Plug loads are common in many buildings, especially office buildings, where use is intensive. Examples include computers, printers, copiers, space heaters, water coolers, task lights, coffeemakers, microwaves, televisions, and vending machines. According to the U.S. Department of Energy, plug and process loads account for one-third of commercial building energy consumption. They are the fastest-growing load types in buildings, in some cases exceeding the lighting load.
Because these devices often remain ON when they are not being used, they are a good target for an automatic shutoff strategy to save energy in both new and existing construction. According to manufacturers, implementing an automatic shutoff strategy to plug loads can generate 15-50% energy savings.
Controlling plug loads is a natural fit for the lighting controls industry, as the same devices and strategies are used for automatic shutoff of plug loads such as task lighting as for general lighting. Similar to general lighting control, the main strategies are scheduling, which is based on vacancy that is predicted, and occupancy sensing, based on vacancy that is detected. A third option is a signal from another system.
Scheduling: Scheduling is relatively simple and well suited to applications with predictable occupancy.
Occupancy sensing: Occupancy sensing offers higher potential for energy savings and less potential for disruption by being directly responsive to occupants, particularly in applications where occupancy is unpredictable and variable throughout the day.
Depending on the type of lighting control system, these strategies could be blended to minimize the potential for disruption to occupants. For example, the lights could be scheduled ON during operating hours, after which occupancy sensor-based control would take priority.
Commercial building energy codes based on the 2010 or later version of the ASHRAE/IES 90.1 energy standard require at least 50% of all 125V, 15A or 20A receptacles be automatically controlled in a list of spaces such as private offices and conference rooms. Compliance options include scheduling (with local override), occupancy sensing (with 20-minute time delay), or automatic signal from another system such as an alarm system (with 20-minute time delay). Starting with 90.1-2013, controlled receptacles must be permanently marked to differentiate them from uncontrolled receptacles. California’s Title 24 has similar requirements. Consult the applicable code for specifics.
Image courtesy of Eaton.
Plug-in power strips: Plug-in power strips controlled by occupancy sensors, smart phones, or other method. Because they are not permanently installed, this option is not compliant with energy codes, though it may be suitable for applications where the code does not apply.
Automatic receptacles: Receptacles controlled by time schedule, occupancy sensor, or automatic signal from another building system. Each receptacle in a duplex receptacle (see photo) can be automatically controlled or one receptacle can be automatically controlled (e.g., top is controlled while bottom is uncontrolled). This allows separate control of plug loads that can be turned OFF (e.g., computer monitors) and those that must remain ON (e.g., computer CPU).
A relay in a power pack wired to an occupancy sensor or residing at a lighting control panel can be used control circuits, with users being given a means to override shutoff for up a period of time (codes impose a maximum of two hours). The plug load controllers may be networked for centralized control, which allows programming, measuring, and monitoring. The plug loads should be controlled separately from the general lighting if a manual ON/automatic OFF control strategy is used.
Alternately, the plug load could be controlled at the receptacle via an integrated timer, relay, and override.
Wireless options are available that simplify installation, particularly well-suited to existing buildings. These receptacles feature an integral wireless receiver and integral relay. A device such as an occupancy sensor sends a wireless signal to a receiver in the receptacle, which disconnects from the branch circuit using an integral relay. This allows the installer to use existing wiring, simplifying installation. The wireless-controlled receptacle may also feature integrated power metering.
As with every other aspect of lighting and control, there is no “one size fits all” for plug load controls. When selecting the right plug load control solution, start with the loads, building, occupants, how the spaces are used, applicable codes and standards. Then select the right approach—scheduling or occupancy sensing, receptacles or power strips, networked or not, hardwired or wireless.
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