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Motors and voltage drop

Chris Pearson
I feel that I ought to know the answer to this one ...


Typically my woodworking machines draw about 6 times the rated current on start-up. So when checking for voltage drop, should I use the rated current (i.e. when on load) or the starting current?


In my particular case, there is no problem because the origin is in the workshop. However, a friend has told me that he has used a whole drum of cable (4 mm 2 for two circuits) and the biggest machines are furthest away. I would be interested to see whether he may expect starting problems. Least cost is his main priority in life.
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    You need to be careful - if dropping 5% at nominal load, then at start up (assuming your 6 times figure is correct - my experiance is that for some kinds of motor that may actually be a bit low) you may drop 30% - which is enough for the no-volt trip on some designs to operate. Equally a cable 6 times the size needed is wasteful being 6 times oversize.

    I'd suggest  a half an half approach and work to have no more than 20% drop on start-up of the largest motor.

    The modern fix to this problem is the variable frequency drive.
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  • 0
    You need to be careful - if dropping 5% at nominal load, then at start up (assuming your 6 times figure is correct - my experiance is that for some kinds of motor that may actually be a bit low) you may drop 30% - which is enough for the no-volt trip on some designs to operate. Equally a cable 6 times the size needed is wasteful being 6 times oversize.

    I'd suggest  a half an half approach and work to have no more than 20% drop on start-up of the largest motor.

    The modern fix to this problem is the variable frequency drive.
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