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Overload setting & cable size to IEC


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Ivanho,

 

 

Welcome to the forum.

 

The thermal overload relay setting is normally kept at 85% of full load current of motor.

 

However after setting at 85% it is preferable to check the normal running current of the motor and set the OLR at 10% above it, as in some cases the running current is as low as 50% of the motor full load current.

 

The conductors should be sized to atleast 2 times the maximum current flowing through the conductor.The upstream breaker should be sized to enable it's tripping if the current flowing through the conductor exceeds its capacity.

 

Chaterpilar

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Thanks Chaterpilar for your suggestions.

 

I am looking at the IEC Standards requirements vs NEC. Motor rated current is generally used in IEC for OL setting but what about acceptable settings if the motors are Nema with service factor > 1 in terms of coordination of the thermal relay to the motor heating characteristics . Likewise, if UL MCCBs are used the coordination with a mixed system could be a problem.

 

Regards

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