The method of cluster laying is more and more commonly adopted in urban distribution cable network. The heat effect among cables is serious and leads to their temperature overbalance

and the middle cable is the hottest one. To improve the section ampacity of cables

a load optimization method based on modified particle swarm and finite element method（FEM） thermal field simulation is proposed

in which to maximize the section carrying capacity is taken as the optimization objective and that the conductor temperature of each cable does not exceed 90 ℃ is taken as the constraint.The results of the load optimization of twelve

sixteen and twenty-four circuits cables laying in ducts reveal that the section ampacity will all be enhanced evidently. And the more the cable number is

the more the ampacity will increase

reaching a maximum of nearly 5%. The method can also improve the carrying capacity in case of changeless load of some lines. Therefore

the load optimization method proposed can extract the optimal scheme of cable load distribution

maximum section carrying capacity of cables

and increase its utilization.