Understanding Bilevel Switching and Occupancy Sensors

Bilevel switching, in which alternate rows, fixtures or lamps are separately circuited and independently controlled, can serve as both a standalone lighting control strategy and an effective complement to automatic occupancy sensing. It has been demonstrated in one study to produce 22% energy savings in private offices. Meanwhile, occupancy sensors, which automatically switch the lights based on occupancy, can produce up to 45% energy savings in private offices, according to the Advanced Lighting Guidelines.

Code Requirements and Control Strategy Conflicts

At least one-half of the energy codes in the United States are based on the International Energy Conservation Code (IECC), which requires light-level reduction controls, such as bilevel switching in enclosed spaces. If an occupancy sensor is used in the space to satisfy the code’s automatic shutoff requirement, an exemption to the light-level reduction requirement is granted. This suggests that these control strategies—manual bilevel switching and automatic occupancy sensing—should be mutually exclusive, per the IECC’s intent to avoid redundant control systems unless a specific energy benefit is demonstrated. But what if they were combined?

CLTC Study on Lighting Control Combinations

In 2008, the California Lighting Technology Center (CLTC) attempted to determine the combination of automation and manual initiative that would produce the highest energy savings in private offices. The study, sponsored by Watt Stopper/Legrand, found that combining bilevel switching and occupancy sensing can produce significantly higher energy savings, outperforming traditional occupancy-only controls by leveraging both automation and user engagement.

Study Design and Office Setup

Researchers selected eight private offices at the University of California, Davis, each lighted by a combination of direct/indirect pendant fixtures and daylight entering through a window with manually adjustable vertical blinds.

The baseline condition was an IECC-compliant private office with its lights controlled by an automatic-on/-automatic-off occupancy sensor, with the occupant having no personal control over his or her lighting levels.

In the other offices, the four-lamp direct/indirect pendants were configured with dual circuiting, with a ballast and two lamps (48W) placed on each circuit. This enabled the researchers to setup three test conditions for study over a period of three weeks each.

Description of Test Conditions

• AUTOMATIC-ON TO 100%: When the office became occupied, an occupancy sensor signaled both relays to turn the lights on to 100% light level. If the occupant desired a lower light level, he or she could flip a switch to 50% or manual off.
• AUTOMATIC-ON TO 50%: When the office became occupied, the sensor signaled one relay to turn half the lamps on to achieve a 50% light level. The user could flip a switch to achieve a light level of 100% or turn the lights off.
• MANUAL-ON: When the office became occupied, the sensor did not activate the lights. The user could turn the lights on to 50% or 100% light level or leave them off.

In each scenario, when the occupant left the room, the occupancy sensor turned the lights off automatically. The study participants were informed that they had complete control of their lighting. They were aware that they were participating in a lighting study but did not know the study’s objectives.

Study Results and Energy Savings by Scenario

All three scenarios saved energy compared to the baseline scenario. The automatic-on to 100% bilevel occupancy sensor design saved 34% of energy consumption. The manual-on bilevel sensor, as might be expected, saved even more energy, cutting energy consumption by nearly half (46%).

The most efficient option? The automatic--on to 50% bilevel occupancy sensor option saved the most energy—52% lower energy consumption than the baseline option.

“Giving individuals control of their lighting is important for achieving both user satisfaction and efficient use of energy,” said Theresa Pistochini, development engineer for CLTC. It also creates opportunities for behavioral energy savings that wouldn’t occur under fully automated systems.

“This study indicates that there is still a lot of room for lighting energy savings in new and existing buildings,” said Pete Horton, vice president, market development for Watt Stopper/Legrand. “If you are looking for energy savings and a good return on investment, combining bilevel switching and occupancy sensing appears to offer one of the highest values a building owner can achieve.”

User Behavior and Lighting Preferences

A major advantage of bilevel switching is that users are given a choice of lighting levels, enabling them to adjust those levels given changing conditions.

Pistochini said that about half the study participants liked the automatic-on to 50% best, while the other half wanted complete control.

“The hypothesis with automatic-on to 50% is that the occupant, when presented with manual-on switches, will not give much thought to the amount of light needed and turn on both of them. With the automatic-on to 50%, the occupant often enters the office, finds the light level acceptable, and continues working. Occasionally, they desire more light and turn on the other switch.”

Impact on Energy Codes and Future Design

Horton said this research will be influential and said, “Because we can demonstrate 52% more energy savings using bilevel lighting over conventional occupancy sensors, we may see energy codes moving in that direction in the future.”