def calcshcs(Zpu, Vnom, Vpu=1.0):
    """Calculates the short circuit current in Amperes.
    Vpu = Pre-fault voltage (optional) 
    Vnom = Vnom in kV
    Zpu = Impedance in per unit on 100 MVA base
    """
    Spu = Vpu / Zpu
    return 100 * Spu / (3 ** 0.5 * Vnom)
