Your questions answered: Lighting: Designing lighting systems with LEDs

The Aug. 17, 2017, “Lighting: Designing lighting systems with LEDs” webcast presenters addressed questions not covered during the live event.


The Aug. 17, 2017, “Lighting: Designing lighting systems with LEDs” webcast presenters addressed questions not covered during the live event.Building owners are becoming more aware of and interested in using light-emitting diode (LED) systems for their lighting systems because of their energy efficiency and long lifespan. Good lighting enhances building design, conserves energy, and increases productivity, safety, security, and personal comfort. According to several government sources, up to 40% of the total energy used in commercial buildings is used for artificial lighting.

Engineers must understand the basics of LED lighting systems and which building types are best suited for them. When designing with LEDs, it is important that engineers understand critical factors, such as rated lifespan, lifecycle costs, efficacy, lumen output, compatibility with dimming controls, color rendering index (CRI), color temperature, and how LED lighting systems affect the electrical systems of the buildings in which they are installed. A basic understanding of the Illuminating Engineering Society and U.S. Department of Energy COMcheck are also necessary.

The Aug. 17, 2017, “Lighting: Designing lighting systems with LEDs” webcast presenters addressed questions not covered during the live event. The expert presenters are:

  • Wanda J. Barchard, Senior Electrical Designer, Metro CD Engineering, Columbus, Ohio
  • Sara Schonour, LC, Assoc. IALD, LEED AP BD+C, CDT, Vice President, CannonDesign, Boston

Question: What’s important to know about LED drivers and dimmer controls? What do you do when multiple LEDS are connected to a dimmer and not all the LEDs go dim when the dimmer is set to low?

Wanda J. Barchard: Make sure they are LED drivers and dimmer controls are compatible. Assuming that you are talking about fixtures with individual drivers, verify that all drivers are the same type, with same settings for lowest dimming available. If they do not all match, you will have to accept the second lowest dimming setting.

Q: What are some key items to consider when specifying LED lighting components for K-12 education projects? What are some of the key things to check for when commissioning these systems for performance?

Sara Schonour: A few ideas come to mind:

  • Correlated color temperature (CCT) tuning/circadian entrainment: We’re seeing more research on light and health, particularly in the realm of color temperature and circadian entrainment. At the moment, it appears that there is a correlation between cooler color temperature and alertness, and warmer color temperatures and relaxation, which have impacts in learning environments. More research is needed to draw definitive conclusions, but there appears to be a preference for color temperature differentiation in these environments.
  • Zoning and flexibility: in addition to designing to an average horizontal footcandle level, consider the importance of vertical surfaces in learning environments, and consider the benefits of providing separate zones for the instructor to bring attention to teaching surfaces (wall washers etc.). As most LED products these days have inherent dimming flexibility, adding in this layer of control typically benefits the users without adding cost.
  • Daylight harvesting: often schools have ample fenestration providing good opportunities for daylighting, even when not mandated by code. Consider these opportunities to reduce energy in classrooms.
  • Commissioning: ensuring all daylight and automatic sensors are properly calibrated is critical, as is training the users and those charged with maintaining the lighting system, so engaging the manufacturer, the users, and the facilities groups are important activities that should occur before turnover.

Q: Many of these codes appear to be designed with commercial applications in mind. How about industrial?

Barchard: The International Energy Conservation Code (IECC) is applicable to Commercial and Industrial Buildings.

Q: What lighting power densities per sq ft. are possible with LEDs for different occupancy types?

Barchard: It depends on your client, budget, and creativity. The technology continues to evolve.

Q: Please address the harmonics that are created. How prevalent is third harmonic current on large installations and is this a problem?

Barchard: Total harmonic distortion (THD) is the measurement of the distortion created from the equipment’s current draw. True resistive loads, such as an incandescent light bulb, do not have THD. Equipment containing coils and capacitors, such as motors, drives, fluorescent lighting, and HID lighting, have some measure of THD. However, solid-state electronic devices have been shown to be the largest contributor to distortion due to the switching of diode bridges producing a discontinuous current, which then causes a distorted sine wave. In four-wire wye systems such as 120/208 V and 277/480 V systems, harmonics may cause a problem with overheating of the neutral wire. The phase wires also should be designed for the increased harmonic current, but because the triplens are additive, the problem is especially critical on the neutral. The third harmonic and other triplens (9th, 15th, etc.) are additive. THD is the percentage of all of these additive values in relation to the total load. The sum of triplen harmonics greater than 33% will result in neutral current greater than individual line currents. The resultant current exceeds the neutral conductor’s rating and causes overheating of the neutral and/or transformer.

There are several articles in the Consulting-Specifying Engineer archives on dealing with THD.

Q: What are some of the pros and cons of LED fixtures having higher lumen packages in the same or smaller apertures as traditional sources?

Schonour: Pros and cons are as follows:


  • Fixtures take up the same or less real estate, both in the reflected ceiling plan and in the ceiling plenum, allowing for more flexibility in the design.
  • In some cases, spacing can be increased to provide similar target levels using fewer fixtures (less cost, less energy).
  • Retrofit solutions that fit into the same footprint can save on costs, material, and energy.
  • Owners can provide higher light levels if desired, or “task tune” LED fixtures to elongate life (run at less than full output day one with the ability to ramp up over time and keep target light levels constant as LED output decreases).


  • Potential for glare: there’s a balancing point between higher output and visual comfort, after which, fixtures become too bright to be comfortable.
  • Potential to over-light the space if additional output is not needed and spacing remains the same.
  • Spacings can potentially change to create optimized solutions, upsetting age-old rules of thumb for spacing/cost estimating, etc.
  • Creating new fixtures in old form factors often does not optimize the optical properties of LEDs, and often create LED versions of fluorescent fixtures which offer little energy benefits as they aren’t designed around the properties of the LED modules inside them.

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