PlantFlow 3D › Guides › Motor full-load current and overload settings
Motor full-load current and overload settings
How to calculate a three-phase motor's full-load current, why the nameplate value wins, and how it is used for overload and cable checks.
The formula
For a three-phase motor the input power is P_in = P_out / η, and the line current is I = P_out / (√3 × V × pf × η). Here P_out is the rated shaft power in watts, V the line voltage, pf the power factor and η the efficiency.
Example: 7.5 kW, 400 V, pf 0.85, η 0.90 gives I = 7500 / (1.732 × 400 × 0.85 × 0.90) ≈ 14.2 A. PlantFlow 3D shows this as a calculated estimate.
Why the nameplate value is better
Real motors differ: efficiency and power factor change with size, speed and design, and the manufacturer has measured the actual rated current. Whenever the nameplate current is known, use it for the overload setting and for checking cables and switchgear. The simulator marks calculated values as estimates and lets you enter the nameplate value.
Where the current is used
The rated current sets the thermal overload relay, guides the contactor size (utilisation category AC-3 for squirrel-cage motors), and is the design current for the cable check together with the cable length, installation method and voltage drop limit.
Starting current is several times the rated current for a direct-on-line start. The exact ratio comes from the motor data sheet. In the simulator it is a teaching assumption that you can change.
Sizing cables needs real data
Cable current ratings depend on conductor size, insulation, installation method, ambient temperature and grouping. These values come from the cable manufacturer or the standard that applies to your project. PlantFlow 3D does not include those tables: you enter the verified rating and correction factors, and it checks the coordination and voltage drop.
Try it yourself
Open Tools → Cable check, link it to motor M1 and enter your cable data. Change the motor power in the Data tab and see the load schedule update automatically.
Open the PlantFlow 3D simulator →Safety: this guide explains principles for learning. Real installations must follow the applicable codes, the equipment manufacturer's instructions and the approval of a competent, qualified person. Isolate, lock off and prove dead before touching any circuit.