Parallel cables — why current sharing is not automatically equal

The cable-sizing/current-carrying-capacity guide covers how to determine the correct cross-section for a single cable. At high load currents (for example a large motor, an EV-charging cluster or a main distribution board), a single cable is sometimes impractical — the cross-section would become too large and too stiff to install. The solution: run multiple cables in parallel per phase. This article covers the conditions under which those parallel cables actually share the current equally.

The conditions for equal current sharing

According to IEC 60364-5-52, equal current sharing between parallel cables is considered sufficiently guaranteed when:

• The cables are of the same material (copper with copper, aluminium with aluminium — not mixed).
• They have the same cross-sectional area.
• They follow approximately the same length, over the same route.
• There are no branch circuits along the length of the parallel set.
• Each parallel set (or group) is effectively bonded at both ends (the same terminal block, the same busbar).

Applying this parallel rule without additional detailed calculation also requires a minimum cross-section — typically 50 mm² for copper and 70 mm² for aluminium per parallel conductor; below that cross-section, paralleling is not automatically considered to share current equally.

Why an unequal arrangement goes wrong

If the parallel cables have a different length, route or relative position, the impedance between the cables differs — and a motor, transformer or supply cable does not "choose" evenly between two paths of different impedance. The result: the cable with the lowest impedance (often the shortest, or the one running closest to a steel cable tray or ladder) carries a disproportionately large share of the total current, while the other cable remains underloaded. Even if the total calculated current comfortably fits within the combined capacity of both cables, the most heavily loaded cable can individually still be overloaded — with overheating and accelerated insulation ageing as a result, without this showing up in a simple total-current calculation.

Note: this effect is reinforced by mutual inductance between parallel conductors — the relative position of phase and neutral conductors within and between the parallel groups affects the impedance distribution, in addition to the purely geometric length difference.

Practical measures

• Run parallel cables via the same route, in the same cable tray or ladder, with a symmetrical relative positioning of the phase conductors within each parallel set.
• Avoid one cable of the set running closer to a steel structure or cable tray than the others — this affects impedance asymmetrically.
• Avoid branch circuits partway along a parallel set; if a branch is unavoidable, an additional calculation is needed to verify the actual current distribution instead of assuming equal sharing.

Practical relevance

When designing a parallel-cable supply (for example for a large motor, an EV-charging cluster or a main distribution board), it must be verified that all parallel cables have the same length, route and cross-section, and that the minimum cross-section for the parallel rule is met — otherwise an additional, detailed impedance calculation is needed to demonstrate that none of the individual cables is overloaded.

Common mistakes

1. Running two parallel cables via a different route or length "because it comes out the same anyway" — the resulting impedance differences can lead to seriously unequal current sharing.
2. Only checking the total calculated current against the combined capacity, without verifying that the individual cables are also actually equally loaded.
3. Adding a branch circuit partway along a parallel cable set without an additional calculation — this breaks the condition for automatically equal current sharing.

Related

• Cable cross-section & current-carrying capacity
• Busbar systems
• Cable support systems — trays and ladders