Case study: Chiller addition adds power complexity

In this case, the building owner needed to increase generator capacity to accommodate a surgery expansion and add an electric centrifugal chiller on emergency power.

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

A 140+ bed hospital facility had two 480/277 V 3-phase 4-wire standby diesel generators at 700 and 900 kW. The existing generators, paralleling gear, and emergency distribution were located in the same room, which is allowed by NFPA 110 The existing paralleling gear was not sized for future expansion.

Design aspects

The existing 700 kW generator set was replaced with a 1,500 kW diesel generator set. Due to the additional generator size, the radiator discharge louver was located at the building exterior wall and a new discharge plenum and louver was placed on the exterior of the existing building (see Figure 3). Additional intake air was required and new intake louvers and air pathways were added to the space, as defined in NFPA 110 7.7. The existing fuel oil storage system was re-used and the project added fuel polishing equipment to maintain fuel quality.

The existing paralleling gear had bus amperage that was too small for the additional generator load. Additional bus layers were added to increase the amperage from 2,000 to 3,600 amps, which allows for a second 1,500 kW generator to be added in the future. Generator beakers were replaced with a larger frame size for sufficient future capacity. An additional distribution section was added to allow for future loads and provides a feed to an external load bank connection cabinet.


The addition of an 800-ton electric centrifugal chiller to the emergency power system meant that additional controls were needed in the event of only the 900 kW generator being available. For this control, the paralleling gear communicated with the chiller control package and provided total available kW to stay less than 80% loaded along with suitable transfer delays. The chiller controller would equate the available kW into possible chiller output and modulate the chiller load accordingly. This allowed for the chilled water system to operate at maximum possible capacity given any combination of connected generators.

Rebuilding paralleling gear requires that it be taken out of service for several months. In order for the campus to be operational, a temporary generator was required. A 2,000-kW temporary generator was supplied and tied into the existing feeds from the paralleling gear. Due to the impact on clinical operations, the design team worked with the electrical contractor to develop outage plans that identified each task by minutes of duration for submission to the state health department. Switch-overs were conducted with on fair-weather days without scheduled surgical procedures. The outages to and from the temporary generator occurred without incident.

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