Generators and transfer switches: Emergency power system solutions

Generators can provide emergency power to a facility’s electrical power system. As a whole system backup, gensets minimize disruption in the event of normal power loss.


Learning objectives

  1. Select the appropriate generator based on prerequisite conditions.
  2. Develop a load study to size the generator.
  3. Code check and coordinate with utility.
  4. Determine the fuel source of the generator.
  5. Address generator safety issues, including power system overcurrent protection coordination and settings.

Generators are proven solutions for providing emergency power to a facility's electrical power system. As a whole system backup, they minimize loss of production or service in the event of normal power loss.

For a project that does not require a generator by code, a first-cost, return on investment, and lifecycle-cost analysis is necessary to determine which type of emergency system is most effective and appropriate for the project. If the analysis reveals that a battery backup is cumbersome or expensive to maintain, a generator could be the best option.

Generators still require maintenance and exercise, but unlike individual batteries or battery bank systems, generators do not have to be replaced every couple of years.

For some projects, data centers or health care facilities for example, an uninterruptable power supply (UPS), which consists of a flywheel or battery bank system, may still be necessary to bridge the time gap between normal power loss and the generator being able to assume the load. Many designs include both a generator and UPS to avoid losing time during an outage, as generators take about 10 seconds to achieve full voltage and frequency. By not having the entire system fed from the UPS, the system size can be decreased so that only the critical loads are powered by the UPS and the remaining loads have the 10-second outage; this also reduces construction costs. A UPS also adds to the reliability and quality of the power system by transforming and regulating the output power.

Load and service planning

In designing an incoming normal power feed to a facility, the engineer determines its size based on a load study of the facility. To conduct a load study, the engineer must consider both the type of load and the type of facility. The types of loads that have energy consumption associated with them are resistive, capacitive, inductive, or any combination thereof. Facility types can be new, existing renovated, or a combination of both. The kilovolt-ampere is the apparent power that is often totaled unless the power factor is known. If the power factor is known, then the real power, or kilowatts, can be used. Once the loads are totaled, demand factors, based on NFPA 70: National Electrical Code (NEC) requirements, are applied to determine the anticipated kW load of the facility. This demand kW is considerably less than the full connected load. See Table 1.

Table 1: Electrical engineers typically use spreadsheet software to compile information for load studies because mathematical formulas can be written into cells for conversion and summation. Courtesy: SSOE Group

After all panels have been totaled and the demand load is known for the facility, contact is established with the utility to coordinate interconnection and service-size. This load study, along with identifying what will be served by the generator, will help determine the generator size needed. If the load is so large that a single generator does not have enough capability, then several generators can be connected in parallel. This opens the door to the concept of cogeneration, or multiple generators working in conjunction to carry the emergency load. This requires switchgear with relays and protection to avoid back-feeding the utility source from the generator. Code required emergency loads that typically are segregated from other loads are egress lighting, exits, and fire alarms. Optional standby loads are determined by the owner of the facility and typically include loads such as telecommunications equipment, security systems, or process equipment.

Code impact

All applications and installations should follow the most current version of the NEC and building codes. The generator is integrated into the electrical power system as a separately derived power source through a transfer device. The type of transfer device is determined by the application and is either automatic or manual transfer. The number of transfer devices needed depends on the application and loads being served. Most generators can be supplied with up to two breakers, and in many situations, this allows the design to feed two transfer switches to segregate loads and still meet NEC requirements. The use group classification of a facility will determine the codes that apply to the emergency power system and the most appropriate type of generator with transfer device needed. Coordination with the utility company is necessary when using transfer switches that have bypass isolation and can "make before breaking" contact between the sources. Net metering may not be required, but the trip settings and coordination of interconnection to the grid will need to be planned as advised by the utility.


The project's geographic setting is a factor in determining the primary fuel source of its generator, as is the availability of the fuel and its price. Common fuel sources for generators are diesel, natural gas, or liquid propane. Each fuel type requires different maintenance procedures to ensure the generator's longevity. The location of a generator, which can be either in the interior or the exterior of the facility it serves, will dictate additional requirements for a code-compliant installation. Early coordination with the authority having jurisdiction (AHJ) can help in selecting the fuel source. Although diesel is widely accepted, natural gas is usually permitted, with liquid propane as a backup. When there are two fuel sources, make sure to call for the automatic gaseous dual fuel option which may not be standard for the manufacturer of the generator. Also, the altitude at which the generator is installed will impact generator performance. Seismic activity of the location should be checked to guarantee that the correct fasteners and associated accessories are specified. 


Coordination of breakers in a power system is essential to minimize short circuit current and arc flash potential. The purpose is to open the breaker closest to the fault in the system to minimize response time and magnitude of energy. One complex aspect of coordination is the settings of the generator breakers and the transfer device. Settings in the generator breakers should be coordinated with other breakers to open and minimize short circuit current in the event of a fault. Similarly, settings like pickup and dropout in the transfer switch affect the safety of the electrical system. Depending on the application and configuration of the system, breakers with ground fault capability can be used in the emergency system. Education and light commercial typically have their main service entrance breaker GFI for over 1000A to follow NEC 2011 230.95. In health care applications, the use of ground fault trip elements for circuit breakers is not permitted. If they are used, coordination is required with the normal power source ground fault detection so a differential does not cause nuisance tripping. For all applications, it is important to coordinate with the equipment manufacturer to ensure code-driven features exist. Among these features are proper vent stack height for all generator types and fuel spill pans with UL 142 double-wall tanks when supplying diesel.

Coordination with architects is critical to determine generator locations in and around the facility that are inconspicuous yet accessible. Designers are also encouraged to coordinate with structural engineers since the pad design needs to bear the weight of the generator and fuel tank when filled to capacity. The designer should also coordinate with the mechanical engineer for two reasons. One is to make sure the exhaust isn't too close to fresh air intakes for building air supply. Another is to make sure the line is sized appropriately if supplying a natural gas feed to the generator.


A safe generator installation is critical to the operator and to anyone who comes in close proximity to the unit. Appropriate signage should be posted to alert individuals that the generator can start, run, and stop automatically. Bollards should be installed to protect the generator from traffic when installed close to service drives or other accessible drivable surfaces. Fencing may be appropriate to protect the generator from untrained personnel if the housing is not lockable. A solid grounding system for the generator housing and power system is critical in case of a fault or accident. Most control panels come with an emergency stop button, but it should be located in an accessible location so that maintenance personnel can easily reach it in the event of an emergency. When possible, all electrical equipment should be de-energized prior to work in accordance with NFPA 70E lockout/tagout. Proper ventilation is necessary around the generator to provide air needed for combustion as well as to exhaust any harmful emissions. This is important whether the generator is installed in the interior or exterior of the facility.

Using a generator to provide emergency power to an electrical system can be an excellent solution when it is properly sized and installed based on the facility's specific needs and code requirements. It is important to understand the issues that affect selecting the appropriate transfer switches, choice of fuel, and setting of generator breakers. Lastly, multiple factors should be addressed to assure that the generator is installed safely and runs properly.

Combined heat and power

A newer concept that has evolved with the green movement is combined heat and power (CHP). The idea is that when a generator is running, it gets hot from the combustion of the fuel source. Special engine jackets have been designed to hug the engine and allow the circulation of liquid for heat transfer and energy recovery. This liquid is then transferred for use by another process or system in the facility. This option is typically considered for an emergency system that would run more frequently than a standby application. The application of CHP is typically considered when generators are already planned on a job and the cost to run the generator continuously is a cost savings for monthly operation. The energy recovery option is a benefit and further investment to reduce costs in another area of the facility's operation or production process.

Corey Zachel has nearly 15 years of experience and works with SSOE's Industrial Automotive division as a lead electrical engineer to produce and review electrical documents.

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