Fusing a Motor Starter Control Circuit

Thread Starter


Joined Aug 5, 2007

I need help on how to fuse a Motor Starter control circuit. How to fuse the Primary side and how to fuse the secondary side.....I think I know....Is it-

480VAC Motor Starter w/ 120 VAC Control Circuit.

VA rating of control transformer / 480 = amp draw of primary side of control transformer ( What kind of fuse to use?) Time Delay? or Fast Acting? Seems logical to use a time delay for "in-rush".

VA rating of transformer / 120 = amp draw on secondary side of control transformer (What kind of fuse to use?) Time Delay? or Fast Acting?

Thanks in Advance!!!!



Joined Aug 4, 2007
I would certainly agree with your line of thought regarding using an anti-surge (delayed) characteristic fuse for the primary side, this would be essential if the transformer is of toroidal design since these exhibit very high magnetising inrush currents.

As for the input fuse current rating you need to bear in mind that small transformers of around 20VA rating and above typically have 10% impedance and hence a short circuit at the output will draw around 10 times full load rating, the fuse obviously needs to have a significantly lower rating than this (very small transformers may have an impedance as high as 25% resulting in a short circuit current of only 4 x rating).

Regarding the output fuse; if there are contactors in the outgoing circuit remember that these also have magnetising inrush currents, if the outgoing side is purely solid-state circuitry then a fast fuse will minimise damage under fault conditions.



Joined Jun 4, 2007
Entirely agree with whats been said. Would like to add, if you total max draw is say 100W, then fuse = >100/440~ .25A. Can you actually get 440V fuses this small?, the smallest I have seen is 6A?