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Preparing a motor feeder means settling four things together: the protection device's setting range, the contactor rating, the cable size and the fuse. The table below is a practical starting point for standard 400 V three-phase induction motors. The exact value always comes from the rated current on the motor nameplate.
| Motor power | Approx. current | Protection range | Contactor (AC-3) | Cable size |
|---|---|---|---|---|
| 0.75 kW | ~1.9 A | 1.6–2.5 A | 9 A | 1.5 mm² |
| 1.5 kW | ~3.5 A | 2.5–4 A | 9 A | 1.5 mm² |
| 2.2 kW | ~5.0 A | 4–6.3 A | 9 A | 1.5 mm² |
| 4 kW | ~8.5 A | 6.3–10 A | 12 A | 2.5 mm² |
| 5.5 kW | ~11.5 A | 9–14 A | 18 A | 2.5 mm² |
| 7.5 kW | ~15.5 A | 13–18 A | 18 A | 4 mm² |
| 11 kW | ~22 A | 17–23 A | 25 A | 6 mm² |
| 15 kW | ~30 A | 23–32 A | 32 A | 10 mm² |
| 18.5 kW | ~36 A | 30–40 A | 40 A | 10 mm² |
| 22 kW | ~43 A | 37–50 A | 50 A | 16 mm² |
| 30 kW | ~58 A | 50–63 A | 65 A | 25 mm² |
| 37 kW | ~71 A | 63–80 A | 80 A | 35 mm² |
Three cases that break the table
- Long cables — voltage drop takes over as the deciding criterion. Above 50 metres, the size can exceed the table's value.
- High ambient temperature — both cable capacity and device current fall. Above 40 °C, apply the catalogue derating table.
- Heavy starting — crushers, hoists and compressors take long to start; the protection device is chosen in a class (20/30) that tolerates it.
What changes on star-delta and drive feeders
In star-delta starting, the main and delta contactors are sized for about 58% of motor current and the star contactor for 33%; the thermal relay sits in the delta line and is again set to the 58% figure. On a drive feeder a contactor is usually unnecessary; the fuse or breaker specified by the manufacturer goes on the drive's input, and motor protection is done with the drive's parameters.
Which family in which brand
For the same setting range there is more than one brand option. The TeSys family at Schneider Electric, SIRIUS at Siemens, and the Sigma Elektrik and Himel ranges in domestic production all cover the same current steps. The differences are breaking capacity, auxiliary contact options, mounting dimensions and lead time. Where the specification does not mandate a brand, we set those four side by side.
Send your motor list or nameplate photos; we will draw up protection, contactor, cable and fuse line by line and quote the same business day. However many lines the list has, it ships on one delivery note.
Frequently asked questions
The nameplate shows two currents — which one do I use?
Nameplates usually show two values, for 230/400 V. Take the row matching how the motor will be connected on your 400 V three-phase supply, star or delta. On a standard 400 V three-phase installation this is the 400 V row, usually the smaller figure. Where in doubt, check the link arrangement in the motor's terminal box.
Is it harmful to set protection above the rated current?
Yes. The most common response to repeated tripping is to turn the setting up, which leaves the motor unprotected. The setting should equal the nameplate current. If it keeps tripping, find the cause: a mechanical bind, phase imbalance, poor cooling or genuinely increased load. Raising the setting hides the problem and burns the winding.
Does it make sense to buy the next contactor size up?
On frequently switched applications, yes. A contactor's mechanical and electrical life is limited by operation count, and one size up lasts longer on the same load. On a motor starting dozens of times an hour that difference shows up in years. On rarely switched feeders the extra cost does not pay back.
Is taking the cable size from the table enough?
On short runs it usually is. But size follows whichever of three criteria is most demanding: current-carrying capacity, voltage drop and short-circuit withstand. On long runs voltage drop decides; where many cables are grouped, heating does. Send the lengths and installation method and we will check it with you.