Case study: Emergency lighting design

The following case study looks at a ground-up office building with a floor plate of 60,000 sq ft, 3 stories, and a total building square footage of 180,000. Three types of power sources for emergency illumination were considered for this project. The power sources consisted of unit batteries, a centralized emergency lighting UPS system, and a natural gas emergency generator system. The purpose of this case study will be to compare how the different lighting layouts can achieve code-required illumination while also comparing the installation and maintenance cost of each of the systems. Given the occupancy type of a 3-story office building, no one specific system is code-required. The designer had the flexibility to discuss and review these options with the owner and contractor in making a decision.

The first option was to install dual-head emergency-unit battery packs throughout the space to provide emergency illumination at the path of egress. This option is the most straightforward from the standpoint of design, engineering, and contractor installation. Emergency-unit battery packs are selected and spaced according to a photometric calculation. One consideration for the unit battery-pack fixture is that very little additional wiring in conduit is required for this type of installation. Having selected an emergency-unit battery fixture, a photometric calculation was run to determine the spacing of the fixtures. After reviewing the output and cost of two different types of fixtures, one was selected based on the photometric output and ceiling heights for the facility. It was determined that, based on the light output, an emergency battery-pack fixture was needed at about every 30 ft along the path of egress to maintain the code-required illumination levels. Each floor required approximately 35 emergency battery-pack units. At approximately $120/fixture and with 3 floors, the fixture cost alone came to $12,600. For the purpose of this comparison, similar light fixture layouts will be reviewed and only the material cost will be reviewed, as the installation cost is assumed to be comparable (see Figure 1).

The second option was a centralized emergency lighting UPS. The battery system associated with the UPS needed to be fairly large in scale due to the amount of fixtures required to supply for emergency egress lighting. The intent of the emergency lighting UPS was to use the already planned LED fixtures throughout the space as the primary source for emergency lighting. Using general 2×4 light fixtures as emergency lights will save on cost, eliminating the need to purchase individual unit battery packs. This cost will be offset by the purchase of the emergency lighting UPS. For this particular project, a 2×4 recessed LED fixture was the primary light fixture specified throughout the entire new office building. In the path of egress, photometric calculations were reviewed to select the number of appropriate fixtures required to be backed by the UPS. It was calculated that a total of 28 light fixtures were needed to supply emergency lighting per floor, totaling 84 fixtures for the building. It was then determined a 5-kW emergency UPS system would be needed to supply the 84 fixtures at full load. The cost of purchasing a 5-kW system directly from the distributor was $8,000. The one factor that can make or break this comparison to dual-head emergency-unit battery packs is the light fixture’s circuiting and the need for additional conduit. This should be reviewed in conjunction to obtain a fair cost comparison. When comparing the material cost for the dual-head unit batteries and the central UPS, the UPS was actually more affordable in this case (see Figure 2).

The third option reviewed during the time of the design was an emergency generator system. This option had the highest upfront cost, with the purchase of a new emergency generator and the associated underground conduit runs and natural gas connection. The emergency generator was sized to handle the emergency lighting and additional life safety loads. The emergency generator was selected at 25 kW, given the smallest reasonable size most major manufacturers carry. The cost of the generator alone, not including the installation, was $10,000. Much to the owner’s surprise, the premium to install the generator including conduits and additional equipment was about $20,000, which wasn’t a substantial cost increase over the other two options when also considering reliability and safety. 

Taking the time to carefully review all options for emergency lighting selections is an important part of adding value to the project. The generator system was determined to be more costly than the battery options. In the realm of battery options for emergency power, every project should be closely considered for a UPS central battery system over unit batteries. Reviewing these costs on every project can help add value to other projects. 

Matt Zega is an associate with RTM Engineering Consultants. He has been designing electrical power and lighting systems for more than 12 years, and his diversified portfolio encompasses projects within all major market sectors. 

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