In terms of operating principle, emergency and standby generators are similar, since both provide a backup power supply when the electric utility service is interrupted. However, the loads served by each type of system are different.
- An emergency generator provides backup power for loads that can create life-threatening conditions if they cease to operate, while a standby generator provides backup power for loads that cause inconvenience or discomfort if their 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 requests it explicitly. Therefore, there is no list of loads for an optional standby power system - the building owner is free to select them.
To determine which loads must be connected to emergency or standby power, first it is 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 under Group R-2, and congregate dwellings managed by the government or a non-profit organization |
| R-2 | Buildings or building portions that contain either sleeping units or more than two complete dwelling units, used for permanent residence. Apartment buildings are classified under Group R-2. |
| R-3 | Residential buildings with one or two dwelling units intended for long-term use (periods of at least one month) |
Once the occupancy group has been determined, the emergency and mandatory standby power requirements can be checked in the in the NYC Building Code.
Emergency Power and Legally-Required Standby Power
The emergency and standby power requirements established by the NYC Building Code change by occupancy type, and the 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 standby power systems; reviewing the NYC Building Code is recommended for detailed requirements.
| Group | Emergency Power | Mandatory Standby Power |
| R-1 |
1) Exit signs and means of egress lighting 2) Elevator car lights 3) Emergency voice and alarm systems, including Auxiliary Radio Communication, even if installed optionally 4) Fire protection systems: Automatic fire detection, fire alarms, electrical fire pumps |
1) Ventilation systems for smoke venting or 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 means of egress lighting 2) Elevator controls, cab lights and ventilation. Additional loads for high-rise R-2: 3)Emergency voice communication systems 4)Electrical fire pumps, unless powered ahead of the main breaker |
1) Smoke control systems 2) Horizontal sliding doors 3) Smokeproof enclosures 4) Pressurized elevator shafts Additional loads for high-rise R-2: 5)Fire command center power and lighting 6)At least one elevator serving all floors, or one elevator per bank if different building areas are served by different banks. |
| R-3 |
1) Exit signs and means of egress lighting 2) Elevator controls, cab lights and ventilation. |
1) Smoke control systems 2) Elevators 3) Horizontal sliding doors 4) Smokeproof enclosures 5) Pressurized elevator shafts |
Optional Standby Power: Additional Requirements
Optional standby power applies for all loads not listed in the table above. However, if the decision to install it is taken, the generator must be able to power the following loads in addition to the optional loads selected:
- Emergency lighting
- Fire alarm systems
- Elevators in Group R-2 buildings with a height of at least 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, since 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 NYC Building Code provides no guideline, given the optional nature of these loads, there are many appliances and systems for which it is highly convenient to have a backup power supply:
- Refrigerators: At least one receptacle per dwelling, to prevent food from being spoiled during a long service interruption.
- Water booster pumps: In high-rise buildings where water booster pumps are required, an electric service interruption can also cut the water supply. This can be prevented by providing optional standby power.
- Communication technologies: They play a key role in the modern world, and are a relatively small load when compared with 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 in project. Keep in mind that it may not always be feasible; for example, in a building that uses electric heating, the capacity and cost of a standby generator can be significant if that load is included.
If an optional standby generator is installed, the NYC Building Code allows the fuel supply to be shared with emergency generators, as well as common complementary equipment necessary for generator operation.
