Overload Relay Sizing for Star-Delta Starters
What is overload relay sizing for a star-delta starter? In a star-delta (wye-delta) starter the thermal overload relay normally sits in the delta leg of the motor circuit and sees only the delta-winding current — line full-load current (FLC) divided by √3, roughly 0.58 times the nameplate FLC — so a relay dialed to the motor's full nameplate FLC is set about 42% too high. Get the reference current wrong and the relay either lets a genuine overload run through the star-to-delta transition undetected, or nuisance-trips at every start if someone tries to compensate the wrong way. This article covers where the relay sits in the circuit, the √3 calculation with a worked example, how the star-to-delta transition timer interacts with trip class, wiring mistakes that put the relay in the wrong leg, and how the sizing differs from a direct-on-line (DOL) starter.
Where the Overload Relay Sits in a Star-Delta Starter
A star-delta starter uses three contactors instead of one: a line contactor, a star contactor, and a delta contactor. During the star (Y) phase, the motor windings are connected in wye, dropping the voltage per winding to 1/√3 of line voltage and cutting starting current and torque to roughly a third of a direct-on-line start. After a timed interval — commonly 2 to 10 seconds, sized to the load's run-up — the star contactor drops out and the delta contactor closes, reconnecting the windings line-to-line at full voltage.
The thermal overload relay is wired in series with the delta contactor and the motor's delta-connected windings, not in the incoming line ahead of the star/delta split. That placement matters: it means the relay's bimetal strips carry the phase (winding) current, not the line current the meter on the panel door shows. Some panel builders wire the relay ahead of the transition circuitry out of habit from direct-on-line jobs — that puts the full line current through it and defeats the sizing logic below.
The √3 Calculation: Line FLC vs Delta-Leg Current
Line current and winding (phase) current are not the same number once the motor is connected in delta. For a delta-connected three-phase load, line current equals winding current times √3 (about 1.732). Flip that around and the winding current — the value the relay in the delta leg actually sees — equals line current divided by √3, about 0.577 times line current.
Formula: Overload Relay Setting for Star-Delta Starters — Source: IEC 60947-4-1, motor starter application practice
IOL = FLC / √3
| Symbol | Description | Unit |
|---|---|---|
| IOL | Overload relay dial setting (delta-leg / winding current) | A |
| FLC | Motor full-load current from the nameplate (line value) | A |
| √3 | Constant, approximately 1.732 | — |
Set the dial to IOL, not to FLC. A relay set to the full nameplate current in this position would need roughly 73% more current before it reacts — by the time it trips, the motor has been overloaded for a stretch the bimetal was never asked to time.
Worked Example: Sizing the Relay for a 30 kW Motor
Take a 400 V, 30 kW motor with a nameplate FLC of 56 A, started star-delta. Divide 56 by 1.732 and the delta-leg current comes out at 32.3 A. The overload relay goes in a range that covers that figure — a 23-32 A or 24-36 A dial range, whichever the selected frame offers, set at 32 A. Nothing in this calculation changes because the motor is large or small; the ratio holds at any FLC.
What we see in the field: technicians who size the relay off the line-side ammeter reading during commissioning, rather than off the nameplate arithmetic, land close to the right number by accident — because the ammeter clamped on the delta-leg conductor already reads the lower phase current. The mistake shows up when someone clamps the incoming supply conductors instead and copies that number onto the dial.
Transition Timing and Trip Class in Star-Delta Starts
The star-to-delta transition timer is set for the load's run-up time, typically long enough for the motor to reach 80-85% of synchronous speed before the delta contactor closes. Too short a timer and the delta contactor slams in while the motor is still accelerating, producing a current and torque transient that can exceed the direct-on-line starting current for a fraction of a second. Too long a timer wastes the reduced-torque benefit and can stall a load that needs more torque than the star connection delivers.
That transient is brief — well under a second — and a correctly sized Class 10 or Class 10A overload relay rides through it the same way it rides through a normal start, provided the thermal memory has not already been loaded up by a previous failed start attempt in quick succession. High-inertia loads — large fans, centrifuges — that take longer than a Class 10 curve allows need the same Class 20 or 30 selection logic used on a direct-on-line starter, just applied to the delta-leg current instead of the line current.
Common Wiring Mistakes That Break the Sizing
Three mistakes turn up repeatedly on panels sent in for rework. First, the relay lands in the line ahead of the star/delta split instead of the delta leg — the dial is then correctly set to full FLC, but the whole point of star-delta protection at reduced current is lost, and most manufacturer coordination tables for this configuration assume the delta-leg position. Second, someone sizes the relay off the star-connection current instead of the delta-connection current — the two are different values on the same motor and neither substitutes for the other. Third, the relay range is selected for the line FLC and then the dial is set to the divided value at the bottom of that range, which reduces trip accuracy because bimetal relays read most consistently mid-scale.
Check the star-delta contactor wiring before troubleshooting a sizing complaint — a relay landed in the wrong leg will read a plausible-looking current and still protect the wrong quantity.
Star-Delta vs Direct-On-Line: How Sizing Differs
The relay itself does not change between the two starting methods — the same bimetallic or electronic thermal overload relay family works in either circuit. What changes is the reference current used to pick the dial setting and, in some cases, the frame size needed to cover that lower current range.
| Criteria | Direct-On-Line (DOL) | Star-Delta |
|---|---|---|
| Relay location | In series with the single contactor and motor | In the delta leg, behind the delta contactor |
| Current the relay sees | Full line current = FLC | Delta-leg current = FLC / √3 |
| Dial setting | Motor nameplate FLC | FLC / √3 (about 0.58 × FLC) |
| Typical frame range needed | Sized to full FLC | Often one range smaller, since the current is lower |
| Nuisance-trip risk from transient | Inrush at start only (6-8x FLC) | Inrush at start, plus a brief spike at star-to-delta transition |
Coordination between the overload relay, the three contactors, and the upstream motor protection circuit breaker or fuse still applies in star-delta the same way it does in a single-contactor starter — check the manufacturer's IEC 60947-4-1 coordination tables for the Type 1 or Type 2 rating declared for the specific breaker-contactor-relay combination, since a star-delta arrangement uses three contactors against one SCPD and one relay.
Frequently Asked Questions
Where does the overload relay go in a star-delta starter?
In the delta leg, wired in series with the delta contactor and the motor's delta-connected windings. It carries the winding current after the star-to-delta transition, not the incoming line current.
Why is the overload relay set below the motor's full-load current?
Because it measures delta-leg (winding) current, which is line FLC divided by √3 — about 58% of the nameplate value. Setting it to the full nameplate FLC leaves the relay under-sensitive by roughly 42%.
What happens if the relay is set to the full nameplate FLC by mistake?
The relay under-protects the motor. It needs about 73% more current than intended before the bimetal reaches trip position, so a genuine overload can run for longer than the Class 10 or 10A curve was meant to allow.
Does the star-to-delta transition timer affect overload relay trip class?
Indirectly. A correctly timed transition produces a brief current spike that a properly selected trip class rides through. A transition timer set too short worsens that spike and can push a marginal Class 10 selection into nuisance tripping.
Can an electronic overload relay be used in a star-delta starter?
Yes. Electronic relays are sized the same way — divide the nameplate FLC by √3 and set the electronic relay's current range to that value — and their wider 1:4 setting ratio makes it easier to land mid-scale on the lower delta-leg current.
Conclusion
Star-delta sizing comes down to one number: line FLC divided by √3. Put the relay in the delta leg, calculate the delta-leg current, and set the dial there — not at the nameplate FLC, and not at the star-connection current. Everything else — trip class selection, coordination with the contactors and upstream SCPD, ambient compensation — follows the same rules used on a direct-on-line starter once that one number is right. For the full mechanism behind the relay itself, see the thermal overload relay engineering guide, and for the setting procedure on a standard motor starter, see how to select and set an overload relay for a motor.
