MH Controls Corner: Time-Based vs. Occupancy-Based Control. Which Strategy Should You Use?

There are multiple strategies for automatic lighting control to save energy while meeting the requirements of applicable energy codes and, in particular, IECC 2015. To meet this code, either time-based or occupancy-based automatic controls are required in nearly every space. The choice of which strategy to use is based on the particular needs of the project, taking into consideration functionality and system cost.

Time-Based Control

Time-based control is typically accomplished using a switching or dimming lighting control panel. These panels house relays and dimmers that operate the lights based on a time schedule, with local override control via low voltage switches.

Relay panel control is typically used for larger spaces and open areas within the facility. Time schedules are programmed to control the lights based on the operational hours of the facility. Local override switches are necessary to offer the flexibility of turning the lights on outside of normally programmed hours. Override switches are also required by IECC 2015 for every 5,000 square feet of floor area.

For smaller spaces, it is possible to meet code requirements using a simple single zone wallbox timeclock (Wattstopper RT-200, for example) connected to a contactor. The timeclock is installed in a standard wall junction box and has the ability to include different programming schedules based on specific times or astronomical times, and has an integral override switch. It is important to keep the override switch in mind as this device will need to be installed near the entry door to the space so that occupants can turn the lights on outside of the set schedule. Connecting the output of the time switch to a contactor allows multiple circuits of lights to be controlled together. This solution can avoid the high costs of multiple occupancy sensors or lighting control panels for small spaces, meeting code at a fraction of the cost.

Occupancy-Based Control

Occupancy-based control utilizes occupancy sensors to detect motion within the space. The IECC requires that lights turn off a maximum of 30 minutes after occupancy is no longer detected. Most spaces utilizing this control strategy are required to operate as manual on or vacancy sensing (automatic on is allowed to 50% light level only). This requires the occupant to turn the lights on via a wall switch with the lights automatically turning off, based on the sensors. The thinking behind the manual-on requirement is that occupants will not turn on the lights if they don’t need to, thus saving energy. It should be noted that the only way to accomplish manual on control with ceiling mounted occupancy sensors is to use low voltage switching. Spaces including corridors, restrooms, stairways, entrances, and lobbies, are exempted from the manual on requirement, allowing the lights to automatically turn on once motion is detected.

Considerations for Which Control Strategy to Use

Either time-based or occupancy-based controls can be used to save energy and satisfy energy code requirements for most spaces. In general, smaller spaces, such as private offices or other spaces where sensor coverage can be accomplished with only a few devices, are more cost effectively controlled via occupancy sensors. Time-based control can be used in these areas. However, the requirement for override switches and the need for a relay for each space will make time-based control more expensive.

It is typically more cost effective to use time-based controls for large spaces. Consideration for these areas include the location for the override switches and the ability to combine time-based controls with daylighting controls where required by code or when desired for additional energy savings. Occupancy sensing can be used in larger spaces. However, the number of sensors required to achieve coverage may cause the cost to exceed that of a time-controlled strategy.

Many light fixtures can be purchased with integral occupancy sensors. In order to be in compliance with the IECC, spaces are required to operate as manual-on. Fixture-based sensors typically do not have a switch input, and therefore cannot meet the manual-on requirement. Additionally, in large spaces, fixture-based occupancy sensor control may create a strange appearance with some lights on and some off. Fixture-based sensors can be a cost-effective approach for spaces where automatic-on lighting control is allowed.

Very rarely is it advisable to use a control strategy that incorporates both time-based and occupancy-based control in the same space. Having occupancy sensors and a lighting control panel, in most cases, creates unnecessary system redundancy and will increase the complexity of the overall system, requiring more complex programming and adjustments. Any time complexity is increased, the potential for operational problems can occur.

There are many ways to save energy and meet code for any given space. In order to choose the optimal system, it is critical to understand the cost and operational impacts of the choice as a starting point for the design. Controls codes and technologies are evolving rapidly. The experts at MH Controls spend every day working on these issues and are available to help with design considerations and answers to questions at any time.

This article was written by Clint Conley, our Lighting Controls Department Manager. Clint can be reached at 720.904.8554 or clint@mhlighting.com.