Parallel Cables Don't Share Current Equally — And Here's When That Destroys Your Installation

A joint failure on a 500mm² cable feeding a SAG mill motor at a major mining operation led to a $240,000 repair bill and 36 hours of lost production. The investigation revealed that the feeder consisted of three parallel 500mm² XLPE cables, and one cable was 4.6 metres longer than the other two due to routing around a structural column. That 4.6-metre difference — on a 95-metre run — caused enough impedance imbalance that the short cable carried 38% of the total current while the long cable carried only 28%. The short cable's joint operated above its rated temperature for two years before it failed.

Parallel cables are a common solution when a single cable can't carry the required current. But the assumption that current divides equally between parallel cables is valid only under idealised conditions that rarely exist in practice.

The Physics of Current Division

Current divides between parallel paths inversely proportional to their impedance — not equally by count:

If two parallel cables have impedances Z₁ = 0.095 Ω and Z₂ = 0.105 Ω (a 10% difference), the current splits:

• Cable 1: 52.5% of total current
• Cable 2: 47.5% of total current

A 10% impedance difference produces a 5% current imbalance. That sounds small — but for large cables operating near their rated capacity, even 5% overcurrent raises the conductor temperature and accelerates insulation ageing.

For THREE cables in parallel with different impedances, the calculation requires solving simultaneous equations, and the worst-case imbalance can be much larger than the two-cable case.

Three Causes of Unequal Current Sharing

1. Different Route Lengths

The most obvious cause. If one cable is routed around an obstacle while the others take a direct path, it has more impedance (both resistance and reactance). The longer cable carries less current; the shorter cables carry more.

For small cables (up to about 50mm²) where resistance dominates impedance, the current imbalance is proportional to the length difference:

Where ΔL is the length difference, n is the number of parallel cables, and L_average is the average length.

For the SAG mill example: ΔL = 4.6m, n = 3, L_avg = 95m:

• Approximate current imbalance: 4.6 / (3 × 95) ≈ 1.6% per cable

This seems small — but for large cables (500mm²), reactance dominates over resistance, and the route difference also changes the mutual inductance geometry, amplifying the effect. The actual measured imbalance was 10%.

2. Installation Arrangement (Flat vs Trefoil)

This is the cause most engineers don't know about, and it's the dominant factor for large cables.

When three single-core cables (one per phase of a three-phase circuit) are installed in flat formation (side by side), the outer two cables have different mutual inductance to the centre cable than they have to each other. This creates different effective reactances for each cable position.

For PARALLEL cables installed in flat formation, the problem compounds. If two parallel cables per phase are laid flat (six cables total), the outer cables have different reactance from the inner cables. The resulting current imbalance can be 10–20% even with identical cable lengths.

3. Termination Resistance

A slightly loose termination, a corroded lug, or a lug crimped with the wrong die creates additional resistance in that cable's circuit. The cable with the higher termination resistance carries less current; the others carry more.

This is an installation quality issue that's invisible to the designer but can cause 5–10% current imbalance. It's also a deteriorating condition — a warm termination oxidises faster, increasing resistance further, shifting more current to the other cables. Eventually one termination overheats and fails.

BS 7671, Regulation 523.5 — Cables in parallel

What the Standards Require

All major standards have specific requirements for parallel cables:

BS 7671 Regulation 523.5

Parallel cables must be:

• The same type and cross-sectional area
• Approximately the same length
• Have no branch circuits along their length
• Arranged to carry substantially equal current

IEC 60364-5-52, Clause 523.7 — Cables in parallel

IEC 60364-5-52 Clause 523.7

Similar requirements, with the addition that the method of installation should be such that the current is distributed equally between the cables. This implies trefoil arrangement for AC circuits.

NEC 310.10(H)

NEC/NFPA 70, Section 310.10(H) — Conductors in parallel

NEC 310.10(H) requires parallel conductors to be:

• The same length
• The same conductor material (all copper or all aluminium)
• The same size in circular mil area
• The same insulation type
• Terminated in the same manner

NEC also requires that conductors be arranged to prevent differences in inductive reactance, which in practice means balanced arrangement (trefoil or transposed).

AS/NZS 3008.1.1 Clause 3.6

AS/NZS 3008.1.1, Clause 3.6 — Cables connected in parallel

AS/NZS 3008 requires parallel cables to be of the same type, cross-section, and length, with symmetrical installation arrangement. Clause 3.6 specifically notes that cables should be transposed for long runs to equalise the impedance.

Solutions for Equal Current Sharing

Trefoil Arrangement

For three-phase circuits with parallel cables, arrange each phase's cables symmetrically. The classic arrangement for 2 cables per phase is:

• Phase A1, B1, C1 in trefoil (or flat)
• Phase A2, B2, C2 in trefoil (or flat)
• Both groups in the same cable route, same formation

This ensures each parallel cable "sees" the same magnetic environment from the other phases.

Transposition

For long cable runs (>50m with large cables), transposition involves swapping the physical positions of parallel cables at one or more points along the route. This averages out the differences in mutual inductance.

A single transposition at the midpoint of the run is usually sufficient. The cable in the "outer" position for the first half moves to the "inner" position for the second half, and vice versa.

Equal Route Lengths

The easiest specification to enforce: all parallel cables must follow the same route and be cut to the same length (within ±2% of the total length). Any cable routed differently due to site constraints must be lengthened to match — coil the excess neatly at the termination end.

Identical Terminations

Specify the same lug type, crimping tool, and torque for all parallel cable terminations. Verify termination resistance with a micro-ohmmeter during commissioning — the resistance of parallel cable terminations should be within 10% of each other.

Why More Than 4 Cables in Parallel Is Problematic

As the number of parallel cables increases, current sharing becomes progressively worse:

With 6 parallel cables, the most loaded cable can carry 30% more current than the average — effectively reducing the benefit of paralleling.

The Economic Calculation

When considering parallel cables vs alternatives:

• 2 × 240mm² vs 1 × 500mm²: the single cable is easier to install, has no current sharing issues, and is often cheaper per ampere when installation labour is included. But 500mm² is near the maximum practical cable size for manual handling.
• 3 × 300mm² vs busbar trunking: for currents above 1,000A and short distances (within a building), busbar trunking has lower losses, no current sharing issues, and easier maintenance access. The breakeven distance varies but is typically 20–50m.
• 4+ parallel cables: almost always worth reconsidering the design approach. Multiple parallel cables create complex termination requirements, take up more cable tray space, and have reliability implications (a failure on one cable overloads the others).

Key Takeaways

1. Current sharing is never exactly equal — design with margin for 10–15% imbalance on well-installed parallel cables
2. Route length differences matter — keep all parallel cables the same length within ±2%
3. Installation arrangement matters more for large cables — use trefoil or transposition for cables above 120mm²
4. Termination quality is critical — verify with micro-ohmmeter during commissioning
5. More than 4 in parallel: reconsider the design — busbar or busduct may be more appropriate
6. Monitor termination temperatures during operation — thermal imaging of parallel cable terminations should be part of the maintenance programme

Related Resources

• Cable Derating: 12 Cables in a Tray at 40°C — Grouped cable derating across standards
• The Complete Cable Sizing Comparison — Parallel cable rules differ between standards
• Grenfell Tower: Cable Tray Fire Spread — When cables in close proximity overheat
• View all worked examples →