Plug load controls

I thought we were specifying lighting controls, so why are we also talking about plug load controls?

By Brian Fiander, PE, LEED AP BD+C, MIES, Harley Ellis Devereaux September 24, 2015

Requirements to automatically switch plug loads are an attempt to reduce energy associated with phantom loads. Phantom loads are devices that are plugged in and draw energy all the time, but that are not needed all the time. These devices would include phone chargers, radios, and task lights, to name a few. Requirements for automatically switching task lights can be accomplished by either integrating the automatic controls into the task lights (i.e., integral motion sensor) or by automatically switching the receptacle directly (via motion sensor or time-of-day control). Because a desk-mounted motion sensor to switch a plug strip is acceptable under some codes but not others, it is important to read codes thoroughly before using this as an option.

Lighting designers used to only have to deal with luminaires and controls. As California Title 24 and ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings started instituting mandatory plug load controls, it became apparent that the sorts of controls required were already in place for lighting systems. While the plug load control system and the lighting control system do not have to be the same, there can be benefits to having one control system versus two.

Plug load controls are not necessarily part of the lighting control budget. But when these controls are specified as part of the lighting control system, the plug load controls often show up under the lighting control system estimate. It is important that those bidding on a project see both the lighting control and the plug load control drawings to ensure that the complete system is budgeted, submitted, and installed.

Cost analysis: sample private office

To get a better sense of the impact of these different lighting control strategies, it is important to look at how the choices made for control strategies affect the initial cost and the lifetime cost. Let us examine a typical 10×10-ft private office with a 10-ft ceiling height and a work surface height of 2.5 ft. This space will use two architectural troffer luminaires to light the space to an average of 30 footcandles at the work surface.

For this example, we shall assume that we must meet the standards for California’s Title 24, Part 6 (version 2013), which would require multilevel lighting, motion sensors or time-of-day control, daylight harvesting, demand load response, and plug load control. The multilevel lighting requirement would necessitate that we use fluorescent luminaires with four lighting steps (approximately 20%, 50%, 80%, and 100%) or LED luminaires with continuous dimming down to 10% illumination. Looking at Table 5, we can see that the sample dimmable LED luminaires are, on average, lower in cost than the sample dimmable fluorescent luminaires. Furthermore, we can see that the LED products are more energy-efficient than the fluorescent products.

Let’s assume an average of 10 hr of operation per day, 5 days/wk for 52 wk, for two luminaires per office. At an estimated utility rate of $0.10/kWh, the energy savings from using LED versus fluorescent can be seen as ranging from $2 to $8 for one office annually.

While the actual energy savings potential from the use of motion sensors may vary, various research studies have shown an energy saving reduction of up to 25% for private offices (Illuminating Engineering Society Handbook 10th Edition, Section Taking into account this reduction factor, a potential energy savings for a private office could save an additional $6 to $7.50 annually.

Potential savings from daylight harvesting rely on so many factors that it is necessary to use a daylight calculation software package to appropriate estimate savings. The building orientation, surrounding structures, distance from the windows, building location, and local weather conditions all play into the amount of energy that could be saved by using a daylight harvesting system. From a review of several of my recent projects, let us assume a potential energy savings of 20% annually for a project in southeastern Michigan. This value is based on a review of specific projects and should not be used to indicate any trends in savings from the use of daylight harvesting.

The requirements for demand load response for both California Title 24 and for ASHRAE 189.1-2014 indicate that the system needs to be able to reduce the overall load by 10%. The demand-response system is initiated by the utility to shed load across many building. While savings can be achieved through the demand-response system, this savings cannot be accounted for as a large-scale savings, as the amount of time that the system is operating in demand-response mode is unknown and should, in theory, only operate for a short time.

Brian Fiander is an electrical engineer at Harley Ellis Devereaux, specializing in lighting design and specification. He serves as one of the firm’s sustainable design champions, acting as a resource for other staff members regarding green design, LEED certification, lighting, and electrical systems. He has designed lighting systems for various projects across the country.