Where the system peak fault current and the cable diameter are known, the following formula, taken from the International standard (IEC 61914), can be used to calculate the forces between two conductors in the event of a three phase fault:
Ft = 0.17 x ip 2 / S
Ft = force in Newton/metre (N/m)
ip = peak short-circuit current in kiloamp (kA)
S = distance between the centrelines of the conductors in metres (m)
Once Ft in N/m has been determined then the force for each potential cable cleat can be calculated.
Metric ladder typically has rungs at 300mm intervals, so cable cleat spacing is usually a multiple of this distance. So, Ft x 0.3 gives the force a cable cleat will see if spaced at 300mm,
Ft x 0.6 for 600mm etc. Ft x cleat spacing can then be compared to the maximum recommended mechanical loop strength of the cable cleat and then the cable cleat type and spacing can be selected.
Please refer to the Ellis Patents Black Book for more examples and information on the calculation of cable cleat spacing and selection of cable cleat type.
ALWAYS REMEMBER: Whole job cost should always be considered as costs can often be reduced by using a stronger, more expensive cable cleat at a wider spacing than a cheaper option at more regular intervals.
Learn more about our cable cleats on our downloads page