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.

By Wanda J. Barchard and Sara Schonour August 23, 2017

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.

Q: What may be the largest wattage (such as 100 W, 500 W, etc.) for a single LED lamp?

Barchard: Please keep in mind that what is of importance in lamp selection is the lumen output and directionality, not the wattage. The technology continues to evolve. To my personal knowledge, there is a 200 W replacement lamp for a 1,000 W HID for open highbays, 25,000 lumen output. There may be something larger.

Q: Is the inrush for LED fixtures for a significant amount of time to where it would affect the size of wire or breaker? What is the typical inrush current, in percent, for current LED technology? Is there a typical duration for this inrush? There is no typical. Each driver is unique. Ask the manufacturer for the information for the driver.

Barchard: The line impedance has a significant effect on the peak and duration of the inrush current. Transformer type and size, wire size, length of runs, and other devices in the circuit path are a few things that could lead to high impedance.

The inrush current and the duration of the LED driver differs from manufacturer to manufacturer and from model to model. As this current duration is very small, sophisticated instruments with very high sampling rates need to be used for exact calculations. The inrush current also depends on the exact moment of switching during the alternating cycle.

For a typical 150 W, 0.7-amp LED driver by one manufacturer used in various luminaires, the inrush current is stated as 130 amps for 165 microseconds. It is suggested that between seven and 10 of these drivers be used per 20-amp C curve MCB, however it will depend on the individual circuit impedance.

For a typical 220 W, 1.05-amp LED driver by another manufacturer in high output luminaires, the inrush current is stated as 80 amps for 1,000 microseconds. It is suggested that between six and eight of these drivers be used per 20-amp C curve MCB, however, it will depend on the individual circuit impedance.

A general recommendation is to use C curve circuit breakers with the highest current rating that is allowed by the circuit cable size.

Q: Have LED lamps made inroads to show-business like theatrical lighting like spot or flood lighting?

Schonour: Yes, several manufacturers with strongholds in the theatrical world now have LED offerings for traditional theatrical products, and the same is true for large-scale flood lighting applications, such as sports arenas and industrial projects.

Q: On a standard 120 Vac power supply, what is the actual voltage the LED is seeing?

Barchard: It depends on the driver. Check the data sheet to determine the output voltage range of the driver.

Q: What options are available for low THD drivers?

Barchard: We cannot recommend specific manufacturers. Ask your local IES section if it has any programs planned on this topic; it is a great source of knowledge.

10. Because LEDs don’t like hot environments, will they work in the hot aisles of a data center?

Schonour: They can, if their thermal management is designed correctly. The key is ensuring the heat generated at the junction point is minimized, which requires a well-designed heat sink. Quality products will list the operating temperature range that the product is rated for. If the design temperature of the application is outside this range, discuss options with the manufacturer or with your local rep.

Q: Please clarify the difference between constant current driver and constant voltage driver?

Barchard: Constant current drivers provide a steady current to the LEDs despite changes in voltage. It accomplishes this by varying the voltage across the electronic circuit. Constant current drivers are useful because they prevent burnout and thermal runaway because they ensure that the current never surpasses the specified maximum current. Here are a few factors to consider when comparing constant current LED drivers versus constant voltage drivers. If your LED or array isn’t specified to take a certain voltage, you will need to use a constant current driver. A constant current driver provides a more consistent brightness and overall more reliable performance.

Constant voltage drivers have a fixed voltage, usually either 12 Vdc or 24 Vdc. You should use only constant voltage drivers with an LED or array that has a designated specific voltage. If you’re using LED strings that are connected in parallel, it may be difficult to match the current to all units properly. A constant voltage driver forces an equal current across all LED units. If your lighting system does specify the voltage, you may be able to save money by using a constant voltage driver, especially in large-scale LED systems. Constant voltage technology also is more familiar and streamlined, but constant current drivers provide more control and flexibility.

Q: What is the inrush current for LEDs as compared to incandescent, which I think is about 17 times operating current?

Barchard: The inrush current for an incandescent is zero, the inrush current for LEDs is specific to the driver and differs from manufacturer to manufacturer (there is no standard).

Q: What is the maximum distance between a remote LED driver and the LED lamp?

Schonour: This distance will vary from product to product and also can change based on the wiring gauge used in the application, but this information is usually listed on a cut sheet for a product using a remote driver. If not, the manufacturer can provide this information.

Q: Can you use constant voltage (CV) or constant current (CC) drivers to compensate for voltage drop (say for site lighting)?

Barchard: LEDs usually provide more lumens for less watts and would have an overall lower load on the circuit for your voltage drop calculations.

Q: When can low voltage wiring be used with LED fixtures?

Barchard: When the input voltage of the driver is low voltage (line side), constant voltage drivers are usually low voltage output, thus wiring to the LEDs would be low voltage (load side).

Q: What testing has been done on the effect of flicker rate of LEDs on the occupants of a space? What were the results?

Schonour: This is an important emerging topic in our field, but we have yet to see a definitive metric emerge to evaluate and qualify flicker. The Pacific Northwest National Laboratory (PNNL) and the Lighting Research Center are doing a lot of research in this area, and are good resources to understand the effects of and the conversation about flicker.

Q: What are the typical power factor values for LED lighting?

Barchard: There is no “typical” yet. You should check the specification sheet (or data sheet) for this information on each product.

Q: What is the significance of daylight harvesting in energy savings even with low-power LEDs?

Barchard: The significance is integrating natural light in the space. Remember that LED lighting is readily available with 0-10 V dimming, which can be tied into daylighting. Also, remember that there is more than just dimming the lighting in response to daylight penetrating the space through the fenestrations. There also is the control of the amount of the light penetrating into the space depending on the different years of the standards. Read your applicable codes and standards.

Q: Lighting controls and LED drivers: How do we know what type of dimming to use?

Schonour: Quality LED products will list the dimming protocol that their drivers use to modify light output, and the lighting control needs to be specified to match. For fixtures, this information usually is found on the cut sheet or can be obtained by querying the manufacturer. For retrofit LED lamps (incandescent or CFL screw-in replacements), many readily available control device manufacturers have produced compatibility tables or online tools that list the products they have been tested with for dimming compatibility.

Q: What are some new lighting control technologies for outdoor lighting (dimming, wireless controls, etc.)?

Barchard: There are wireless applications for the individual fixtures to report their status (much like Dali when it first came out). There are setback dimming afterhours, integral occupancy sensors to return to full-on.

Q: What are the failure rates for LED lighting and what is the worst application you have found to date?

Barchard: TM-26 Methods for Projecting Catastrophic Failure Rate of LED Packages from the IES. The accepted industry standard is 3% at this time. The worst application I have had is due to substitution of the basis of design over which I had no control.

Q: Can you recommend LED fixtures for underwater use?

Schonour: Look for the IP or “ingress protection” rating of the fixture. IP ratings that end in a “7” (ex: IP67) mean the fixture is protected against the effects of temporary immersion (30 minutes) between 15 cm and 1 m. IP ratings that end in an “8” (ex: IP68) mean the fixture is protected against long periods of immersion under pressure.

Q: What are the life safety code regulations for using LED luminaires for emergency/exit applications?

Barchard: Be aware of the dimming capability of downlights in audience areas during performances, because fixtures are to be dimmed to a fractional level of illumination to always provide illumination of the path of egress. It is critical to specify a driver with a range of 100 to 0% in this application.

To my understanding, as long as the fixture meets ADA limitations in dimension if it is in the path of travel, actually illuminates the path of travel to the point designated by the authority having jurisdiction.

Q: Are LEDs more susceptible to lightning than other sources?

Barchard: Being electronics, they would be susceptible to transient voltage surges, or power surges thus it is smart practice to have surge protection on your electrical service to protect the electronics installed.

Q: There have been problems with radio frequency interference (RFI) from some LED drivers. What should one look for and/or consider to minimize these problems?

Barchard: Shorten the length of conductors, verify the conductors are shielded cables, EMI filter, or suppression at the input/output of the transformer (if low voltage).

Q: Can high quality LED lighting be obtained from most overseas manufacturers, or is caution still needed?

Schonour: Quality products, just like sub-par products, can come from anywhere. So, it’s important to check for quality metrics when evaluating products regardless of origin. Quality products will clearly list their material and performance characteristics via product information/cut sheets or Lighting Facts labels, and will carry UL or ETL listings at a minimum. Obtaining samples and performing side-by-side functionality and performance tests is the best way to ensure quality when evaluating lighting products.

Q: Is there driver noise with LEDs?

Barchard: Only if there is a fan to cool the driver because it is high output.

Q: Is it possible to combine LED-based wireless lighting control and Internet of Things (IoT)?

Barchard: Yes, it all depends on the protocol (or language) communicated.

Q: Wanda, you mention drivers having a shorter life. What is the typical life? And are facilities stocking backup drivers or hoping that those drivers will be available at time they fail? Or are facilities planning on doing whole fixture replacement when driver fails?

Barchard: Drivers are more likely to fail before the LEDs themselves, which is usually after the 50,000 hours (or more – check the data sheet) of minimum testing for the 70% lumen output of the LEDs. In a perfect world, you may get a certain percentage that keep on going for double that period. Depending on the facility, you may specify an additional 10% or more extra drivers be provided for each type used to the facility’s department for future use. If the facility has a good budget for capital improvement, it may plan for fixture replacements every 20 years.

Q: Are there life safety code regulations on using LEDs fixtures for emergency/exit lighting?

Barchard: There is no mention not to use LEDs for life safety. If you do not have a standby generator, or emergency generator, nor space for inverters and must use individual emergency driver units, be aware of the limited output of these emergency units. One may need to double up emergency lighting in some areas to meet the prescribed footcandles for various codes and standards.

Q: What, if any, is the de facto CCT standard for indoor and outdoor applications?

Schonour: There is no de facto CCT standard for indoor or outdoor applications. Typical architectural applications range from 2,700 to 4,100 K. There are some emerging trends, but CCT selection should be a function of understanding the material palate and the potential effects of light color on the occupants, as well as considering the proximity to daylight.

Q: What anticipated technology improvements are in the process for LEDs?

Barchard: Stay tuned, as this tunable light thing shows up everywhere. More high-power drivers with high-output LEDs. Controls. We will all be amazed.