When Disaster Strikes, a Generator Rules: Optional standby systems for new and old dwellings

Shutterstock / Steven Phraner
Shutterstock / Steven Phraner
Published On
Jul 15, 2022

Loss of electrical power is never convenient, but can be especially catastrophic during hot summers or cold winters. This interruption could come from power company brownouts or blackouts, bad weather such as tornadoes or hurricanes, or failed transformers and other electrical equipment. Installing a generator may make the difference between being comfortable or totally overwhelmed when disaster strikes.

Determining a source of power is critical for the location and generator type. Unlike an emergency generator, an optional standby power generator can be connected to a natural gas utility company supply line, or to a propane, gasoline or diesel fuel tank that will last for the duration of the electrical power outage. Purchase a generator suitable for the location where it will be installed and the type of fuel available. It must be tested by a nationally recognized testing laboratory.

Generator requirements

The generator must have a nameplate with the manufacturer’s name; rated frequency; number of phases, such as single- or three-phase; rating in kilowatts and kilovolt-amperes; the power factor rating of the generator; the normal volts and amperes corresponding to the generator’s rating; and the rated ambient temperature at which the generator can be operated.

Rated ambient temperature is very important, since the generator should be placed in an area where there is shade or the generator is otherwise protected against direct sunlight in hotter parts of the country—such as desert areas in Arizona, California, Nevada, New Mexico, Texas and Utah—and sheltered during extreme cold and heavy snow.

Article 445 covers generator installations and requirements and Article 702 covers optional standby systems. The articles work together to ensure compliance with the National Electrical Code , so review them before designing and installing a system.

Conductors should be appropriately sized to the load and must follow the requirements in 445.13. This section requires the conductors from the generator to the first overcurrent protective device to be sized at not less than 115% of the nameplate rating of the generator, unless the generator has a listed overcurrent device built into it with appropriate load terminations for connection of feeder conductors.

Generators—other than cord- and plug-connected portable ones—must have one or more disconnecting means so each one can simultaneously open all associated ungrounded conductors, and they must be able to be locked in the open position. Permanent generators for one- and two-family dwellings must have an emergency shutdown device located outside the home and in a readily accessible location, based on the 2020 NEC . A permanent plaque or directory must be provided, based on 225.37, at each service, feeder and branch circuit disconnect location denoting all other services, feeders and branch circuits supplying that building or structure to ensure total disconnect of power.

Standby power systems

An optional standby system is intended to supply power to public or private facilities or property where life safety does not depend on the system’s performance. These systems are intended to supply on-site generated or stored power to selected loads that are either automatically or manually controlled.

Optional standby system equipment is required to be suitable for the available fault current it may be subjected to at its terminals. If a manual transfer switch is used, the standby system must have adequate capacity and rating to supply all electrical equipment intended to be operated at one time. The system’s user is permitted to select the load connected to the system. The generator cannot be overloaded, even during an emergency situation.

There are two choices available when an automatic transfer switch is used. The first choice is to size the generator large enough to carry the full load transferred by the automatic transfer switch with appropriately sized conductors. The second choice is to use a load management system to automatically manage the connected load, and the generator is large enough to supply the maximum load that can be connected by the load management system. If there is an override switch, then take care to ensure the load does not exceed the generator’s capacity.

When a customer asks for an optional standby system, be sure to appropriately size the generator to carry the maximum amount of load the customer needs, but do not install a system that can be overloaded and could result in danger.

About the Author

Mark C. Ode

Fire/Life Safety, Residential and Code Contributor

Mark C. Ode is a lead engineering associate for Energy & Power Technologies at Underwriters Laboratories Inc. and can be reached at 919.949.2576 and Mark.C.Ode@ul.com.

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