CableDataSheet, Cable and Wire Technical Consulting Service

What is the adiabatic equation for cables?

When calculating the fault ratings of a cable, it is generally assumed that the duration is short enough that no heat is dissipated by the cable to the surroundings.  Adopting this approach simplifies the calculation and errs on the safe side.

The normally used equation is the so-called adiabatic equation.  For a given fault of I, which lasts for time t, the minimum required cable cross-sectional area is given by:

t = (A^2*k^2)/I^2

where:    A - the nominal cross-section area, mm2
        I - the fault current in, A
        t -  duration of fault current, s
        k - a factor dependant on cable type (see below)

The factor k is dependant on the cable insulation, allowable temperature rise under fault conditions, conductor resistivity and heat capacity.  Typical values of k are:













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