The keys to designing emergency lighting systems

There are numerous building codes in various editions in use around the country for engineers designing emergency illumination systems. The most widely used codes in effect today are NFPA 101: Life Safety Code and International Building Code.
By Steven Eich, PE, CDT, REP, LEED AP, Environmental Systems Design, Chicago March 20, 2017

This article is peer-reviewed.

Learning objectives

  • Outline the codes and standards that define how to design emergency lighting systems.
  • Understand when and where emergency illumination is needed in commercial buildings.
  • Assess prices, testing, maintenance, and other variables when specifying emergency lighting systems.

Emergency lighting systems play a very important role in keeping buildings safe for public use. When in an unfamiliar place like a store, hotel, theater, or restaurant, sudden darkness presents a challenge for exiting a building. In darkness, attempts at finding an exit in an unfamiliar place could likely prove futile. If the building is a high-rise, imagine making the way down a stairwell in complete darkness. The situation can be especially worse where crowds have gathered, leading to unsafe panic conditions.

Figure 1: This is an example of a typical office-corridor lighting scheme using recessed fluorescent luminaires for emergency egress lighting. Emergency power is provided to three of the seven luminaires installed in the corridor. This lighting layout results in actual illuminance readings at the floor of 2.4 fc (25.9 lux) average, 6.7 fc (72.4 lux) maximum, with a minimum reading of 0.24 fc (2.59 lux). The emergency lighting provided in this corridor is sufficient to meet the requirement of the NFPA 101-2015: Life Safety Code. All graphics courtesy: Environmental Systems Design

For these reasons, emergency lighting systems must be carefully designed and properly constructed to provide a highly reliable system ready to illuminate the exit path when a power outage or other failure occurs. Fortunately, these systems are straightforward to design and simple to operate. While building codes, in regard to emergency lighting, are also fairly straightforward and easy to apply, proper specification is critical to optimal operations and reliability.

It’s all about the building codes

Usually, the code applicable to the design of the building—like the International Building Code (IBC), for example—sets the requirement to include an emergency lighting system as an element of the project design. The building code, alternatively, might invoke NFPA 101: Life Safety Code. NFPA 101 and IBC are written to coordinate with each other and define photometric performance. NFPA 70: National Electrical Code (NEC) defines installation requirements. In special building types, such as hospitals and health care facilities, occupancy-specific requirements may come into play. Always compare NPFA 101 with the applicable local building code as some have more stringent or different requirements.

Other codes referenced by NFPA 101 are likely applicable to the project as well. In particular, NFPA 110: Standard for Emergency and Standby Power Systems establishes performance and testing requirements for emergency and standby power systems including energy sources, converters, inverters, transfer switches, and controls. Similarly, NFPA 111: Standard on Stored Electrical Energy Emergency and Standby Power Systems does so for electrical stored-energy emergency power systems.

According to the IBC and NFPA 101, the following occupancy types require emergency lighting systems:

  • Ambulatory
  • Assembly
  • Commercial
  • Correctional
  • Day care
  • Educational
  • High-rise
  • Health care
  • Hotels and dormitories
  • Industrial
  • Lodging and rooming
  • Mercantile
  • Multifamily
  • Residential board and care
  • Storage
  • Underground and limited-access structures.

Figure 2: An illuminance calculation shows a stairwell. Illuminance values are shown in footcandles for the occupied condition. The where, how, and when of emergency lighting codes

Where: The basic requirement is to provide emergency lighting systems in all exit paths including stairwells, aisles, corridors, ramps, elevators, escalators, and passageways leading to an exit and to the public way. Code writers did leave some discretion to designers in regard to where emergency lighting is required. The 2015 edition of the IBC doesn’t explicitly say to provide emergency lighting in mechanical rooms or open-office areas, but these areas should be provided with emergency lighting systems.

A good practice is for designers to imagine themselves in an unfamiliar mechanical room, open-office area, or any other area of a building. Would lighting be required to make a safe exit? It is possible that power is lost only to lighting feeders leaving mechanical and electrical equipment still in operation? Exiting an unfamiliar mechanical or electrical room could be dangerous in these conditions.

For similar reasons, emergency lighting should be included in open-office areas. There should be adequate emergency lighting to light the path from the desk areas to the corridors. Similarly, don’t forget about exterior emergency lighting. The code requires emergency lighting in the exit path to the public way. Exterior lighting is very straightforward for an urban high-rise building located right on the property line. Consider what exterior lighting is required in a suburban setting, where the public way is remote from the structure. It’s best to clarify these situations with the authority having jurisdiction (AHJ) during the early stages of the project.

How: For most commercial building projects, NFPA 101 requires emergency lighting systems to use artificial lighting when emergency lighting is required. Natural daylighting cannot be relied upon for life safety applications. NFPA 101 does allow some industrial applications to use natural lighting, provided emergency lighting is required only during daylight hours.

When: Emergency lighting systems must be operable whenever the conditions of occupancy require means of egress to be available. It is possible to shut off emergency lighting systems when a structure is not in use; however, most emergency lighting systems are not provided with an off feature, resulting in higher reliability.

Figure 3: In this stairwell rendering, one two-lamp fluorescent fixture is wall-mounted on each side of the stairwell. The building is shown in an occupied condition. Requirements for power density, emergency lighting levels

Emergency lighting distribution systems must be designed to provide adequate power to supply light fixtures that can maintain at least minimum lighting requirements in a space, as specified by the code in footcandles (fc) or lux. The ratings of distribution equipment selected is a function of light fixture efficacy. Some building codes, however, like that of the city of Chicago, contain a requirement that distribution systems be designed for a minimum emergency lighting power density, which is measured in watts/square foot (W/sq ft) or watts/square meter (W/m²). City of Chicago code, for example, requires the building total area to be used in the calculation to determine the emergency lighting system distribution ratings. The value of 0.1 W/sq ft is generally adequate; however, values of 0.2 to 0.3 W/sq ft might be warranted for large public spaces like ballrooms, conference rooms, convention center halls, etc.

NFPA 101-2015 gives a lighting level requirement of 1 fc (10.8 lux), measured at the floor. Lighting levels will vary over the length of an exit path due to the location of the light fixtures. NFPA 101 allows lighting levels to decrease to a minimum of 0.1 fc (1.08 lux) for floor areas between light fixtures (see Figure 1). Initial lighting levels are given to allow for light fixture output degradation over time for unit battery equipment. Lighting levels are allowed to drop to an average of 0.6 fc (6.48 lux) at the end of the required run time of 1.5 hours. The lowest light level allowed between light fixtures at the end of the run time is 0.06 fc (0.648 lux). As always, refer to the applicable building code. Chicago’s building code, for example, does not allow for degradation of light levels in between light fixtures and requires a minimum of 1 fc (10.8 lux) at all points along the exit path.

Emergency lighting levels can be designed to be higher than the code-minimum levels. However, care must be taken to keep light-level ratios less than 40:1. Exceptions are given for stairwells and assembly occupancies. In stairwells, emergency lighting levels are required to be 10 fc (108 lux) during conditions of use and allowed to decrease to 1 fc when occupants are not in the stairwell (see Figures 2 and 3). In assembly occupancies, lighting levels along the exit path can be dimmed to a minimum level of 0.2 fc (2.2 lux) during periods of performance or projection.

Figure 4: This functional diagram depicts an emergency lighting dimming-override controller. Loss of normal power or actuation of the fire alarm system will cause the emergency light fixtures to go to full brightness. Controllers like this are used in theaters, auditoriums, conference rooms, and similar applications. NFPA 101 allows emergency light levels to be below even the minimum requirements for conditions where operations or processes require lower light levels. In the event of a failure of a single emergency light fixture, light levels in the affected area must not fall below 0.2 fc (2.2 lux). To meet this requirement, emergency light fixtures must be arranged to allow for some overlap in floor coverage.      

In the case where emergency lighting is dimmed for a theater performance, emergency lighting controls must be listed and must monitor the power source for the normal lighting circuits. Upon loss of the normal power supply, the dimmed emergency light fixtures must automatically return to the full brightness state (see Figure 4).

The power of two

One nuance sometimes overlooked is the fact when codes require emergency lighting systems to be provided, there is an assumption that a normal lighting system already exists. This typically is not an issue. However; in the case of a small elevator vestibule, for example, where there would otherwise only be one light fixture, two are necessary. In the case where an emergency lighting system is required in addition to the normal lighting system, a full additional light fixture must be installed. Similarly, think of an exit stairwell in a high-rise building. Emergency lighting is required here because it is an exit path. Emergency light fixtures need to be provided in addition to the normal lighting fixtures. A typical design approach is to provide normal light fixtures for one side of a stairwell and emergency light fixtures on the other side.

Figure 5: A typical emergency lighting system uses a utility as the preferred source and a generator as a backup. This can be found in high-rise buildings and large commercial projects in the United States. Emergency lighting goes out while the generator comes up to rated speed and voltage. Emergency lighting systems are also required to have two sources of power. The two sources may be two utility sources—preferably from two separate substations. Another option is a utility source and a storage battery or unit battery equipment—an option typically used in small commercial projects.

A third option uses a utility source with a generator as a backup source (see Figure 5). This option is most commonly used in large commercial projects like high-rise buildings. The building code applicable to each project will determine which option to follow. Some building codes—IBC, for example—require a generator to be used as the backup source for specific occupancies like high-rise buildings. Some building codes limit generator fuel to diesel for large projects or high-density occupancies allowing natural gas or liquid propane fuel for smaller projects. The Chicago code requires three sources of power—two utility and one generator—for certain occupancies like hospitals, large assembly areas, and theaters.

The process of switching from the preferred source to the standby source and back to the preferred source must be fully automatic, capable of repeated operations without manual intervention. The emergency lighting system must be automatically initiated upon any of the following three conditions:

  • Utility or other outside power source failure
  • Opening of a circuit breaker or fuse
  • Manual or accidental opening of a lighting switch.

The first two conditions are generally met through the use of an automatic transfer switch or emergency lighting inverter. The third condition is met by having the emergency lighting system engaged at all times or monitored through the use of a listed transfer device. A typical listing standard is UL 924: Standard for Emergency Lighting and Power Equipment.

Energy code exemption

Energy codes like ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings and the International Energy Conservation Code (IECC) permit life safety or code-required lighting to be exempt from the energy code requirements. Emergency lighting is not exempt from the IECC power-density requirements unless it is normally off during occupied hours (e.g., unit batteries). It is exempt from the controls requirements in certain areas only.

Light fixtures providing illumination for a means of egress can be outfitted with lighting controls, provided several stipulations are met. All emergency lighting control devices must be listed by an organization acceptable to the AHJ. Lighting controls used to switch emergency lighting fixtures must automatically turn on lighting when occupants are sensed. The lighting-controller timer must keep the light fixtures on for a minimum of 15 minutes after occupants have left the area. The lighting control device also must be connected to the fire alarm system so that light fixtures can be automatically turned on in the event a fire alarm signal is activated. Finally, emergency lighting controllers must monitor the normal lighting circuit and turn on the emergency lights in the event of failure of the normal power supply.

Emergency light fixtures cannot be switched off if those fixtures are used for the activation of photoluminescent exit signs or markers.

If unit battery equipment is used in the area where emergency lighting fixtures are automatically controlled, the unit battery equipment must be wired upstream of the automatic switch.

Figure 6: This typical emergency lighting system uses a utility as the preferred source and a generator as a backup. It can be found in high-rise buildings and large commercial projects internationally. Emergency lighting remains on while the generator comes up to rated speed and voltage.

Timing is everything

The building code will define how long the emergency lighting system must operate. NFPA 101-2015 requires emergency lighting systems to operate for a minimum of 1.5 hours. Local codes should be checked. Some buildings, like supertall high-rises (>984 ft), should be provided with systems having run time capabilities significantly longer than 1.5 hours. Supertall buildings often employ protected areas of refuge floors and evacuation elevators. Evacuating supertall buildings could take longer than NFPA 101’s 1.5-hour requirement.

Typically, high-rise buildings are equipped with life safety generators that supply power to fire pumps, smoke exhaust fans, and stairwell-pressurization fans—as well as emergency lighting and other life safety loads. Of all the loads connected to the life safety electrical system, fire pumps have the longest run time requirement. Stipulated by NFPA 20-2016: Standard for the Installation of Stationary Pumps for Fire Protection, fire pumps require a run time of 8 hours. The emergency power system should be designed to operate all life safety loads including emergency lighting for the entire 8-hour fire-pump-required run time.

A typical emergency system specification—Type 10, Class 1.5, Level 1—is defined by NFPA 110 with the following specifications:

  • Type 10: The maximum time to switch from a failed utility source to an operational emergency system is 10 seconds.
  • Class 1.5: The minimum time the emergency system shall operate is 1.5 hours.
  • Level 1: A failure of the emergency system could lead to loss of human life.

As always, be sure to check the local building code requirements. Many international projects require emergency lighting systems to switch to the emergency supply without delay. This requirement will significantly affect the system design. To meet an immediate switchover requirement, an emergency lighting inverter system will be required (see Figure 6). If the project is a high-rise, a backup generator system will be required, too, as it is sometimes impractical to provide an inverter system when run times exceed 1.5 hours.

Figure 7: Shown is the proper connection of unit battery equipment and battery-backed light fixtures connected upstream of the switch for the normal lighting fixtures. Available lighting sources

For generator- or inverter-backed systems, typical in large projects, emergency lighting is incorporated into the overall lighting layout or scheme. Most lighting sources used for general illumination can be used for emergency applications; care and thought must go into the selection of emergency fixtures to ensure reliability. For instance, incandescent lighting sources, rarely used now due to energy code requirements, are not good selections for emergency lighting due to the short life of the lamps.

Fluorescent lighting sources are better choices due to their longer lamp life. Consider fluorescent-fixture temperature limitations when selecting fixtures for outdoor or exposed parking garage applications. During cold temperatures, fluorescent-fixture light output can be significantly reduced, especially during start-up.

LED lighting is becoming standard for large commercial projects. Most LED fixtures are excellent choices for emergency lighting systems due to their instant-on characteristics and extremely long lifetime.

High-intensity discharge (HID) fixtures need special attention when used in an emergency lighting system. Because HID fixtures have a long start-up cycle, they do not provide any light output after cycling on after a power outage. Some HID fixtures are available with quartz restrike elements. The quartz element will provide some fixture output during the time the main HID lamp is coming up to operating temperature. A better design is to incorporate another source—fluorescent or LED—to maintain emergency levels while the HID fixtures are starting up. Incorporating fluorescent or LED fixtures within an HID lighting layout is challenging due to varying lamp color temperature and fixture aesthetics.

Unit battery fixtures are self-contained equipment with a battery, lamp, and control/charging equipment. Unit batteries are easy to apply and are generally used in smaller commercial projects. Because they aim their light output directly at the floor, they are very efficient and effective in lighting the egress path. Unit batteries are often employed in addition to generator-backed lighting in areas like generator or switchgear rooms to ease troubleshooting in the event of a generator or emergency lighting system failure. Unit batteries have gained a reputation as being unsightly and are often referred to as “bug eyes.” However, there are many aesthetically appealing options available now, which include units that retract into the wall when not in use. Unit battery equipment includes all the control, test, and charging circuitry required for proper operation.

There is sometimes confusion about how unit battery equipment should be connected to the power system. Unit battery equipment should not be connected to an emergency power panel or to dedicated circuits. Unit battery fixtures must be wired to the circuit used to power normal lighting in the vicinity of the unit battery fixture (see Figure 7). To avoid operation when normal lighting is turned off, unit battery fixtures must be connected between the branch circuit breaker and the lighting control device.

Battery-backed lighting fixtures are available and can be a good choice. With the exception of a test switch or maybe an indicator light, these types of fixtures look like normal lighting fixtures and blend well with the normal lighting layout. Battery-backed fixtures contain batteries, an emergency ballast or driver, and controls to allow for switchover and testing. Upon failure of power to the light fixture, a transfer device within the fixture will switch over to the battery-backed ballast/driver, and the fixture will maintain a partial level of light output.

Care must be taken in the design in regard to light output during emergency operation. Depending on the battery ballast specified, the fixture light output can be only 40% to 60% of the normal ballast/driver output, sometimes less. For some low-wattage fixtures, it’s difficult to tell the light fixture is on when connected to a battery ballast.

Another option is a lighting inverter. Lighting inverters are available in sizes from a few hundred watts to 3-phase units with ratings up to 50 kW. Battery times are rated at 1.5 hours at full load. Lighting inverters offer several advantages over other available options. First, the switchover from normal to battery is immediate and seamless. Second, light fixtures supplied by inverter systems will output their full rating as opposed to battery-backed fixtures, which only give partial output when operating in battery mode. The disadvantages include cost, size, weight, limited operation time, and maintenance requirements.

Don’t forget to test it

Testing and maintenance of emergency lighting systems are required to keep emergency lighting systems in a constant state of readiness. NFPA 101 allows three options for the testing of emergency lighting systems.

  1. This requires functional testing to be conducted monthly for a minimum of 30 seconds. Annual testing is required to be performed for a period of 1.5 hours.
  2. This option can be used when self-testing/self-diagnostic equipment is used. Along with a visual examination, a self-test must be performed every 30 days. As with option 1, an annual 1.5-hour test is required.
  3. This is similar to option 2 but involves the use of computer-based self-testing/self-diagnostic emergency lighting equipment. Option 3 negates the requirement for a visual examination. The annual 1.5-hour test is still required.

Through careful design and by paying close attention to emergency lighting codes, emergency lighting systems can be designed and constructed to provide extremely reliable systems that will function when normal lighting goes out. It is important for system designers and constructors to be knowledgeable of all the codes and pertinent information to keep the public safe in the buildings with which we are involved.

Steven Eich is a vice president and electrical technical director at Environmental Systems Design in Chicago. His expertise includes 29 years of designing electrical systems for industrial and commercial projects including high-rise buildings, hospitals, schools, theaters, museums, hotels, convention centers, manufacturing facilities, water treatment plants, and nuclear power facilities.