Case study: Hospital generator upgrade

A hospital needed to upgrade emergency power equipment with a new generator while maintaining some current equipment.


For this project, the owner wanted to increase emergency power capacity by adding a third diesel generator and correct NFPA 110 7.2.3 code and 2008 NEC 700.9 B (now 2017 NEC 700.10 B) violations associated with emergency power system (EPS) and emergency power supply system (EPSS) locations and configurations.

Existing conditions

A 300+ bed hospital facility was being served by a pair of 1,000 kW standby diesel generator sets with remote radiators. The generators and paralleling gear were located in the same room as the 15 kV normal incoming service and distribution. This facility operated the EPS system at 480 V 3-phase 3-wire. This meant that a neutral conductor was not present in the EPSS equipment. All of the 480 V loads were 3-phase motor loads and all single-phase loads were 120 V. More than 25 automatic transfer switches were served by the EPSS distribution.

Design aspects

The project needed to maintain the existing emergency system while systematically moving transfer switch feeds to the new distribution equipment. The existing paralleling gear was in good condition, and required replacement in order for a rated wall to be built to separate the EPS equipment from the 15 kV normal service entrance equipment. A room adjacent to the existing EPS location was part of a make-ready remodel effort. Care was taken to maintain egress paths and allow for equipment replacement. The new paralleling gear was located in the same room as separate distribution panels for life safety, critical, and equipment branch loads.

A third 1,000 kW diesel generator was added to the system and located in the same room as the existing generators. The 3-phase 3-wire EPS required a low-impedance neutral-ground bond to stabilize the wye-configured generators.

The existing remote radiator feeds came from the paralleling gear. This has a negative side-effect of preventing immediate operation upon generator start-up. This configuration required that the paralleling gear breakers be closed before generator cooling function would begin. The new configuration used a fused combination starter directly off the associated generator output lugs to allow for immediate cooling operation.

Existing on-site fuel storage was retrofitted with new pumps and fuel oil piping to account for the increased demand and to improve system reliability. Existing room ventilation was sufficient for the third generator, helped by the use of remote radiators, which meant the ventilation system only needed to provide combustion air and not radiator cooling air


When paralleling generators with different winding pitches, the resulting harmonic distortion is a combination of both pitch designs. This distortion affects the voltage waveform. A 2/3 pitch design has no 3rd harmonic distortion while a 4/5 pitch design has no 5th harmonic distortion. The reason for allowing different pitch configurations was to allow for competitive bidding on the new generator.

An existing uninterruptable power supply (UPS) was adversely affected by paralleled generator operation. Due to the increase in harmonic distortion from the multiple pitch configuration, the UPS identified the incoming waveform as unacceptable and would not allow battery charging or connection to the EPSS. The solution was to install a 480 V 480/277 V isolation transformer ahead of the UPS to provide filtering of harmonics, which was an effective solution.

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

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