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Currents on Star-Delta Motors


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Dear Sir,


I have a 3phase motor of 100HP; 74.6KW; 0.8FP; 380V; 50Hz and I would like to know which would be the maximum current of normal functioning at Star and Delta situations. The idea is to know which current should I consider for the cable dimensioning task (both for Star and Delta connections), as the star-delta box will be far away from the motor, and I have to use two cables with three conductors each one for making the star-delta connections at the motor. Should I consider the following scenario for both cables dimmensioning ?. The cable length will be of 30 meters.


** I= I (nominal, at maximum speed and torque) at delta funcioning, i.e.: 74.6KW / (sqt 3 * 0.38 * 0.8)= 142A


or should I consider the current for the cable dimmensioning of 142A/sqt3= 82.6A ?


Thanking you in advance for your favourable reply, I remain.


Best regards,



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Hello John


Welcome to the forum.


The cables should be sized for the steady state current to the motor, you can ignore the start currents as the thermal capacity of the cable is much higher than the thermal capacity of the motor, and the protection should have a lower thermal capacity than the motor!!


If the motor was connected by three cable, the minimum cable size would be rated for at least 142 Amps continuous.

As this installation is inside delta with six cable between the star delta starter and the motor, each cable is carrying the winding current only, so each cable between the starter and the motor can be rated at 142 x 0.58 or 83Amps.


If you have a problem with voltage drop, you may wish to up the cable size to compensate.

The cables from the supply to the starter are a three wire circuit and must be rated at the full 142Amps capacity.


Best regards,

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  • 6 months later...



Just to clarify, cables coming into the top (line side of the starter) must be fully rated ie 142 amps continuous. Cables from the line and delta contactors to the motor can be 58% or 83 amp rated.




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