Section 94I. In each base distribution rate proceeding conducted by the department under section 94, the department shall design base distribution rates using a cost-allocation method that is based on equalized rates of return for each customer class; provided, however, that if the resulting impact of employing this cost-allocation method for any 1 customer class would be more than 10 per cent, the department shall phase in the elimination of any cross subsidies between rate classes on a revenue neutral basis phased in over a reasonable period as determined by the department.