I am designing a device that will be placed between a 220VAC 50Hz power source and a 1100W 7.7A motor load. The device switches power to the load given a relay input and the motor load draws a ~20A inrush current during startup (20A LRA and 7.7A FLA).

The device is powered from the same 220VAC 50Hz source, and I want to know if it is safe / OK to design the device with connectors/cables rated for 16A? This includes a 'CEE 7/7 plug to C-13' power cable and an 'IEC Type F' power socket (which powers the load).

For reference, the motor load's power cable is manufactured with a 250VAC/16A CEE 7/7 plug, which leads me to believe that I would be fine designing my device with the 16A-rated components. I already want to place a 16A slow-blow fuse in-line with the device power...



2 Answers 2


Short answer: possible ok.

Long answer: Probably not.

If the mechanical load is such that the amp-seconds of accumulated time does not trip the breaker time profile ( fast vs slow) then the I^2R heat rise time constant should not impose a significant aging or safety issue on the wire getting too hot. Current ratings are steady state for thermal reasons.

However, if you pull the plug during startup, it will arc and wear the contacts prematurely.

Normally Relays contacts and plugs are all derated by some OEM spec for surge loads like motors and light bulbs and battery loads. This depends on the solver contact or plating material resistance. (DC is derated even more due to arc extinguishing durations are longer)

However, you might not want a 50m extension cable because although the heat loss (I^2R=Pd) is distributed the resulting V=IR (rms) drop on the cable might reduce the voltage and hence the motor starting current and torque and result in poorer performance and a bit more self heating temperature in the motor windings from extended startup times and thermal time constants from the heat velocity and mass and thermal resistance for cooling with much less air circulation at startup.

Although many AC motors are designed for reduced surge currents like yours with ratio of 20LRA/7.7FLA many varieties of induction motors are 2~8:1 and DC motors up to 10:1


I believe your European Ratings are 16A resistive and 4A inductive, thus normalized to induction motors with 4:1 surge rating, you need a plug rated for 32A(resistive)/(8A inductive)

your connector contacts are undersized.


The typical European rating will distinguish between resistive and inductive load ratings. Below is an example of a typical European rating: 16(4)A 250V ~ T85 µ

In this example the 16 = resistive load amperage; (4) = inductive load amperage; A= amperage; 250V= voltage; ~ = AC; T85= Maximum operating temperature in centigrade; µ = micro-gap (<3mm) approved.

If there is less than 3mm of air space between a switch's contacts in the open position, a micro-gap approval (µ) may be granted. This mark indicates that the switch has general application approval with a qualifier that another device, such as a cord and plug, must provide an alternate means of disconnection from the main power source. < end quote >


From what I have been able to research and find, it is enough to design a circuit with 1.3x the full-load amperage (FLA) of a motor. Therefore, 1.3 * 7.7 = 10.01A, so designing circuitry with components rated to 16A should be safe enough.

LRA can be anywhere from 5-8x the FLA with a short-enough duration not to trip a thermal breaker, which is suggested (and I am planning) to include - therefore, the 16A slow-fuse should suffice.

  • 1
    \$\begingroup\$ Do you have links to your sources? \$\endgroup\$
    – CL22
    Jun 10, 2017 at 15:57

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