Case study: Generator replacement

A hospital needed to increase its emergency generator capacity to 4 MW and provide for future expansion while using the existing generator building.

By Richard Vedvik, PE, IMEG Corp. December 6, 2017

A 600+ bed hospital facility had two energy centers supplying heating, cooling, and normal and emergency power to the campus. A previous project new paralleling gear that allowed for four 1,000 kW generator sets and one temporary generator set to parallel to half of the campus. This new paralleling gear was in a dedicated room, replacing existing gear that was co-located with the generators. The other half of the campus was served by two 600 kW and one 1,000 kW standby diesel generator sets operating at 4,160 V 3-phase 3-wire.

This paralleling gear operated at 4,160 V and distributed to unit substations throughout the campus. The existing paralleling gear suffered water damage and the enclosure frame was weakened from rust. The paralleling gear was in the same room with the generators and decades of moisture and dirt were causing premature wear. The generator building consisted of three rooms, one room contained the two 600 kW and the other contained the 1,000 kW. Existing 30,000 gallons of fuel storage was sufficient for the additional generating capacity.

Design aspects

The hospital project included upgrading the controller on the 1,000 kW generator set and replacing the two 600 kW generator sets and providing two new 1,000 kW generator sets for a total of 3,000 kW of capacity. Room for a fourth 1,000-kW generator set was included in the design for future expansion. The existing 1,000 kW generator room was re-purposed for the paralleling gear. The two 600 kW generators were replaced in the same location. The system operating voltage was maintained at 4,160 V.

The design team and owner agreed that the new paralleling gear should be in a dedicated room, separate from the generators to reduce exposure to moisture and dirt while providing a quiet environment to review system function. The new paralleling gear featured four generator inputs and seven distribution outputs (see Figure 4). Due to the operating voltage, a separate cabinet was located elsewhere in the room to allow for operation of the system without standing in front of the switchgear.

Additional intake air was achieved with roof-mounted louvers. The third and future fourth 1,000 kW generators were located outside in custom walk-in enclosures. The use of walk-in enclosures was chosen over a building addition due to cost. Larger generators required larger discharge louvers; an exterior discharge plenum and louver was added to the building. Because the paralleling and distribution system was 3-wire, an external neutral-grounding resistor was chosen to balance the wye-configured generators. These resistors were located within the generator building for all three generators.

The existing fuel oil system was completely re-piped and new day tanks were added. The day tank size was limited due to NFPA 110 7.9.5 requirement for no more than 660 gallons of fuel storage in the emergency power system (EPS) room. New duplex fuel oil pumps were provided in each of the storage tanks and a fuel-polishing system was added. Trenches were added in the existing floor to allow for gravity-drainage of the return fuel oil. The sprinkler system features high-temperature heads with dry-piping for freeze protection.

The existing 600 kW generators had a smaller radiator opening than the required clearance to install the new 1,000 kW generators. The existing exterior wall could be removed and rebuilt or the existing intake air louver could be removed to allow for generator installation. The design and construction team chose the latter for cost and minimal impact to the structure.


The project needed to provide temporary emergency power during construction. The design team and electrical contractor developed a plan for locating two temporary generators on the campus allow for the existing equipment to be taken out of service and to provide the entire generator space at once. Drawings identified the phases of construction efforts and included temporary power connections and configurations. The new generators would be in different physical locations, which resulted in adjustments to exhaust routing. Existing rooftop equipment was relocated and structural openings were modified for exhaust system compliance with NFPA 110_7.10 (see Figure 5).

Temporary generators in the project area are not available in 4,160 V. The design and construction team discussed providing a temporary step-up transformer to allow for connection of a 480 V generator. Due to the concerns of this transformer being normally unenergized and condensing when energized, the decision was to provide raceway and a pad for a temporary transformer. Raceway is provided into a temporary generator section of the paralleling gear.

The temporary generator had to meet the requirements of NFPA 110, which identifies the requirement for Type 1 systems to have alarm monitoring at a continually attended location in addition to local to the EPS. The existing annunciator cabling could not be reused and temporary annunciators were provided for the temporary generators due to the duration they would be in service.

Richard Vedvik is senior electrical engineer and acoustics engineer at IMEG Corp. He is a member of the Consulting-Specifying Engineer editorial advisory board.