Commercial lighting load centers can be quite a challenge in modern electrical installations. A simple definition of a commercial lighting load center would be, a panel board used to distribute 480 or 208 volt three phase power to individual 277 or 120 volt single phase lighting circuits. A typical panel will be populated with switch rated over current devices while more sophisticated systems will include integrated controls to utilize devices such as photocells, timers, motion sensors, and smart switches.
One of the most common issues with lighting load centers is the use of newer energy efficient fixtures. While new lighting technologies have reduced energy use in the form of overall wattage, this same technology introduced a problem termed “Harmonics” largely due to electronic ballasts and switching power supplies used in the fixtures. Let me explain.
When analyzing AC power, we look at two component waveforms, voltage and current. We analyze how they relate to each other in each separate phase (A, B, and C). In a perfect world, these two components would be in sync with each other so when phase “A” voltage reaches its peak so would phase “A” current. Using this theory, we can use a single return (neutral) conductor rated for the primary conductor amperage for all three phases since only one phase is peaking at a time (each phase of a 3 phase system is 33.3333 degrees shifted from each other) the return current will always be within safe limits of the conductor. We all know the world isn’t perfect and neither is the relationship of these two electrical components. The measure of phase difference between voltage and current of the same phase is termed “Power Factor”. Circuits that are primarily inductive loads will have a current wave form that lags behind the voltage wave form and the opposite is true of circuits that have capacitive loads and power factor is the percent difference measured between voltage and current wave forms. Power factor has been an issue with electrical systems for many years and is a topic for a separate discussion but in older properly designed lighting systems, power factor was generally the same for each of the three phases therefore the total sum of the return (neutral) load was still safe for the conductor. It was not until we started seeing electronic controlled lighting enter the picture when we introduced a new issue termed “Harmonics” to lighting circuits. Harmonics in this instance are a byproduct caused by high speed switching characteristics of electronic circuits found in devices such as ballasts and switching power supplies used to power high efficiency lighting. The explanation of the “Why’s and How’s” this is possible can be staggering but In a nutshell, a harmonic effect is the generation of current wave forms at several harmonic frequencies to the base frequency of our electrical system of 60hz. What this means is that we now see current flow for a phase at several frequencies rather than just 60hz and the end result is that now we can get a harmonic current wave form from phase “A” adding to the primary current waveform of phase “B” and also means is that the practice of sizing the neutral conductor for the same current rating as the primary conductors is no longer valid as we can easily see current on the return conductor 1.5 to 2.0 times the primary current often leading to damaged neutral conductors in existing installs. Unfortunately high efficiency lighting only relates to wattage vs lumen output not power transfer between circuit power to lamp power through the ballasts and switching power supplies that are integrated into the fixtures although these values are improving as well.
Most new lighting load center installs are designed with high efficacy lighting in mind so problems are few. Problems end up coming from the desire to save energy retrofitting older installs. We remove our old wasteful energy hog fixtures and install super deluxe high efficiency fixtures that are rated at two thirds the current. No Brainer, right? Sometimes this works out fine, sometimes not so much. Although the lumens per watt have increased and each new fixture consumes fewer amps, we don’t think too much about the wire or the load centers for that matter, supplying power to the fixture and there could be a problem lurking in the background waiting to strike months or possibly years later. Harmonic power problems can be heating up the neutral conductors of your system and if not corrected, can fail causing a lighting outage or in the worst cases, a fire. A typical indicator of this problem is all the light fixtures in a circuit go out and the breaker is on. It is also possible to test voltage from line to ground at a problem fixture and read line voltage 277 or 120 AC volts depending on the source. This is a clear indicator of an open neutral in the circuit and if once found, the conductor shows signs of overheating, it is reasonable suspicion of harmonic problems. Further testing with a power analyzer will be needed to figure out the cause, severity, and correction. Be warned, this can be a time consuming and expensive endeavor.
There are several things that can be done to ensure a retrofit project results in success prior to install:
- Check load balance between phases of existing lighting load centers.
- Differentiate between continuous use and periodic use (such as night time exterior) lighting. This will also help in balancing loads.
- Check for overloaded circuits (circuits near or slightly above full amp rating and yes, a 20 amp breaker can pass 21 amps for extended periods of time under the right conditions). This generally occurs when adding fixtures to existing circuits over time.
- Check all connections in existing load centers to ensure no damage exists prior to install. Thermal imagery works well for this.
- Work with lighting vendors to assure the selected fixtures will work in the desired application. Ask about noise ratings such as RFI, EMI, and Harmonics. This is important information for deciding the suitability of a fixture for the environment it will be used in.
If new fixtures have been installed and harmonics cause a problem, solutions can be as simple as reducing the numbers of fixtures per circuit to installing harmonic filter systems. This of course depends largely on the type and severity of the problem.