A voltage sag (often called a dip) is a short-duration reduction in RMS voltage, typically dropping between 10% and 90% of nominal for a duration of half a cycle to one minute. When you are troubleshooting flickering lights, unexplained PLC resets, or variable frequency drive (VFD) trips, voltage sags are almost always the culprit. Unlike a total blackout, a sag leaves the circuit energized but starves the load of the electrical pressure it needs to operate correctly, often causing digital logic to brown out or magnetic contactors to chatter and drop out.
The Anatomy of Power Quality Disturbances
To diagnose a power issue, you first need to classify it. The IEEE 1159-2019 standard provides the definitive framework for power quality monitoring. It is critical to distinguish a sag from other disturbances because the mitigation strategies are entirely different. A surge protector will not fix a sag, and a standard UPS might not have the runtime to handle a sustained undervoltage condition.
| Disturbance Type | IEEE 1159 Definition | Typical Magnitude | Typical Duration | Common Root Cause |
|---|---|---|---|---|
| Voltage Sag (Dip) | Decrease in RMS voltage | 0.1 to 0.9 pu (10%-90% drop) | 0.5 cycles to 1 minute | Motor starting, utility faults |
| Voltage Swell | Increase in RMS voltage | 1.1 to 1.8 pu (10%-80% rise) | 0.5 cycles to 1 minute | Large load switching off, capacitor banks |
| Interruption | Loss of voltage | < 0.1 pu (>90% drop) | 0.5 cycles to 1 minute | Blown fuse, tripped breaker, grid fault |
| Transient (Spike) | Sub-cycle high-frequency impulse | Up to 4.0 pu (400%+ spike) | Microseconds to 2ms | Lightning, inductive load switching |
| Undervoltage (Brownout) | Sustained low voltage | 0.8 to 0.9 pu | > 1 minute (often hours) | Utility grid overload, tap changer failure |
Calculating the Drop: A Worked Numeric Example
Voltage sags are governed by Ohm's Law and the impedance of your source. Think of source impedance like a kink in a garden hose: when you suddenly open the nozzle (inrush current), the pressure (voltage) at the nozzle drops because the kink restricts flow. Let us calculate a real-world sag caused by a compressor motor starting on a 120V branch circuit.
• Circuit: 120V nominal, 10 AWG THHN copper in conduit.
• Distance: 100 feet one-way from the subpanel.
• Load: 3 HP single-phase compressor with a Locked Rotor Amps (LRA) of 75A.
• Source Impedance (Utility transformer + service drop): Assume 0.15 Ω.
Step 1: Calculate Wire Resistance
According to NEC Chapter 9, Table 8, 10 AWG copper has a resistance of roughly 1.21 Ω per 1,000 feet at 75°C. Because current must travel to the load and back, our loop length is 200 feet.
R_wire = (200 / 1000) × 1.21 Ω = 0.242 Ω
Step 2: Determine Total Source Impedance (Z_total)
Add the wire resistance to the utility source impedance.
Z_total = 0.242 Ω (wire) + 0.15 Ω (utility) = 0.392 Ω
Step 3: Calculate the Voltage Sag
When the compressor starts, it draws 75A. The voltage dropped across the impedance is:
V_drop = I_start × Z_total = 75A × 0.392 Ω = 29.4V
Step 4: Determine the Sag Magnitude
The voltage at the compressor terminals during startup will be:
V_terminal = 120V - 29.4V = 90.6V
This represents a 24.5% voltage sag (the voltage drops to 75.5% of nominal). If this compressor shares the circuit with a sensitive CNC controller, that 29.4V drop will almost certainly cause the controller's internal power supply to reset, halting your machine mid-cut.
Where You Meet Voltage Sags in Practice
On the jobsite or in the workshop, you rarely see a sag on an oscilloscope unless you are actively hunting it. Instead, you see the symptoms. Here is where voltage sags manifest in real installations:
- Lighting Flicker: Incandescent and halogen lights will visibly dim when a large load kicks on. LED drivers with poor internal capacitance will also flicker or strobe during a sag.
- Contactor Chatter: HVAC systems and industrial motor starters rely on magnetic contactors. If the voltage sags below the contactor's holding threshold (often around 80% to 85% of nominal), the magnetic field collapses, the contacts open, and the motor shuts off. As voltage recovers, it pulls back in, creating a destructive rapid cycling known as chatter.
- IT and Logic Resets: Power quality experts at Fluke note that PC power supplies and PLC 24V DC logic supplies will drop out if a sag dips below 70% nominal for more than 20 milliseconds (just over one cycle on a 60Hz grid).
- Utility Capacitor Switching: When the utility switches a capacitor bank online to correct grid power factor, it can cause a brief, high-magnitude sag followed by a high-frequency transient. This often resets digital clocks and sensitive audio/video equipment across an entire neighborhood.
Sags vs. Brownouts: Clearing Up the Confusion
The most common mistake DIYers and junior technicians make is confusing a voltage sag with a brownout. While both result in low voltage, their timescales and mitigation strategies are entirely different.
What exactly is the difference between a sag and a brownout?
A voltage sag is a transient event lasting from a few milliseconds up to one minute, usually caused by a local high-inrush load (like a motor starting) or a distant utility fault clearing. A brownout (technically classified by IEEE 1159 as a sustained undervoltage) is a utility-wide reduction in nominal voltage that lasts for minutes, hours, or even days, usually implemented intentionally by the grid operator to prevent a total blackout during peak summer demand.
Why does the distinction matter for my equipment?
Because the protective gear is different. To survive a sag, you need a double-conversion online UPS or a constant voltage transformer (CVT) that can bridge a 200-millisecond gap using stored energy in capacitors or batteries. To survive a brownout, you need a buck-boost transformer or an automatic voltage regulator (AVR) that can continuously step up a 104V line back to 120V for hours without overheating.
How do I measure and confirm a sag is happening?
A standard digital multimeter (DMM) is useless for catching a 3-cycle sag; it samples too slowly and averages the reading. You must use a power quality logger (like a Fluke 435 or Hioki PQ3100) set to capture RMS voltage at a 1-cycle interval. Connect it at the point of common coupling (PCC) or directly at the failing load's terminals, set the sag threshold to 90% of nominal (108V for a 120V system), and let it log for a week.
Understanding the exact magnitude and duration of a voltage sag allows you to stop guessing and start applying the right fix—whether that means upsizing your feeder wire to reduce impedance, installing a soft-start VFD to limit inrush current, or adding an online UPS to protect your most sensitive logic boards.
