Currently, variable-frequency drives (VFDs) are alwaysapplied on the motors in variable flow systems to modulate thepump water flow. Meanwhile, multiple parallel pumps arerequired if a single pump cannot satisfy the required water flowrate. Therefore, the water flow and head demand in variableflowsystems with parallel pumps may be satisfied by eitherfewer operating pumps at a higher speed or more operatingpumps at a lower speed. Parallel pumps should be sequencedbased on the water-to-wire efficiency of a VFD-motor-pumpsystem, which combines the efficiencies of the pump, motor,and VFD at any water flow rate and head. However, the motorefficiency is currently not well modeled under variablefrequency. The purpose of this paper is to develop an optimalpump sequencing control with an accurate water-to-wire efficiencymodel. First, the wire-to-water efficiency of a VFDmotor-pump system is improved by applying a motor equivalentcircuit method to calculate the motor efficiency at variablefrequency, and then the improved wire-to-water efficiency isapplied to develop the optimal operating regions for a secondarychilled-water system with three parallel pumps. The simulationresults show that the developed optimal pumpsequencing control can reduce the system power input by upto 25% over the maximum flow sequencing control and by upto 3% over the best pump efficiency control.