def thermal_time_constant_at_rated_load(C, P, dTOr):
    """ Returns the average oil time constant in minutes (for rated load)
    As per IEEE C57.91-2011
    C = Thermal capacity of oil
    P = Supplied losses (in W) at the load considered
    OilRise = The average oil temperature rise above ambient temperature
    in K at the load considered
    """
    tau = (C * dTOr * 60) / P
    return tau