Size the contactors, overload relay setting and cable current for a direct-on-line (DOL) or star-delta starter from the motor rating, and compare the starting current. Results update as you type.
| Direct-on-line (DOL) | Star-delta | |
|---|---|---|
| Starting current | 6–8 × FLC | about 2–2.7 × FLC (one third of DOL) |
| Starting torque | Full (100%) | About one third |
| Contactors | 1, rated ≥ FLC (AC-3) | 3: main and delta ≥ 0.58 × FLC, star ≥ 0.33 × FLC |
| Overload relay setting | FLC | 0.58 × FLC (relay in the phase circuit) |
| Motor cables | 1 three-core cable, ≥ FLC | 2 three-core cables, each ≥ 0.58 × FLC |
| Typical use | Small motors, most motors up to about 7.5 kW | Fans, pumps and compressors above about 7.5 kW that start with light load |
NEC installations size the motor branch-circuit conductors at 125% of the FLC from Table 430.250 (430.22) and set overload protection from the nameplate current (430.32). Short-circuit protection (fuse or MCCB) should follow the manufacturer's Type 2 coordination tables.
Average catalogue values for 4-pole motors at 400 V, 50 Hz, and the same scaled to 415 V and 380 V. For guidance only; use the nameplate for selection.
| kW | HP | FLC @ 380 V (A) | FLC @ 400 V (A) | FLC @ 415 V (A) | DOL contactor (A, AC-3) | Star-delta main/delta (A) |
|---|---|---|---|---|---|---|
| 0.37 | 0.50 | 1.2 | 1.1 | 1.1 | 9 | — |
| 0.55 | 0.74 | 1.6 | 1.5 | 1.4 | 9 | — |
| 0.75 | 1.01 | 2.0 | 1.9 | 1.8 | 9 | — |
| 1.1 | 1.5 | 2.7 | 2.6 | 2.5 | 9 | — |
| 1.5 | 2.0 | 3.6 | 3.4 | 3.3 | 9 | — |
| 2.2 | 2.9 | 5.1 | 4.8 | 4.6 | 9 | — |
| 3 | 4.0 | 6.7 | 6.4 | 6.2 | 9 | — |
| 4 | 5.4 | 8.8 | 8.4 | 8.1 | 9 | — |
| 5.5 | 7.4 | 11.6 | 11 | 10.6 | 12 | 9 |
| 7.5 | 10.1 | 15.8 | 15 | 14.5 | 18 | 9 |
| 11 | 14.7 | 22.1 | 21 | 20.2 | 25 | 18 |
| 15 | 20.1 | 29.5 | 28 | 27.0 | 32 | 18 |
| 18.5 | 24.8 | 36.8 | 35 | 33.7 | 38 | 25 |
| 22 | 29.5 | 43.2 | 41 | 39.5 | 50 | 25 |
| 30 | 40.2 | 57.9 | 55 | 53.0 | 65 | 32 |
| 37 | 49.6 | 69.5 | 66 | 63.6 | 80 | 40 |
| 45 | 60.3 | 84.2 | 80 | 77.1 | 80 | 50 |
| 55 | 73.7 | 102.1 | 97 | 93.5 | 115 | 65 |
| 75 | 100.5 | 138.9 | 132 | 127.2 | 150 | 80 |
| 90 | 120.6 | 168.4 | 160 | 154.2 | 185 | 95 |
| 110 | 147.5 | 205.3 | 195 | 188.0 | 225 | 115 |
| 132 | 176.9 | 242.1 | 230 | 221.7 | 265 | 150 |
| 160 | 214.5 | 294.7 | 280 | 269.9 | 330 | 185 |
| 200 | 268.1 | 368.4 | 350 | 337.3 | 400 | 225 |
| 250 | 335.1 | 452.6 | 430 | 414.5 | 500 | 265 |
Contactor columns show the next common AC-3 rating at or above the current. Manufacturer frame sizes differ; check the catalogue.
Next steps: size the cable with the Cable Size Calculator, the protective device with the Breaker Size Calculator, convert kW or HP to amps with the Load Current Calculator, and check generator starting capacity with the Generator Sizing Calculator.
Find the motor full load current (FLC). Choose a contactor with an AC-3 rating at least equal to the FLC, set the overload relay at the FLC, and size the cable for at least the FLC (125% of FLC under NEC 430.22). Expect a starting current of about 6 to 8 times FLC.
The main and delta contactors carry the phase current, about 0.58 × FLC, and the star contactor carries about 0.33 × FLC during starting. The overload relay, normally fitted in the phase circuit, is set at 0.58 × FLC. Two cable sets run to the motor, each carrying 0.58 × FLC.
In star, each winding sees line voltage ÷ √3, so starting current and starting torque both fall to about one third of the DOL values. This suits fans and pumps that start with light load, but not loads that need high starting torque.
Typically for motors above about 7.5 kW where the supply authority or a standby generator limits starting current. The motor must be rated for delta connection at the line voltage, for example 400 V Δ / 690 V Y on a 400 V supply.
About 15 A at 400 V three-phase for a typical 4-pole motor. Always use the nameplate value for final selection.
AC-3 is the IEC 60947-4-1 utilisation category for squirrel-cage motors that are started and switched off while running. Contactor catalogues give the AC-3 current and kW rating at each voltage; select by AC-3, not by the thermal (AC-1) rating.
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