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This paper presents an efficient method for optimizing the design of power/ground (P/G) networks by using locally regular, globally irregular grids. The procedure divides the power grid chip area into rectangular subgrids or tiles. Treating the entire power grid to be composed of many tiles connected to each other enables the use of a hierarchical circuit analysis approach to identify the tiles containing the nodes having the greatest drops. Starting from an initial configuration with an equal number of wires in each of the rectangular tiles, wires are added in the tiles using an iterative sensitivity based optimizer. A novel and efficient table lookup scheme is employed to provide gradient information to the optimizer. Incorporating a congestion penalty term in the cost function ensures that regularity in the grid structure does not aggravate congestion. Experimental results on test circuits of practical chip sizes show that the proposed P/G network topology, after optimization, saves 12%-23% of the chip-wiring area over other commonly used topologies.