Overload or Not? Manual Transfer of Optional Standby Loads

Can a manual transfer switch, a trapped key interlock system (commonly known as a Kirk Key system), an emergency automatic transfer switch and a permanently mounted generator be used at an industrial installation to provide both emergency power and optional standby power? These questions arise periodically during electrical plan review and during construction of both small and large facilities with serious cost and safety at stake.

Getting a resolution for these issues often involves multiple articles and sections in the National Electrical Code (NEC). This article provides detailed information with regard to optional calculations as well as the use of a trapped key interlock system.

Before dealing with the questions above, one must first understand the system’s design. Transfer switches can be manual or automatic. Automatic transfer switches are either both magnetically actuated and held or magnetically actuated and mechanically held, depending on the transfer switch’s function and the use within the system. A common form of manual transfer switching is the use of a double-throw, double-pole safety switch used for single-phase systems or a double-throw, three-pole safety switch for three-phase systems.

An additional method of manual transfer switching can be the trapped key interlock system, where two circuit breakers are installed with the trapped key system permitting one circuit breaker to be on while the other is locked off. Single- or multiple-key systems will permit only one breaker to be in operation at a time. As an example, one circuit breaker will be connected to normal power and the other connected to the generator with a key-and-lock system controlling which circuit breaker is operational.

Manual transfer switches are permitted to be used for optional standby loads, based on 702.4(B) covering the capacity and rating of standby sources that was new in the 2008 NEC. The text states: “(B) System Capacity. The calculations of load on the standby source shall be made in accordance with Article 220 or by another approved method. (1) Manual Transfer Equipment. Where manual transfer equipment is used, an optional standby system shall have adequate capacity and rating for the supply of all equipment intended to be operated at one time. The user of the optional standby system shall be permitted to select the load connected to the system.”

This was inserted in the Code to permit standby system generators to be manually transferred with a limited amount of load with a smaller generator than would be required for an entire building load. To answer a question above, a manual transfer switch system can be used for the optional standby load if the standby load and source will permit the emergency load to be automatically transferred and the generator is large enough to carry both loads. One method of ensuring this is to mark the optional loads at the disconnecting means to indicate these optional loads are manually transferred with the assurance that the emergency loads are fully maintained.

In addition to limiting the optional load by manually turning on circuit breakers, adding a new load to the existing normal and emergency load on the generator can be accomplished by recalculating the new and existing load with 220.87. Determining the existing load based on the demand according to 220.87 ensures the new load and the emergency load won’t overload the generator.

Three specific requirements are necessary for compliance with this optional calculation: (1) the maximum demand data from the utility company must be available for a one-year period, or if the demand data is not available for a year, an exception permits the calculated load to be based on the maximum demand (the highest average kilowatts reached and maintained for a 15-minute interval) continuously recorded over a minimum 30-day period using a recording ammeter or power meter connected to the highest loaded phase of the feeder or service, based on the initial loading at the start of the recording. This recording must reflect the maximum demand of the feeder or service while the building is occupied and must include by measurement or calculation of the larger of the heating or cooling system load as well as other loads that may be periodic in nature due to operation during seasonal or other conditions.

The other requirements are: (2) The maximum demand is at 125 percent of the new load, and (3) the feeder or service has adequate overcurrent protection.

Following these simple rules will help you comply with Article 700 for emergency systems and Article 702 for optional standby systems.

About the Author

Mark C. Ode

Fire/Life Safety Columnist 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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