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 the safety of occupants and where interruption of 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 interior 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 residences), as well as residential units owned by nonprofit organizations and government agencies.
- Tall residential buildings that are not rated R-3 (one or two dwelling units). The exemption also applies to R-2 buildings with a maximum height of 125 feet.
- Underground residential buildings, excluding one- or two-family residences 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 fewer than 10 occupants.
To provide a reliable source of backup power for critical building systems, an emergency generator must be appropriately 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 backup power system, which provides backup power to building systems that cause discomfort if they stop working, but not in life-threatening conditions. Keep in mind, however, that backup power systems are still required by the New York Building Code in many cases, even if the equipment they power is less critical than that covered by emergency power systems.
General Requirements for Sizing Emergency Generators
Emergency generators must be sized with sufficient capacity to power all equipment as required by the New York Building Code for the type of occupancy, assuming they operate simultaneously at full power. The emergency generator and its associated equipment must also be capable of tolerating the highest possible fault current that may occur at its terminals. If the emergency generator uses an internal combustion engine, it must be equipped with a local fuel supply with sufficient capacity to operate for six hours at full demand.
The New York Electrical Code allows emergency lighting systems to be powered by batteries, and if so, their load does not count in calculating the generator's total capacity. However, the batteries must be capable of providing sustained power for the entire emergency lighting load for at least 1.5 hours, and their voltage must not fall below 87.5% of the nominal value. All other building systems that require emergency power must be connected to the emergency generator; Batteries may be permitted in some cases, but special permission from the New York City Department of Buildings is required. Batteries must be provided with an automatic recharging system and car batteries are not acceptable.
Emergency Generator Capacity in R-1 Buildings
As stated previously, an emergency power system is mandatory in all R-1 residential buildings, regardless of other characteristics. The New York Building Code provides a list of all types of equipment to consider when sizing your emergency generator:
- Exit signs and all lighting systems associated with the means of egress.
- Elevator car lighting.
- The entire communications infrastructure used by voice and emergency alarm systems. This applies to auxiliary radiocommunication systems, even if voluntarily installed where not required by code.
- Fire protection systems: automatic fire detection, alarm systems and electric fire pumps of all types – including manual, automatic and sprinkler booster pumps.
As stated previously, emergency lighting does not count toward 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 dropping below 87.5% of the nominal 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 R-1 or R-2 occupancy. Keep in mind that R-3 occupancies are exempt as stated in the New York Building Code. For R-1 tall buildings, the list of requirements provided in the previous section applies. On the other hand, the list of loads requiring emergency power in R-2 occupancies is shorter:
- Exit signs and means of exit lighting systems.
- Communications infrastructure used by emergency voice systems.
- Electric fire pumps, except those whose power is supplied prior to the main service switch.
R-2 occupancies are exempt from the requirement to maintain on-site fuel supplies for six hours of continuous operation; Utility natural gas is accepted as a fuel source.
Emergency Generator Capacity in Underground Buildings
When sizing an emergency generator in an underground building, the loads to be considered are the same as in R-1 occupancies, with the exception of fire pumps:
- Exit signs and all lighting systems associated with the means of egress.
- Elevator car lighting.
- The entire communications infrastructure used by voice and emergency alarm systems. This applies to auxiliary radiocommunication systems, even if voluntarily installed where not required by 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 check whether 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, taking into account that there are differences between R-1, high-rise and underground constructions.
The New York Electrical Code clearly states that all equipment requiring emergency power must be connected to a generator of sufficient capacity. The only exception applicable in all cases is for emergency lighting systems powered by suitable batteries. Battery-based emergency power may be permitted for other systems, but only with direct approval from the Department of Buildings.
