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Three-phase motors between 30 and 90 kW draw roughly 52 A to 161 A at 400 V. Cables and breakers are sized starting from this nameplate current, taking the starting method and the cable route into account.
| Power (kW) | 3,000 rpm (A) | 1,500 rpm (A) | 1,000 rpm (A) |
|---|---|---|---|
| 30 | 52.1 | 53.8 | 56.2 |
| 37 | 64.0 | 66.1 | 68.1 |
| 45 | 76.8 | 80.2 | 81.6 |
| 55 | 93.5 | 97.6 | 98.1 |
| 75 | 127 | 129 | 136 |
| 90 | 152 | 155 | 161 |
Values are the nameplate ratings of the models in the HEM Motor catalogue; they may differ for other brands or series.
How are breakers and contactors sized?
With direct-on-line starting, the motor protection breaker, contactor and thermal relay are chosen for the motor's line current, i.e. the value in the table. With star-delta starting, the main and delta contactors carry the winding current: Iwinding = Iline / √3 ≈ 0.58 × Iline. The star contactor sees roughly Iline / 3.
Example: the 45 kW 1,500 rpm 225M-4 motor has a nameplate current of 80.2 A. Assumption: 400 V supply, star-delta starting, thermal relay at the output of the main contactor. The main and delta contactors must carry at least 80.2 / √3 ≈ 46.3 A in category AC-3 and the star contactor at least 80.2 / 3 ≈ 26.7 A; the thermal relay is set to 46.3 A. The supply breaker and supply cable are chosen for the full line current of 80.2 A.
Cable cross-section
The cross-section cannot be read from the current alone: installation method (cable tray, conduit, underground), ambient temperature, the number of cables running side by side and the voltage drop due to length are assessed together. With star-delta, two separate three-core cables run to the motor and each carries the winding current. The calculation should be done by an electrical engineer; for the method see the question on motor cable cross-section.
Models in these ratings are in the cast iron frame electric motors category. For panel preparation, the article on panel checks when ordering a 30 kW motor is also useful. Send a photo of the existing motor's nameplate on WhatsApp and we will confirm the equivalent model and its nameplate current together.