Before installing an IE3 motor in the field, the first question to answer is simple but often skipped: have I correctly selected the cable, fuse and contactor for the current this motor draws? A wrongly sized cable heats up and wastes energy for its whole life; an undersized contactor fails early; a wrongly set thermal relay either leaves the motor unprotected or trips constantly. In this article we work step by step, with a practical installation view, from the rated current (In) on the motor nameplate to the selection of cable cross-section, fuse, contactor and thermal relay.

IE3 electric motor nameplate, rated current and contactor with thermal relay in panel

What Does the Nameplate Rated Current (In) Tell Us?

The rated current on the motor nameplate is the current the motor draws when running continuously at rated power and rated voltage. In a three-phase motor this current is roughly a function of power, voltage, power factor (cos φ) and efficiency. The nameplate usually lists two current values, one for 230 V (delta) and one for 400 V (star); you must use the correct one according to the field supply voltage. We explained the logic of these two values and star-delta bridging in detail in terminal connection and 230/400V voltage selection.

Reading the nameplate correctly is the foundation of a correct calculation. Rated current, power factor, efficiency class, pole count and duty type all play a role in selection. The article on nameplate matching, where we gathered nameplate reading and ordering information, is a good checklist for avoiding the wrong motor. For the shaft and frame table of IE3 motors, see the IEC 56-355 shaft and frame table.

Starting Current: Transient but Decisive

Rated current describes the steady state; but when the motor starts it draws a much higher current. In direct-on-line (DOL) starting, this starting current can rise to about 5-8 times the rated current. This transient surge matters for the fuse not tripping by mistake, the contactor contacts withstanding it, and the cable carrying it. We covered why the starting current is so high and how it can be reduced in starting current (LRA) and starting methods.

To reduce the starting current, star-delta or a soft starter is used; star-delta lowers the starting current to roughly one third. You can find which method suits which case in star-delta or soft starter and the wiring diagram in the star-delta wiring diagram. The starting method directly affects both the number of contactors and where the thermal relay is connected.

Cable Sizing: Current, Voltage Drop and Distance

Three main factors determine the cable cross-section. The first is current carrying capacity: the cable must continuously carry the motor's rated current without overheating. This capacity varies with cable type, installation method (conduit, tray, free air) and ambient temperature. The second is voltage drop: the longer the cable, the more voltage drops across it; if the voltage reaching the motor terminal falls too low, the motor struggles, heats up and loses efficiency. Common practice is to keep the voltage drop on the motor feeder below a reasonable limit (for example 3-5 percent). The third is distance: the longer the run between panel and motor, the larger the cross-section needed for the same current.

A practical approach to cable selection

  • First determine the correct rated current of the motor (according to the field voltage).
  • Select the cable current capacity above this current, with installation and temperature derating factors.
  • If the run is long, calculate the voltage drop and increase the cross-section one step if needed.
  • Consider the transient surge of the starting current and any future power increase.

The correct cable gland and IP protection at the terminal box where the cable connects to the motor also matter; we examined this in terminal box and cable connection.

Cable cross-section, thermal magnetic breaker and contactor selection in a motor feeder panel

Fuse and Thermal Magnetic Breaker Selection

Protection in the motor circuit must do two jobs at once: fast disconnection against short circuit and time-delayed protection against overload. Short-circuit protection is provided by the fuse or the magnetic stage of the thermal magnetic breaker; this stage is chosen so it does not trip on the transient starting surge but cuts instantly on a real short circuit. Motor protection devices (motor protection circuit breaker, MPCB) are designed exactly for this purpose; the setting range is based on the motor's rated current. We gathered the logic of thermal, relay and fuse selection in electric motor protection: thermal, relay and fuse.

A common mistake here is selecting the fuse by "the nearest value at hand" rather than by the motor. An oversized fuse cuts late even on a short circuit, risking both cable and motor; an undersized fuse trips on every start. The right approach is to evaluate the motor's rated current and starting method together and select the protection device accordingly.

Contactor Selection: AC-3 Utilisation Category

The contactor is the switch that energises and de-energises the motor, and the most critical concept in its selection is the utilisation category. The standard category for motor loads is AC-3; this category is defined for drawing the motor starting current and breaking at rated current under normal conditions. The contactor must be sized in the AC-3 category to comfortably carry the motor's rated current (and the starting surge). The same contactor is rated for a lower current in an application with a heavier breaking condition such as AC-4 (frequent breaking, plugging, jog); so the breaking character of the application directly affects contactor size.

In star-delta starting, three contactors (line, delta, star) are used and their rating differs from DOL; therefore contactor selection cannot be finalised before the starting method is decided. We listed motor commissioning and first-start checks step by step in commissioning and first start.

Thermal Relay Setting

The thermal relay is the element that protects the motor against sustained overload, and setting it correctly is vital. The setting value should be equal to or very close to the motor's rated current on the nameplate; you must use the correct current for the field voltage. A thermal relay set too high does not act when the motor is overloaded and allows the winding to burn; one set too low trips even at normal load and stops production. The connection point of the relay also changes with the starting method; in star-delta, if the relay is placed before the delta contactor to measure phase current, the setting value is calculated differently.

Protecting the motor from overheating and burning is not only about the thermal relay but also about correct sizing and the right load ratio. We covered at what load the motor should run in load ratio and correct sizing, and the role of temperature monitoring (PT100/thermistor) in temperature monitoring. You can find the effect of the IE3 motor insulation class (F/H) on thermal endurance in insulation class F and H.

Starting With the Right IE3 Motor

For all this calculation to rest on the right foundation, the motor itself must be selected correctly. The IE3 efficiency class is legally mandatory at most power levels; we summarised which power requires IE3 or IE4 from which date in efficiency mandate and regulation. For a stock guide covering power and speed options of IE3 motors, see the IE3 motor stock guide, and for the wide product range our efficient electric motors page. We compared the effect of the correct pole count (speed) on current and torque in pole selection.

Other Factors That Affect the Current Calculation

Rated current is not a number on its own; many environmental and application-dependent factors affect this current and therefore the selections. The first is power factor (cos φ): part of the current the motor draws is the active component doing useful work, and part is the reactive component building the magnetic field. A low power factor means a higher current for the same power; this matters for both the cable and the reactive penalty. We examined power factor and reactive penalty in high-efficiency motors in power factor and reactive penalty.

The second is environmental conditions: high ambient temperature and high altitude reduce the carrying capacity of both the motor and the cable. In a hot boiler room or a high-altitude plant, the cable and motor must be re-evaluated with derating factors; we covered this derating topic in derating at high altitude and hot environments. The third is duty type: the thermal load of a continuously running (S1) motor differs from a frequently starting-stopping (S3, S4) motor; frequent starting is a heavier condition for both the contactor and the thermal relay.

The fourth factor is harmonics: if the motor is fed from a frequency drive, the current waveform is distorted and both cable and motor are subject to extra heating. In drive applications you therefore need to leave some margin; we explained drive-motor compatibility in frequency drive with asynchronous motor. All these factors require a selection that accounts for the real field conditions, rather than "running a cable exactly to the nameplate current". We gathered typical mistakes made when buying a motor in mistakes when buying a motor.

Frequently Asked Questions

Should I size the cable by the motor power or by its current?

Always by current. Power is a starting figure, but cable heating and voltage drop relate directly to current. A motor of the same power draws a higher current at 230 V and a lower current at 400 V; so you must determine the correct rated current for the field voltage and select the cable above this current, the installation conditions and the run length.

Why do we select the contactor slightly above the motor's rated current?

Because the contactor must carry the continuous rated current and also withstand the transient surge on every start and the contact wear. AC-3 utilisation category rating is made exactly for this motor character. Under heavy conditions such as frequent breaking, jog or braking (AC-4), the same contactor is rated for a lower current, so one size larger is chosen.

What current should I set the thermal relay to?

You should set it to the rated current on the nameplate corresponding to the field voltage. In star-delta starting, the setting is calculated by phase current depending on where the relay is connected. The general rule is to set the relay as close as possible to the rated current so it protects the motor under overload but does not trip during normal operation.

Get a Quote

You can consult us to make the right decision on the panel, protection and cabling suitable for your IE3 motor's current. Share your motor nameplate data, starting method and run length; let us choose the right motor and protection solution together. Call +90 (532) 345 49 86 now or request a quote through our contact page. For the product range, visit our efficient electric motors page and our hemmotor.com homepage.

IE3 Motor Installation Checklist

  • Did you read the nameplate rated current correctly for the field voltage (230/400 V)?
  • Have you decided the starting method (DOL, star-delta, soft starter)?
  • Did you select the cable cross-section by current capacity, voltage drop and run length?
  • Is short-circuit protection (fuse/MPCB) set so it does not trip on the starting current?
  • Did you select the contactor in AC-3 category, above the rated current?
  • Did you set the thermal relay to the rated current (at the point suitable for the starting method)?
  • Are the correct cable gland and IP protection provided at the terminal box?
  • If required, has temperature monitoring (PT100/thermistor) been added?