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An ac shunt calibration system at the National Metrology Institute of Japan (NMIJ) has been developed. The uncertainties of the system were estimated 3.4 μΩ/Ω for in-phase and 8.6 μrad for quadrature at 55 Hz/5 A. Meanwhile, the enhancement of the system is planned up to 10 kHz/10 A. For this goal, the accuracy and high-frequency performance of the buffer amplifier, which have the largest influence on the uncertainties, have been improved by developing a load effect reduction (LER) circuit. The LER circuit cancels out the effect of output impedance of the buffer amplifier. The effect of the circuit on the buffer amplifier allows the uncertainties of the ac shunt calibration system to be reduced into 2.6 μΩ/Ω and 8.2 μrad at power frequencies.