather than the family name — see the range breakdown in ABB PSTX vs Schneider ATS480 vs Siemens 3RW55.

When to Choose In-Line vs Inside-Delta

In-line stays the default whenever the starter frame already covers the motor's line current with acceptable margin — it is simpler to wire, simpler to fault-find, and has no pairing risk. Inside-delta earns its place in three situations: the motor is large enough that the next in-line frame size is a real cost or enclosure-space jump, the motor already has six leads available from a star-delta or other prior installation, or a smaller starter is already on hand and the motor's inside-delta rating covers the job. It is rarely worth choosing inside-delta purely for its own sake on a small or mid-size motor where the in-line frame was already going to be small.

For the broader selection process — duty rating, starts per hour, ambient derating — see the soft starter selection guide. Stoklink stocks soft starters across the ABB PSR/PSE/PSTX, Schneider Altistart, and Siemens SIRIUS 3RW ranges, with inside-delta ratings listed on the relevant product pages.

Conclusion

In-line and inside-delta are two ways to put the same SCR pairs into the same motor circuit, and the only real difference is which current each pair has to carry — full line current in-line, roughly 58% of it (line/√3) inside-delta. That difference lets inside-delta shrink the starter frame on a big motor, provided the motor has six accessible leads and the model's catalog explicitly supports it. Get the phase pairing right from the manufacturer's own diagram, size from the model's inside-delta current table rather than a back-of-envelope √3 calculation, and default back to in-line whenever there is no clear frame-size or wiring-reuse reason to do otherwise.