"Around 1 metre should be fine" is an acceptable approximation for plenty of mechanical engineering problems. For cable cleat spacing on a high-fault-current power circuit, it is an engineering error. Spacing and short-circuit withstand are paired parameters; changing the spacing changes the effective protection level of the installation even if the cleat is unchanged.
§ 01 Three factors that govern maximum spacing
Maximum spacing between adjacent cable cleats is constrained by three independent limits — the most restrictive governs:
- Short-circuit electromagnetic force — the primary design driver for most power circuits. Longer span → greater cable deflection under the lateral impulse → higher bending moment at each cleat. The manufacturer's type-test report declares the maximum spacing at which the rated kA level is valid;
- Cable self-weight (vertical runs) — large-section cable can weigh 5–10 kg/m. Over a 100 m vertical drop, the cumulative axial tension on the lowest cleats is significant. For large-conductor vertical runs, self-weight can govern spacing before the fault-force limit is reached;
- Manufacturer's declared rating — the product datasheet maximum spacing value integrates both considerations and must not be exceeded regardless of engineering judgement.
§ 02 The spacing–kA relationship
This is the most commonly misunderstood aspect of cable cleat selection. A given cleat model does not have a single kA rating — it has a kA rating at a specified installation spacing. Increasing the spacing reduces the effective short-circuit withstand of the installation, even though the cleat body is identical.
Spacing 500 mm → withstand 63 kA peak
Spacing 750 mm → withstand 40 kA peak
Spacing 1 000 mm → withstand 25 kA peak
Always obtain the actual spacing–kA table from the manufacturer's type-test report for the specific product being specified.
The correct selection sequence is: determine peak fault current iₚ first, then find the maximum spacing that supports that kA level from the test report — not the reverse.
§ 03 Vertical vs. horizontal runs
Vertical runs experience both the lateral short-circuit impulse and the axial self-weight tension simultaneously. For large-section cable on long vertical drops, the self-weight constraint can require spacing considerably tighter than the short-circuit limit alone would suggest. Bottom-section cleats (highest cumulative weight) may need closer spacing than top-section cleats.
Horizontal runs experience the short-circuit lateral force without the axial self-weight component — gravity is perpendicular to the cable axis and is carried by the support structure. Spacing for horizontal runs is governed by the short-circuit limit and can often be somewhat larger than the equivalent vertical-run requirement for the same fault level.
§ 04 Practical calculation workflow
- Obtain the prospective steady-state fault current Isc (RMS) from the protection coordination study;
- Calculate peak current iₚ = κ × Isc (use κ = 2.5 for typical HV systems, or derive from X/R ratio);
- From the manufacturer's test report, find the maximum spacing for which the product's kA rating ≥ iₚ;
- For vertical runs, additionally check self-weight axial loading against cleat mechanical rating;
- Record the derived spacing in the installation drawings — it is a design parameter, not a site instruction left to the installer's discretion.