A plug load is equipment powered by plugging into an ordinary AC receptacle. Plug loads such as computer monitors, task lighting, coffeemakers, and vending machines are common in many buildings, especially office buildings, where use is intensive. They are a significant and the fastest-growing load type in buildings, in some cases exceeding the lighting load.
Because these devices often remain ON when they are not being used or draw power in an OFF or standby state, they offer an attractive target for automatic removal from power to reduce energy consumption by 15% to 50%. A growing number of energy codes now require plug load control, using the term “automatic receptacle control.” Controlling plug loads is a natural extension for lighting control, as the same strategies and devices used for automatic switching of general lighting can be deployed for plug load control either at the receptacle or circuit level.
This article, based on the Lighting Controls Association’s new Education Express course EE202: Automatic Plug Load Control, provides an overview of approaches used to automatically control plug loads in commercial buildings.
PLUG LOADS: PREVALENCE AND TYPE
Plug loads include computer monitors, printers, cell phone chargers, task lights, copiers, personal fans/heaters, A/V equipment, and more, all of which can be switched OFF at night without negative consequences. A 2008 California study (Moorefield, L., et al) found that computers and monitors accounted for 66% of plug load devices, with the remainder split between office electronics and miscellaneous devices such as task lights, telephones, and coffeemakers. The researchers discovered that offices contained an average 30 plug load devices per 1,000 sq.ft., or seven per office worker, combining to account for about 30% of electric energy consumption.
Today’s office equipment typically operates with a range of power levels or modes corresponding with user activity, though it is usually ON at some level. In active mode, the user is operating the equipment, which at certain times can draw its maximum power. If standby mode is featured and enabled, the equipment will go idle after a period of no user activity but remain ON at a lower power level. The equipment may power down to OFF or be manually turned OFF by the user. Even when the device is OFF, however, it may continue to draw a small amount of power as long as it is connected to a socket, so as to start quickly; this is called a “vampire, “phantom,” or “parasitic” load.
According to the National Renewable Energy Laboratory, plug and process loads accounted for 40% of commercial building energy consumption in 2017. Another study (Poll, s. and C. Tuebert, 2012) estimated plug loads at a maximum of 25% energy consumption in less-efficient buildings and 50% or more in high-efficiency buildings. Due to the size of these loads that are either used in relation to occupancy or on a predictable schedule, they are attractive targets for reducing energy consumption using automatic shutoff long in use for control of general and task lighting, with the highest energy savings achievable in buildings where plug loads are intensive.
ENERGY SAVINGS POTENTIAL
In a 2012 study conducted by the General Services Administration (GSA) as part of its Green Proving Ground program, GSA evaluated automatic plug load control in eight office buildings. A dozen standard power strips were replaced with advanced power strips controlling more than 295 devices, which were monitored over three test periods each a month long. All workstation devices already had power management in effect. The advanced power strips de-energized the circuit based on a schedule, de-energized connected devices based on load sensing, or a combination of the two.
Even with power management enabled, GSA recorded significant energy savings: 26% in workstations and nearly 50% in kitchens and printer rooms. The most successful control strategy was scheduling, which generated an average 48% energy savings. Highest savings were generated with devices operating 24/7, including printers, copiers, and kitchen appliances.
In new construction, permanently installed plug load controls are required by commercial building energy codes based on the ANSI/ASHRAE/IES 90.1 energy standard as well as California’s Title 24, Part 6 energy code. Additionally, some states—such as Florida and Washington—have amended their adoption of the International Energy Conservation Code (IECC) to include plug load control. As of the 2018 version, the IECC otherwise does not include automatic plug load control, though future versions are likely to include it.
The 2010 and later versions of the 90.1 standard require at least 50% of all 125V, 15A or 20A receptacles be automatically controlled in a list of spaces including private offices, classrooms, conference rooms, and others, in some cases based on square footage. Since the 2013 version, at least 25% of circuit feeders installed for modular furniture not shown on the construction documents must be automatically controlled for future furniture installation use. Compliance options include scheduling (with local override of no more than 5,000 sq.ft. or one floor), occupancy sensing (with 20- or 30-minute time delay, depending on the version of the standard), or automatic signal from another system such as an alarm system.
CALIFORNIA TITLE 24, PART 6
California requires a mix of controlled and uncontrolled 120V receptacles be provided in private and open offices, office kitchenettes, and other spaces. For each uncontrolled receptacle, there must be a controlled receptacle within six feet; duplex units with one controlled and one uncontrolled receptacle comply. Additionally, for open offices, at least one controlled receptacle must be provided per workstation. Electric circuits feeding controlled receptacles must be equipped with permanently installed automatic shutoff controls operating using occupancy sensing or scheduling (countdown timers do not comply).
Additionally, at least 50% of receptacles in hotel and motel guest rooms must be automatically turned OFF within 30 minutes of vacancy via an occupancy sensor, captive key switch, or other control.
NATIONAL ELECTRICAL CODE (NEC)
Both 90.1 (2013+) and Title 24 require that controlled receptacles be permanently marked to distinguish them from uncontrolled receptacles. Starting in 2014, the NEC specifically required that all nonlocking-type 125V, 15A and 20A receptacles that enact or are used to enact automatic removal from power for the purpose of energy management must be permanently marked with the symbol shown below (green here, though manufacturers use different colors, often black). Beginning in the 2017 NEC, the word CONTROLLED was added to the symbol. (For reference, this is found in NEC Article 406.3(E).) As a result, controlled receptacles may be found in the field with the symbol but not the word CONTROLLED.
The marking must be on the receptacle’s face and visible after installation. When a duplex or quadruplex receptacle is installed that is only partially controlled, the plug(s) that are controlled must bear the required markings.
Additionally, NEC 2020 required that automatically controlled receptacles being replaced must be replaced with equivalently controlled receptacles, unless automatic control is no longer required.
Utility rebate programs recognize the energy savings potential for automatic control of plug and process loads. In 2020, rebates of varying types were available from scores of programs covering much of the U.S. The most commonly incentivized categories included plug load occupancy sensors, advanced power strips, vending machine controllers, computer power management software, and plug load ENERGY STAR equipment such as computing devices and electronically commutated motor plug fans. The Better Buildings Initiative maintains a database of plug load incentives here.
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 such as building automation or security. Application of these strategies for lighting control is very similar to controlling receptacles, resulting in a high degree of compatibility.
Scheduling is relatively simple and well suited to larger, open applications with predictable occupancy as well as loads that must remain ON during business hours, such as water coolers and shared printers. With this approach, users should (or shall, where certain energy codes apply) be given a means to override shutoff for up to two hours via a manual switch located in the space or on the receptacle itself.
Occupancy sensing can generate higher energy savings by being directly responsive to occupancy. This approach is well suited to smaller, enclosed spaces such as private offices and conference rooms, particularly where occupancy is intermittent and unpredictable. Only loads that can be turned OFF during business hours should be controlled. The sensor functions as its own override by providing automatic ON as well as OFF.
Depending on the type of lighting control system, these strategies could be blended within the same space to maximize energy savings while minimizing potential disruption for occupants. For example, the lights could be scheduled ON during operating hours, after which occupancy sensor-based control would take priority.
Now that we understand the strategies involved in plug load control, we can look at equipment. The core element of this type of control is the automatic receptacle, a receptacle that is capable of responding to a control signal or is fed by a branch circuit that is capable of responding to the signal. Each receptacle in a duplex receptacle can be automatically controlled, or only one (e.g., top is controlled while bottom is uncontrolled). This allows separate control of plug loads that can be turned OFF (e.g., a computer monitor) and those that must remain ON (e.g., a computer CPU).
Controlled receptacle circuits can be switched using one of three methods: controllable circuit breaker panelboard, lighting control relay panel, or an individual relay in a local plug-load dedicated power pack, lighting control panel, or at the receptacle itself. For scheduling, the control panel or receptacle with integrated timer features timeclock functionality and should be capable of override via a switch in the space or on the receptacle. The control panel should be able to respond to external signals via a basic connection. If the system is networked, remote programming, measuring, and monitoring via software or a web page is often enabled. Occupancy sensors are typically standalone but may be connected as well. Although the plug loads may utilize the same occupancy sensor used to control lighting, plug loads should be controlled separately from the general lighting if a manual switch is used. Power relays used must be properly rated for receptacle control.
One approach is advanced power strips. These are power strips controlled by occupancy sensors, smart phones, or other method, and as such they are ideal for plug load control in workstations. Shown here is an eight-receptacle power strip with surge suppression and two uncontrolled receptacles and six controlled via a personal occupancy sensor, also shown. Portable (plug-in) advanced power strips are not permanently installed and therefore are not compliant with energy codes, though they may be suitable for applications where the code does not apply, such as retrofits. For code compliance, the advanced power strip must be permanently installed (hardwired to power).
Plug load signal connections may be wired or wireless. For some applications, notably existing buildings, wireless connection, typically via radio frequency, can simplify installation and reduce installed cost. These receptacles feature an integral wireless receiver and integral relay. A device such as an occupancy sensor sends wireless signals to controlled lighting but also to the receptacle, which connects or disconnects from the branch circuit using an integral relay. This allows expanded utility from the occupancy sensor without any new wiring or changes to existing wiring, simplifying installation. The wireless-controlled receptacle may also feature integrated power metering.
Below are three example configurations. At the top, we see a system in which an occupancy sensor and manual switch operate separate power packs dedicated either to lighting loads or controlled plug loads. Next, we have a lighting control relay panel with timeclock functionality for scheduling as well as inputs and outputs for control of lighting and receptacles via local manual switches and occupancy and light sensors. Third is a wireless system in which the occupancy sensor controls lighting and automatic receptacles via radio-frequency wireless signals.
CHOOSING THE RIGHT APPROACH
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.
PLUG LOAD CONTROL DECISION PROCESS
Conduct a walkthrough or review design plans to determine plug loads eligible for control based on user needs and how the space is or will be used. Plug loads required to operate continuously for business or safety reasons should not be controlled. Identify opportunities for support such as local utility rebates along with their requirements.
As part of the audit, meter the plug loads to be controlled to establish a baseline level of energy consumption. For office buildings, DOE recommends the meter be suitable for the circuit type, able to accurately meter loads up to 1800W, able to measure and log one week of electrical power (W) with a sampling interval of 30 seconds, timestamp each data point, and offer download of stored data.
Install any software needed to configure the meter and analyze the data. Set up the meter to measure at a sampling interval of 30 seconds, if possible, up to 15 minutes. Power down and unplug the device, plug it into the meter, plug the meter into the device, and power on the device. Meter all day, every day, for at least one workweek, longer, if possible, to generate more accurate usage patterns. Download the metered data for analysis and to calculate average load during business and nonbusiness hours.
Note that some control systems incorporate customizable, software-based energy metering, which automatically measures plug load energy consumption, displays it on an app, and shares it as needed.
Based on the energy use data, identify opportunities to reduce energy consumption via scheduling, occupancy sensing, a combination of the two, or a signal from another building system. Along with application characteristics, these strategies will reveal what equipment is required.
The opportunities will depend largely on whether the building is new or existing, and what level of lighting control is already available or will be installed (potentially as part of an LED upgrade). In an existing building, wireless controls and portable advanced power strips with personal occupancy sensors can be simple and cost-effective. In a new building, extending the automatic lighting control system required by energy code to control plug loads can be simply accomplished.
EDUCATING THE USER
As with any energy efficiency control system or strategy, they are most effective when used properly. Plug load controls rely on users plugging the right devices into the controlled and uncontrolled receptacles. Educate users how the plug load control in their space functions, how to distinguish a controlled receptacle from an uncontrolled receptacle, and what devices should be plugged into each.
A best practice is to provide a simple leave-behind informational sheet that cover how to effectively use plug load controls. An example is shown below.