When One Cable Is Not Enough: Running Power Cables in Parallel

A feeder that needs 800 A is sometimes built from two cables per phase because a single conductor large enough is hard to pull, bend or terminate. The arrangement is legitimate, and it is also one of the easier ways to waste copper or aluminium. If the two cables do not share current evenly, one runs hotter than its rating allows while the other idles, and the protective device never sees a problem because the combined current looks acceptable.

Why parallel cables share current unevenly

Current divides between parallel paths in inverse proportion to their impedance. Two identical cables of identical length carry equal current. Change either one slightly and the split changes with it.

Take length first. If one cable is 5 per cent longer than its partner, its resistance is about 5 per cent higher, so it carries roughly 48.8 per cent of the load and the shorter cable carries 51.2 per cent. That imbalance is modest. Conductor size is a different matter. Pair a 240 mm² cable with a 300 mm² cable of the same material and the larger cable takes about 56 per cent of the current. The smaller cable then carries 44 per cent, which may be well above the share its own rating would allow.

With single-core cables there is a second effect. Magnetic coupling between cables makes the reactance of each conductor depend on where it sits in the group. A poorly arranged group can push far more current into one cable than resistance alone would predict.

The conditions that keep the split even

Standard wiring practice sets out the same short list for any parallel group, and APAR recommends treating every item as mandatory rather than preferred.

• Same conductor material in every cable of the group. Copper and aluminium must not be mixed.
• Same cross-sectional area in every cable.
• Same length, and as far as possible the same route, so that each cable sees the same ambient conditions.
• Same construction and insulation type, so that each cable has the same permitted conductor temperature.
• For single-core cables, the same formation for each group, with the phases interleaved in a repeating pattern so that every cable has the same reactance. The trade-offs between trefoil and flat laying are set out in the trefoil versus flat laying guide.

Terminations matter as much as the cables. Each cable should be fitted with a lug of the same type, crimped with the same tool and die, and bolted to the same busbar or terminal face. A high-resistance joint at one end shifts current just as surely as a shorter cable would.

Derate the group, not the cable

A parallel group is still a group of cables, and cables laid close together heat one another. The current rating of each cable must be reduced for grouping, for the ambient temperature and, where buried, for depth and soil conditions. The working method is to size for the load, divide by the number of parallel cables, then check that each cable can carry that share after all derating factors are applied.

A simple illustration, using assumed numbers for the arithmetic only: a circuit must carry 600 A after derating. Two parallel cables must each be able to carry 300 A under the installed conditions, which means the tabulated rating of each cable has to be higher than 300 A before the derating factors are applied. The derating factors themselves are listed in the cable derating factors chart and, for buried runs, in the depth of laying and temperature guide.

The same logic applies to fault current. The short-circuit withstand of each conductor must be checked against the fault current that flows through it, using the method in the short-circuit withstand chart.

When a single larger cable is the better answer

Parallel runs double the number of terminations, double the pulling effort, and make a fault harder to trace. They are best reserved for cases where one cable genuinely cannot do the job. Before accepting a parallel design, check whether a larger single cable is available. APAR’s LV XLPE insulated power cables are made as single-core cables up to 1000 mm², at up to 1100 V, with a 90°C continuous rating, and the same page lists a higher-ampacity 105°C variant. Where the limit is the available conductor size, a larger single cable or a higher-ampacity option may remove the need for parallel runs altogether. Termination space, bending radius and pulling tension still need to be checked, since a very large cable is stiffer to handle.

For medium voltage circuits the same reasoning applies. The MV and HV XLPE power cables range includes single-core cables from 150 to 1000 mm², and sheath bonding then has to be considered for every cable in the group, because circulating sheath currents add loss.

APAR recommendation

Use parallel cables only when the load cannot be met by one cable, and then match the cables exactly: one material, one size, one length, one construction, one formation. Verify the current share of each cable after derating, check the short-circuit withstand per conductor, and use identical terminations. For a feeder that is being specified now, send the load, route length, installation method and ambient temperature with the enquiry, and APAR’s technical team can advise whether a single larger cable or a parallel arrangement suits the duty better.