Sometimes it is necessary or desirable to run cables in parallel. This can help achieve the required cable rating, simplify installation, reduce procurement cost when smaller cables are bought in bulk, or make cable termination more practical.
When deciding whether to use parallel cables, the engineer should consider current capacity, voltage drop and fault rating. These checks are especially important where cable sizes, route lengths or installation conditions are not identical.
Current capacity
In parallel circuits, the phase current is divided between two or more conductors. Where the parallel cables are the same size, have the same length and are installed under the same conditions, the current is normally shared equally between them.
If the conductors, route lengths or installation conditions differ, current may not divide equally. In that case, one conductor may carry more current than another. Where current sharing is unbalanced, it may be necessary to provide separate overload protection for each conductor or to revise the design so that the parallel paths are matched.
Voltage drop
Voltage drop calculations need to take account of the arrangement of the parallel conductors. For equally loaded conductors with the same impedance, the calculation is relatively straightforward.
Where the impedance of each conductor varies, or where the current in each conductor is different, the calculation becomes more involved. The total voltage drop depends on the impedance and current in each parallel path, rather than simply on the number of conductors installed.
Fault rating
Adding cables in parallel does not necessarily increase the fault rating of each individual cable. The worst-case fault may still be towards the source and may apply to only one faulted parallel cable or conductor.
As a general rule, each individual parallel cable should be rated for the expected fault level. The through-fault level may also be increased or decreased by the use of parallel cables, depending on the system arrangement.
Good practice for parallel cable designs
- Use conductors of the same material, size and construction wherever possible.
- Keep parallel cable lengths and routes matched.
- Install parallel conductors under the same thermal conditions.
- Check current sharing, voltage drop and fault rating for the actual route arrangement.
- Confirm overload and fault protection requirements for each conductor or cable set.