While we tend to refer to all of the electrical circuits served from an on-site source as "emergency power," the National Electrical Code (NEC) establishes separate categories of electrical systems intended to supply an alternate source of power on interruption of the normal supply. These are the emergency, legally required standby, and optional standby systems. It is important to know the difference between these systems and to be aware of what equipment may be served from each.
This discussion will focus on emergency and standby systems for typical commercial buildings, and it is not intended to address the specialized requirements of health care facilities.
Power SourcesTo avoid confusion with the emergency system, we refer to the generator or other backup supply as the alternate source. In most cases the alternate source is an engine-driven generator with an on-site fuel supply, although a storage battery system is also acceptable. Engine-generators require fuel supply for 2 hrs of operation at full demand load. Battery systems must carry full demand load for 1.5 hrs.
Use of off-site fuel such as natural gas or use of a second service as an alternate source requires approval of the Authority Having Jurisdiction (AHJ). Connection ahead of the main service disconnect, a practice frequently used in the past, is no longer acceptable as an alternate source for an emergency system, although it may be for a standby system with approval of the AHJ.
The alternate source must have capacity to supply all the loads that will be operated simultaneously. If needed to prevent overload, automatic selective load pickup and load shedding are required to ensure that power is available to the emergency, legally-required standby, and optional standby loads in that priority. Alternate source equipment may also be used for peak shaving if the above requirements are met.
The Emergency SystemThe emergency system is intended to supply systems and equipment required for life safety and has the most stringent requirements. Power must be available to the loads within 10 sec of interruption of the normal source. This system is permitted to supply only those loads required for life safety or those considered emergency loads by the AHJ. This includes emergency lighting, fire alarm, and emergency communication systems and may also include ventilation equipment essential to maintain life, elevators, and fire pumps. All other loads requiring an alternate power source must be connected to a standby system.
Standby SystemsLegally-required standby loads are those systems and equipment required to be connected to an alternate power source by a governmental entity or other authority. An example is sewage pumps whose operation is necessary to prevent overflow of waste to a lake or river. The legally required standby system must make power available from the alternate source within the time required by the load, not to exceed 60 sec after the interruption of the normal source.
Optional standby loads are those systems and equipment connected to the alternate source for the protection of property where life safety does not depend on the performance of the system. This is the correct designation for the majority of owner-requested alternate source connections, such as freezers and coolers, heating systems, data processing, etc. There are no specific requirements for the availability of the alternate source to optional standby loads, and transfer may be automatic or manual.
Keep Them SeparateEmergency system wiring must not occupy the same conduits or equipment enclosures with normal power or standby system wiring. The only exception to this rule is where both a normal and an emergency circuit are required to supply the same item of equipment, such as an automatic transfer switch or a light fixture with a separate emergency lamp. Standby systems do not share this requirement; their wiring may occupy the same conduits and enclosures as normal power circuits.
Further, emergency circuits may not share the same fuses and circuit breakers or transfer equipment as standby or normal power circuits. A separate generator circuit breaker and transfer switch must be provided if standby loads are served from the same alternate source as the emergency system. This prevents a fault in a standby circuit from shutting off the supply to the emergency system. ES