How to Size an Emergency Generator for Residential Buildings

An emergency generator is used to provide backup power to building systems whose operation is necessary to ensure occupant safety, and where interruption of the power supply has the potential to create life-threatening conditions. The New York City Building Code makes emergency power systems mandatory for residential buildings that meet any of the following descriptions:

• All R-1 residential buildings, regardless of their characteristics. The R-1 occupancy classification includes all residential buildings or indoor spaces that will be used for lodging purposes and for periods of less than one month, such as hotels and vacation timeshare properties. The R-1 classification also includes all student dormitories that are not classified as R-2 (more than two dwellings), as well as living units owned by nonprofit organizations and government agencies.
• High-rise residential buildings that are not classified as R-3 (one or two dwelling units). The exemption also applies for R-2 buildings with a maximum height of 125 feet.
• Underground residential buildings, excluding one- or two-family dwellings that are adequately protected with sprinklers. Buildings with only one underground level are also exempt if the area of that level is 1,500 ft2 or less, and there are less than 10 occupants.

In order to provide a reliable source of backup power for critical building systems, an emergency generator must be properly sized according to the loads present. This article will provide an overview of the loads to consider when sizing an emergency generator in the three types of residential buildings described above.

An emergency power system should not be confused with a standby power system, which provides backup power to building systems that cause discomfort if they stop working, but not life-threatening conditions. Keep in mind, however, that standby power systems are still required by the NYC Building Code in many cases, even if the equipment they power is less critical that that covered by emergency power systems.

General Requirements for Sizing Emergency Generators

Emergency generators must be sized with enough capacity to power all equipment as required by the NYC Building Code according to the occupancy type, assuming it operates simultaneously and at full power. The emergency generator and its associated equipment must also be capable of tolerating the highest possible fault current that can occur at their terminals. If the emergency generator uses an internal combustion engine, it must be equipped with an on-site fuel supply of enough capacity to operate during six hours at full demand.

The NYC Electrical Code allows emergency lighting systems to be powered by batteries, and if this is the case their load does not count when calculating the total generator capacity. However, the batteries must be capable of providing sustained power for the full emergency lighting load during at least 1.5 hours, and their voltage must not drop below 87.5% of rated value. All other building systems required to have an emergency power supply must be connected to the emergency generator; batteries may be allowed in some cases, but special permission from the NYC Department of Buildings is required. The batteries must be supplied with an automatic recharging system, and car batteries are not acceptable.

Emergency Generator Capacity in R-1 Buildings

As previously stated, an emergency power system is mandatory in all R-1 residential buildings regardless of other characteristics. The NYC Building Code provides a list of all equipment types to consider when sizing the emergency generator:

• Exit signs and all lighting systems associated with means of egress.
• Elevator car lighting.
• All communications infrastructure used by emergency voice and alarm systems. This applies for Auxiliary Radio Communication systems, even if installed voluntarily where not required by the code.
• Fire protection systems: automatic fire detection, alarm systems and electrical fire pumps of all types - this includes manual, automatic and sprinkler booster pumps.

As previously stated, emergency lighting does not count towards the required generator capacity if it is equipped with batteries that meet code requirements: at least 1.5 hours of service at full lighting power, without falling below 87.5% of rated voltage.

Emergency Generator Capacity in High-Rise Buildings

If a residential building is considered high-rise construction, the emergency generator sizing procedure changes depending on whether the building is an R-1 or R-2 occupancy. Keep in mind that R-3 occupancies are exempt, as stated in the NYC Building Code. For R-1 high-rise buildings, the list of requirements provided in the previous section applies. On the other hand, the list of loads that require emergency power in R-2 occupancies is shorter:

• Exit signs and means of egress lighting systems.
• Communications infrastructure used by emergency voice systems.
• Electrical fire pumps, except those whose power is supplied ahead of the main service switch.

R-2 occupancies are exempt from the requirement of keeping an on-site fuel supply for six hours of continuous operation; the natural gas utility service is accepted as a fuel source.

Emergency Generator Capacity in Underground Buildings

When sizing an emergency generator in an underground building, the loads to consider are the same as in R-1 occupancies, with the exception of fire pumps:

• Exit signs and all lighting systems associated with means of egress.
• Elevator car lighting.
• All communications infrastructure used by emergency voice and alarm systems. This applies for Auxiliary Radio Communication systems, even if installed voluntarily where not required by the code.
• Fire protection systems: automatic detection and alarm systems.

Conclusion

Before a residential building in New York City can be used for its intended occupancy, it is important to verify if the Building Code requires an emergency generator. If this is the case, the unit must then be sized according to the loads specified in the building code, keeping in mind that there are differences between R-1, high-rise and underground constructions.

The NYC Electrical Code clearly states that all equipment requiring emergency power must be connected to a generator of sufficient capacity. The only exception that applies in all cases is for emergency lighting systems powered by suitable batteries. Battery-based emergency power may be allowed for other systems, but only with direct approval from the Department of Buildings.