In terms of operating principle, emergency and standby generators are similar in that they both provide backup power when electrical service is interrupted. However, the loads served by each type of system are different.
- An emergency generator provides backup power to loads that could create life-threatening conditions if they stop operating, while a standby generator provides backup power to loads that cause inconvenience or discomfort if the power supply is interrupted. Emergency power is always mandatory, but standby power can be mandatory or optional depending on the load.
- New York City codes establish different technical requirements for each type of system, emergency and standby. The requirements may match in some applications, but are specified independently.
Basically, all equipment not classified as an emergency load is a standby load, and standby power is optional unless the code explicitly requires it. Therefore, there is no list of loads for an optional backup power system – the building owner is free to select them.
To determine which loads should be connected to emergency or backup power, it is first important to check the type of residential occupancy. In New York City, residential properties are classified into three occupancy groups:
| Group | Description |
| R-1 | Temporary residential space used for periods of less than one month (e.g., hotels, timeshares), student dormitories not classified in Group R-2, and congregate housing operated by the government or a nonprofit organization |
| R-2 | Buildings or parts of buildings containing sleeping units or more than two complete dwelling units used for permanent residence. Apartment buildings are classified in Group R-2. |
| R-3 | Residential buildings with one or two housing units intended for prolonged use (periods of at least one month) |
Once the occupancy group is determined, emergency and mandatory standby power requirements can be checked against the New York Building Code.
Emergency power and legally required standby power
Emergency and backup power requirements established by the New York Building Code change depending on the type of occupancy, and applicable requirements for residential buildings are summarized in the following table. Keep in mind that this table is just an overview and should not be used to design emergency and backup power systems; Review of the New York Building Code is recommended for detailed requirements.
| Group | Emergency power | Mandatory standby power |
| R-1 |
1) Exit signs and exit lighting means 2) Elevator Car Lights 3) Voice and emergency alarm systems, including Auxiliary Radiocommunication, even if installed optionally 4) Fire protection systems: Automatic fire detection, fire alarms, electric fire pumps |
1) Ventilation systems for ventilation or smoke control 2) Stair pressurization systems 3) At least three elevators per building, with manual transfer to other elevators as needed 4) Fire command center power and lighting |
| R-2 |
1) Exit signs and exit lighting means 2) Elevator controls, cabin lights and ventilation. Additional loads for R-2 high-rise buildings: 3)Emergency voice communication systems 4)Electrical fire pumps unless powered before the main circuit breaker |
1) Smoke control systems 2) Horizontal sliding doors 3) Smoke-proof cabinets 4) Pressurized elevator shafts Additional loads for R-2 high-rise buildings: 5) Fire command center power and lighting 6) At least one elevator serving all floors, or one elevator per bank if different areas of the building are served by different banks. |
| R-3 |
1) Exit signs and exit lighting means 2) Elevator controls, cabin lights and ventilation. |
1) Smoke control systems 2) Elevators 3) Horizontal sliding doors 4) Smoke-proof cabinets 5) Pressurized elevator shafts |
Optional standby power: additional requirements
Optional standby power applies to all loads not listed in the table above. However, if the decision is made to install it, the generator must be capable of supplying the following loads in addition to the selected optional loads:
- Emergency light
- Fire alarm systems
- Elevators in Group R-2 buildings with a minimum height of 125 feet: At least one elevator serving all floors, or one elevator per bank.
- Elevators in other buildings with occupied floors at least 75 feet above fire truck access: At least one elevator serving all floors.
The purpose of this requirement is to provide redundancy, as these loads would normally be powered by emergency power systems or mandatory standby power systems. Sizing the optional power system with enough capacity to power them provides an extra safety measure.
Which loads should be supplied with optional standby power?
Although the New York Building Code does not provide any guidelines, given the optional nature of these loads, there are many appliances and systems for which it is highly convenient to have a backup power source:
- Refrigerators: At least one container per household, to prevent food from spoiling during a long interruption in service.
- Water booster pumps: In tall buildings where water booster pumps are required, an interruption in electrical service can also cut off the water supply. This can be avoided by providing optional standby power.
- Communication technologies: They play a fundamental role in the modern world and represent a relatively small load when compared to other systems such as HVAC or water heating. Providing optional standby power for IT infrastructure adds convenience at a reasonable cost.
Optional standby power exceeds code requirements and provides added value to the project. Keep in mind that this is not always feasible; for example, in a building that uses electric heating, the capacity and cost of a backup generator can be significant if that load is included.
If an optional backup generator is installed, the New York Building Code allows the fuel supply to be shared with emergency generators as well as common supplementary equipment required for generator operation.
