Cable Cleat Spacing & Short-Circuit Forces

Walk any cable route and you'll see cleats at 300 mm centres — usually because the last job did it that way. But cleat spacing is a real calculation with a standard behind it (IEC / BS EN 61914), and the default can be wrong in both directions: dangerously wide on high-fault-level runs, wastefully tight on small circuits.

What Cleats Are Actually For

Under normal load, cleats just carry weight. Their real job appears in the first milliseconds of a short circuit: two parallel conductors carrying current in opposite directions repel each other, and the force is proportional to the square of the current and inversely proportional to the distance between conductors. During a fault, single-core cables in trefoil or flat formation try to burst apart — violently. Unrestrained or under-cleated cables have broken free, destroyed containment, and injured people. The cleats — and the spacing between them — are what hold the formation together.

The Numbers Are Bigger Than Intuition Says

Because force scales with current squared, fault level dominates everything. As a first-order approximation for a trefoil formation, the force per metre between conductors is roughly 0.17 × î² / d newtons per metre (peak current î in kA, conductor spacing d in metres). A 32 mm single-core cable in trefoil seeing a 25 kA peak fault experiences about 3.3 kN per metre of route — the weight of a grand piano hanging off every metre of cable. At 40 kA it's 8.5 kN/m. Double the fault current and the force quadruples.

Note the word peak: the relevant current is the first-loop asymmetric peak, which can be ~2.5 times the RMS break fault current your protection study quotes. Using the RMS figure by mistake understates the force by a factor of six.

What Actually Determines the Spacing

• Peak fault current at that point in the network — from the protection study, not a generic system figure; a run near the transformer sees far more than a remote submain.
• Formation and spacing — trefoil vs flat changes both the coefficient and the effective conductor distance; touching cables see the highest forces.
• The cleat's tested withstand — manufacturers publish short-circuit test results (per IEC 61914) for cleat + cable diameter combinations; a cleat is only as good as its test evidence.
• Installation practicality — the calculated maximum then rounds down to the ladder rung pitch, and cleat width vs cable diameter can rule out fixing on every rung.

That last point deserves a picture. A cleat is wider than its cable, so on a fully loaded ladder, cleats on adjacent cables clash if they share a rung — the practical fix is staggering onto alternate rungs:

Calculated vs Tested

IEC 61914 allows two routes: use a cleat/cable combination that has been short-circuit tested at your fault level and spacing, or calculate the force and compare it against the cleat's declared withstand. In practice most projects need the calculation route — real fault levels and cable sizes rarely match the exact tested configuration — which is why a proper spacing calculation, documented with its inputs and assumptions, is part of a competent containment design package.

Getting It Right Without Reinventing It Every Project

The calculation itself isn't exotic — the difficulty is doing it consistently: correct peak current at each location, the right formation coefficient, manufacturer withstand data to hand, and an output your checker can audit. That's exactly the kind of repeated calculation we turn into bespoke Excel tools — pick the cable, pick the cleat from a manufacturer library, enter the fault data, and get a spacing verdict rounded to your rung pitch with the assumptions documented. The fault levels themselves come from a protection and fault study.

Specifying containment on a high-fault-level project? We can produce or check the cleat spacing calculations — get in touch.