This project was a firm fixed contract to install new rooftop heating and cooling units for the entire USPS PDC building. Because of this upgrade, the existing electrical system had to be upgraded to accommodate the new loads. It was critical that the power remain on with very minimal interruption. The main electrical system for this facility consisted of 4 – 4160/480 volt double-ended switchgear. As part of this project the 4160/480 volt dry type transformers had to be upsized from 750KVA to 1250KVA. The existing medium voltage cable had to be removed and replaced with new 5KV 500MCM copper cable to service the new loads. To minimize the number of interruptions and the length of the interruption, the Peachtree Mechanical Electrical Division was able to switch the loads from one end of the double-ended gear to the other by opening the main on one side and closing the tie breaker. This operation was performed in less than 15 minutes and allowed the facility to continue operation with minimal disruption. After the power was switched, the existing 750KVA transformer was removed, the new 1250 KVA transformer was installed, the previously installed 500MCM 5KV cable was terminated, tested, and energized, and the process was repeated for the other end of the gear. This sequence was repeated on each piece of switchgear. In addition to the replacement of the transformers in the switchboards. New 600amp MLO panels were added to the facility for the new loads and to allow the transfer of numerous existing loads from MCC units. These panels were fed from new 800amp breakers with adjustable trips that had to be installed in the existing switchgear. The new breakers were of the draw-out type, so new cradles had to be installed as well as re-working the existing bus in the switchgear. This work did require an 8hr outage, but the work was scheduled on Holiday weekends so there would be no interruptions to the processing of the mail.