When sizing parrallel supply cables, is it a requirement to install three conductors to allow for failure of one, therefore the remaining two conductors are capable of maintaining the full load.

Not usually, no - and it would be a very particular fault that open circuited a single conductor without influencing the other as they should be approx the same length and run generally in close proximity.

The choice of grouping ie 2 x, trefoill, 4 x or 5 x is generally a function of load versus conductor size/route and need for "equal" current sharing by conductors of a group.


Regards

OMS

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Failure is always an option