Release the untapped potential of plug load control

Learning Objectives

• How plug load contributes to a building’s overall performance.
• Understand ASHRAE Standard 90.1 energy mandates and building applications.
• Explore the plug load options that are available to help specifying engineers comply with ASHRAE 90.1.

The benefits of plug load control have been extensively proven in real-world applications. Limiting energy consumption at an electrical outlet can reduce energy waste, support an organization’s sustainability efforts, and save money on utility bills. Plug load control is also required by code in a growing number of states.

In practice, energy efficiency is one of the biggest challenges facing specifying engineers, building owners, and facility managers today. Plug load control is one of the final frontiers to conquer in terms of energy efficiency, but it can also prove the most challenging. With plug load control, one of the issues engineers face is figuring out which control method is best for their application.

Additionally, codes vary by state and even from city to city. For example, some jurisdictions have adopted ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings standards that include plug load control, some have their own codes that are similar to ASHRAE 90.1, some have codes that exceed ASHRAE 90.1, and some do not have plug load control requirements.

This can leave engineers scratching their heads, wondering what are the code requirements and what plug load control solution is right for them. But it does not have to be overly complicated.

Plug load by the numbers

A plug load is defined as the energy source used to power a device or equipment that is plugged into an ordinary ac receptacle. Data found in a U.S. Department of Energy (DOE) study shows plug load consumption is a significant issue:

• Plug and process loads (PPLs) account for 30% of the electricity consumption in office buildings. This includes computers, printers, networking equipment, task lighting, kitchen appliances, and more.
• Performing a plug load inventory and implementing a load-reduction strategy can reduce plug loads by 20% to 50%.

Yet, controlling plug loads does not always get the attention it should. In a National Renewable Energy Laboratory (NREL) paper presented at the 2014 ASHRAE conference, the authors wrote that “In an ultra-efficient office building, plug loads are typically one of the last end uses to be considered for energy conservation and, as a result, can account for more than 50% of the total electrical load.”

That same paper reported that occupancy-based plug load controls at a workstation reduced energy use by 21% relative to the baseline. Timer-based methods brought decreases of 26%, 46%, and 50% for workstations, break rooms, and print rooms, respectively.

Technology use is increasing, and employees in commercial settings are plugging in their personal devices in addition to office equipment. The DOE projects that by 2030 commercial energy consumption will increase by 24% and PPL energy consumption will increase by 49%. These numbers indicate controlling plug loads will be an important strategy to reduce energy consumption and costs, regardless of code requirements.

ASHRAE 90.1 and plug load control

Building and energy codes are a critical piece of the equation. That’s why it’s important for specifying engineers to understand ASHRAE 90.1 and codes like it that establish standards for plug load control.

Plug load control was added to ASHRAE 90.1 in 2010, and its requirements further expanded in the 2013 edition. Section 8.4.2 mandates that at least 50% of all 125 V, 15- and 20-amp receptacles run on an automatic control device that turns power on and off based on a schedule, occupancy sensor, or signal from another control system. It applies to private offices, conference rooms, rooms used primarily for printing and/or copying functions, break rooms, classrooms, and individual workstations. It also stipulates that at least 25% of branch-circuit feeders installed for modular furniture must be controlled.

ASHRAE 90.1, Section 8.4.2, considers three types of automatic receptacle controls:

• Schedule-based—Also called timer-based, this type of control turns off receptacles at specific times. The code requires that an independent program schedule be provided for controlled areas of no more than 5,000 sq ft and not more than one floor, and the occupant must be able to manually override the device for up to 2 hours.
• Occupancy-based—A sensor must turn off receptacles within 20 minutes of all occupants leaving a space. 
• System-based—Another control or alarm system sends an automated signal that shall turn off receptacles within 20 minutes after determining all occupants have left the space.

There are three other things that are important to know about ASHRAE 90.1. First, plug-in devices cannot be used in place of hardwired controls, as they could be easily defeated by end users.

Second, all controlled receptacles must have a permanent marking to differentiate them from uncontrolled receptacles. In fact, Article 406.3 of the 2017 edition of NFPA 70: National Electrical Code goes so far as to say that the universally recognized power symbol and the word “controlled” must be permanently marked on the face of a controlled receptacle.

Third, exempt from the code are receptacles designed for equipment requiring continuous operation and spaces where automatic control would risk the safety or security of the room or building occupants.

While the focus of this article is on ASHRAE 90.1, codes vary by state and municipality. You can find local building codes at energycodes.gov.

The California Energy Code, Title 24, is considered the most stringent energy code in the U.S. Section 130.5 of Title 24 mandates that at least one controlled receptacle must be within 6 ft of an uncontrolled receptacle in private offices, open offices, kitchenette areas, reception lobbies, conference rooms, copy rooms, and hotel/motel guest rooms. The requirement can be achieved using split-wired receptacles.

Three options for plug load control

As discussed above, ASHRAE 90.1 includes three types of automatic control devices: occupancy-based control, time-based control, and system-based control. Let’s break down the pros and cons of each.

Occupancy-based control

This concept should be familiar because of the popularity of occupancy sensors for lighting. When no one is in a space with an occupancy-based plug load control, the equipment connected to the receptacle will turn off. That’s the biggest advantage of this type of device—it will reduce energy consumption when people are not around, as opposed to a timer, which is on for a set period of time no matter what.

Another pro is that, in most cases, scaling plug load control on top of lighting controls is entirely possible. This can be done with either digital or analog controls. For simple retrofits where ceiling sensors already exist for lighting control, one can simply wire a device to the sensor, which in turn transmits a wireless signal to a receptacle in the space. This receptacle is then capable of switching a 15-or 20-amp load on or off based on occupancy.

One can take the same approach using digital controls and a building automation system through a network and a power pack that connects in-line with the receptacles that are to be controlled. In this case, a category cable connects to the building control system, and facility managers can then control the power pack, which in turn controls the receptacles. Note that all lighting control systems cannot support the addition of plug load control. You will need to check whether it was designed with plug load control modules.

The above is an example of a building-level control system. There are also local control options, meaning control occurs at an individual receptacle. To give an example of how this might work at a workstation, an occupancy sensor can be mounted under a desk and connected to an outlet. Some products use passive infrared (PIR) technology to detect occupancy and can have coverage of approximately 120 degrees and a few hundred square feet. After the space is vacant for a programmed amount of time, the device turns off all controlled equipment.

There is not a one-size-fits-all solution for plug load control. In some cases, occupancy-based plug load control requires the installation of occupancy sensors and receptacles, which takes resources.

Time-based control

Time-based control often can be seen as the simplest way to comply with plug load control requirements. In this case, to meet a requirement for 50% control (as required by ASHRAE 90.1), one can make half of the receptacles time-based. No occupancy sensors or buildingwide system is needed.

Timers can serve as another example of local plug load controls, with the programming occurring at the receptacle. There are products that allow for individual control of a single load or single circuit. For instance, a workstation used during the day can be programmed differently than one used by a night-shift employee. Often there is a panel that opens and has buttons for programming. Some products include an LCD display that shows the program status, the current time, and the day. More sophisticated methods, like relay panels, include the possibility to control larger spaces or zones but operate on the same premise.

There are different ways to set the time on the control devices. They could be programmed to shut off equipment from 6 p.m. to 6 a.m., for instance, or after a set amount of time, such as 8 hours. Some use an astronomical clock, enabling them to be controlled based on sunrise and sunset and automatically updating for daylight saving time.

The biggest disadvantage of time-based plug load control is that if a space is unoccupied, equipment is still consuming energy during the time the receptacle is scheduled to be on. While ASHRAE 90.1 mandates that occupants be able to manually override the control device, in practice, employees are not often diligent about this.

System-based

System-based plug load controls are part of a broader buildingwide system. Often, this is a system controlling all of the energy usage in a building—HVAC, lighting, plug load. It’s common for lighting and plug load to use the same occupancy sensors and come bundled, but HVAC may need to be integrated separately.

These systems are usually controlled by web-enabled software with user-friendly interfaces. A big advantage is the data gained from a buildingwide system, which gives a detailed analysis of energy consumption. This data empowers owners and facility managers to implement energy-saving plans to reduce overall operating costs and improve the sustainability of the building. The resources and commissioning needed to set up a building management system are viewed by some as a drawback to this method.

Determining the type of plug load control method

It’s difficult to give generalized advice on which plug load control method is best for certain applications. The first thing to do is to see what is required by code. In California per Title 24, for example, system-based plug load control is more popular, because two-way communication is required in new commercial buildings greater than 10,000 sq ft. Cost, of course, is another consideration. So, pay attention to how a space is used. Perform an energy audit on different circuits throughout the building at different times of the day to understand when and where peak energy use occurs. Instead of viewing the cost as a mere expense, try to understand this in terms of payback, reduced carbon emissions, LEED points, or other meaningful statistics.

Also, plug load control does not need to be an either/or decision. Methods can be used in combination. A facility may determine that time-based control is the best option for the private offices of a building, but in one or two rooms they could choose stand-alone, occupancy-based plug load control.

Another thing to keep in mind is that not all receptacles in commercial buildings should have plug load control. There are some pieces of equipment that should not turn off, such as security systems and refrigerators. In some cases, end users are very concerned about CPUs turning off sporadically but can afford for their peripherals to be switched off. So at a workstation, a task light and monitors are good candidates for controlled receptacles, but a computer may need to be plugged into the uncontrolled receptacle.

Case study

What does plug load control look like in practice? A company in Silicon Valley was asking employees to turn off lighting and equipment when they were not at their workstations. The company had decent participation, but equipment was still being left on after hours.

The company chose to install 3,400 multi-outlet assemblies (one in each cubicle) that featured a personal occupancy sensor. The convenience of the motion-sensing technology factored into their decision. In the first month, with only 60% to 70% of the products in use, electricity costs at the office decreased $15,000. That was a 20% reduction from the year before, when conservation efforts started.

That’s just one example of the potential of plug load control. With a solid baseline understanding of plug load control and ASHRAE 90.1, there’s no reason you cannot achieve similar results on your next project.

James Forte works as a product manager for the Wiremold product line at Legrand. He is a member of several UL standards technical panels that develop safety standards for the electrical industry.

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