What power quality means
Utility power is supposed to arrive as a steady 60 Hz sine wave at the right voltage. Power quality describes how closely the electricity inside your building matches that ideal. Most problems start inside the building, from the equipment you run, not from the utility.
| Problem | What you notice | Common causes |
|---|---|---|
| Voltage sags | Lights dip, drives or computers reset | Large motors starting, faults on the utility or in the building |
| Transients (spikes) | Failed electronics, damaged drives | Switching, lightning, capacitor banks |
| Harmonics | Hot neutrals and transformers, nuisance trips | LED drivers, computers, VFDs, UPS, EV chargers |
| Imbalance | Motors running hot, uneven panel loading | Single-phase loads piled onto one phase |
| Flicker | Visible light flicker | Welders, compressors, cycling loads |
Harmonics in plain language
Older loads like heaters and incandescent lamps draw current smoothly, in step with the voltage. Modern electronic loads do not. LED drivers, computers, variable frequency drives, UPS systems and EV chargers draw current in short, sharp pulses.
A pulsed current can be described as the normal 60 Hz current plus smaller currents at multiples of 60 Hz: the 3rd harmonic at 180 Hz, the 5th at 300 Hz, the 7th at 420 Hz and so on. Those extra currents do no useful work, but they still heat wires, transformers and motors.
Why harmonics cause trouble
In a 208Y/120 V system, the normal 60 Hz currents on the three phases largely cancel in the shared neutral when the load is balanced. The 3rd harmonic currents do the opposite: they add together. In an office full of computers and LED lighting, the neutral can carry more current than any phase conductor, and nothing on a standard breaker protects it from that.
Transformers feeding these loads see the same problem. Harmonic currents circulate in the transformer windings and add heat that a simple ammeter reading does not reveal, which is why a transformer can run hot while appearing lightly loaded. Other effects include:
- Transformers running hotter than their nameplate load suggests
- Breakers tripping when the load looks well within rating
- Power factor correction capacitors failing or causing resonance
- Interference with sensitive controls and instruments
How power quality is measured
Guessing is expensive, so start with data. A licensed electrician connects a power quality analyzer at the main service and at suspect panels, usually for a week or more so the log captures a full business cycle including start-ups and weekends.
The log shows voltage, current, harmonic distortion, imbalance, sags, swells and transients, each with a time stamp. Matching events to what was happening at that moment is how the cause is found. It helps to keep a simple log of your own during the study, noting when big equipment started, when trips happened and when anyone noticed flicker. IEEE 519 is the guideline commonly used for harmonic limits where the building connects to the utility.
Common fixes
| Problem | Typical solutions |
|---|---|
| Harmonics | Line reactors or filters on drives, harmonic mitigating or K-rated transformers, larger neutrals, active filters |
| Sags from motor starting | Soft starters or VFDs, staggered start times, ride-through settings |
| Transients | Coordinated surge protection at the service and at sensitive panels |
| Sensitive loads | UPS systems for servers, controls and point-of-sale |
| Imbalance | Rebalancing single-phase loads across phases |
| Low power factor | Correction equipment designed for the harmonic levels present |
See surge protection and UPS systems for two of the most common solutions.
Replacing a breaker that keeps tripping, or a drive that keeps failing, rarely solves a power quality problem. Measure first, then choose a fix that matches what the data shows.
When to call an electrician
Good design prevents many of these problems. When a new tenant space, data room or production line is planned, the electrical design can account for harmonic loads from the start with properly sized neutrals, suitable transformers and drive filters. That is far cheaper than fixing overheated equipment later.
Book an assessment if you notice warm neutral conductors or transformers, repeated nuisance trips, drives or electronics failing early, flicker that lines up with equipment starting, or after adding a lot of LED lighting, drives or EV chargers. A power quality study pairs well with an energy audit, since the same data often shows where energy is being wasted.
Common questions
Are LED lights bad for power quality?
Individually, no. A building full of LED drivers, computers and drives adds up to a lot of harmonic current. Good quality drivers and a well designed distribution system usually keep it manageable.
Is the utility responsible for poor power quality?
Sometimes, but most power quality problems start inside the building. A logged study shows whether events come from your own equipment or from the supply, which is what the utility will want to see before investigating.
How long does a power quality study take?
Logging usually runs for about a week or more to capture a full cycle of operation. Analysis and recommendations follow once the data is downloaded.
Will surge protection fix flickering lights?
No. Surge protection handles short spikes. Flicker and sags come from other causes, such as large loads starting, and need different fixes.
Codes and permit rules change and every building is different. For work on your system, talk to a licensed electrician and your local permit authority.