Sometimes it is necessary or desirable to run cables in parallel. Reasons may include:
- It is required to achieve the needed cable ratings
- Installation of two smaller cables may be easier than one larger cable
- The cost of buying smaller cables in bulk may make it more economical
- It may make termination of the cable easier
In deciding to use parallel cables, there are some considerations the engineer needs to be aware of, particularly in current capacity, voltage drop and fault rating.
In parallel cables, the phase current is divided between two or more conductors. Typically in paralleling cables, the size and length of each cable are the same, and the current flow is equally divided between the parallel conductors. If the conductors (or lengths) are of unequal size, then the current will not be distributed evenly (i.e. the conductors will carry different currents). Where currents in parallel conductors are unbalanced, it may be necessary to provide separate overload protection for each conductor.
The calculation of voltage drop across a cable needs to consider the arrangement of parallel conductors. For equally loaded conductors the impedance of each conductor is the same, and the calculation is relatively easy. Where each conductor impedance varies, and there are different current flows in each conductor the calculation becomes more involved.
For parallel conductors, the worse case fault may still be towards the fault source and may only apply to a single faulted parallel cable or conductor. Adding additional cables in parallel doe not necessarily increase the fault rating. Generally, each individual parallel cable should be rated for the expected fault level.
Note: the through fault level may be increased (or decreased) by the use of parallel cables.