Voltage Sags and Swells
A ten percent deviation on your line, in either direction, is enough to reset equipment, rob motors of torque, and quietly age your assets. Here is how to find the source and match the fix to the problem.
Who this is for
- ■Plant managers and facility managers at manufacturers, hospitals, data centers, and large commercial buildings.
- ■Operations leaders whose electric bill runs five figures or more each month.
- ■Executives seeing unexplained equipment failures, process errors, or rising repair costs.
- ■Anyone who cannot tell whether the problem is their equipment or their power.
Is the disturbance on my line internal to my facility or coming from upstream, and which mitigation tool actually matches the problem?
A voltage sag, also called a dip, is a drop of 10% or more below your nominal voltage. Picture a 120 V outlet dropping to 90 V. A swell is the opposite: a jump of 10% or more above nominal. Both are defined by magnitude and by how long the RMS voltage stays outside the normal band.
When a momentary event becomes a sustained one
If a sag lasts longer than 1 minute, it is reclassified as undervoltage, and the causes shift to infrastructure: overloaded transformers, undersized conductors. If a swell lasts longer than 1 minute, it becomes overvoltage, and the causes shift to transformer taps set incorrectly, loads positioned near the beginning of a distribution system, or solar feeding back into the network.
Drop below nominal
Surge above nominal
Microseconds, thousands of volts
30%of this guide, read. The rest of it is below.
- 02 The mechanism Where sags and swells come from
The most common cause of a sag is a load turning on. Sags also come from impedance problems inside your building: loose wiring, undersized cable for the load, or temporary short circuits out on the utility line. Swells occur less frequently. They come from loads turning off, switches being activated, or line-to-ground faults on single-phase lines, which cause voltage to swell on the two non-faulted phases.
Event Common internal cause Common external cause Sag / dip Large motor starting; loose wiring; undersized conductor Temporary short circuit on utility line Undervoltage Overloaded transformer; undersized conductor Sustained supply issue upstream Swell Large load shutting off; switch activation Line-to-ground fault on another phase Overvoltage Loads near start of distribution; incorrect transformer taps Solar or other generation feeding back into the network Transient Capacitor bank switching; arcing; faulty contactor Lightning strike The ten-percent rule, in both directionsThe band outside your nominal voltage is where equipment starts getting hurt. The symptoms are not subtle. Lights flicker. Lighting levels drop, sometimes to unsafe levels. Equipment with electronic power supplies resets on its own. Motor-driven processes fail or produce errors because a voltage deviation robs motors of torque. Sensitive electronics need peak voltage inside a tight range to run properly, and when voltage moves outside that range, they shut down, restart, or run degraded.
Short termWhat today looks like
Assets reset unexpectedly. Production slows. Batches get scrapped. Workers file the same complaint about the same machine, again.Long termWhat next year looks like
Motors run below rated torque and burn extra kWh every hour. Out-of-warranty repairs consume capital. Assets fail earlier than they should. The symptoms are not subtle. Lights flicker. Lighting levels drop, sometimes to unsafe levels. Equipment with electronic power supplies resets on its own. Motor-driven processes fail or produce errors because a voltage deviation robs motors of torque. Sensitive electronics need peak voltage inside a tight range to run properly, and when voltage moves outside that range, they shut down, restart, or run degraded.
Short termWhat today looks like
Assets reset unexpectedly. Production slows. Batches get scrapped. Workers file the same complaint about the same machine, again.Long termWhat next year looks like
Motors run below rated torque and burn extra kWh every hour. Out-of-warranty repairs consume capital. Assets fail earlier than they should.Operators often reach for a UPS first because it feels like the most comprehensive answer. A UPS addresses interruptions and short sags. It is not the right tool for sustained overvoltage or high-energy transients, and it carries the highest capital cost of any mitigation option. Match the device to the problem. Not every circuit needs a UPS. Some just need a voltage regulator.
The myth The reality A UPS protects against everything. A UPS handles interruptions and short sags. It does not fix sustained overvoltage or absorb a high-energy transient. Mitigation is a spending ladder. Start cheap and climb until it works. Mitigation is a matching problem. The right tool depends on the disturbance type, not the price point. If the lights only flicker, it is not serious. The same event flickering your lights is resetting controllers and pulling torque out of motors elsewhere in the building. No symptoms means no problem. Chronic low-grade sags degrade motors and consume capital in warranty repairs long before anyone connects the dots. Match the tool to the disturbanceEvery facility starts with connections and grounding. What comes after depends on what the analyzer actually found. - 05 Your leverage The internal-versus-external test, and the sequence around it
The most valuable diagnostic in this whole topic is one test with a power analyzer. Watch what current does at the moment voltage drops. If current rises as voltage falls, a load inside your facility is pulling the line down. The cause is internal. If current stays flat or drops while voltage drops, the disturbance is coming from upstream. That single test separates your problem from the utility's problem, and it tells you where to direct your resources.
- 1 Interview every stakeholder in the affected areas, not just electricians. Map whether symptoms are isolated to one zone or spread across the building.
- 2 Inspect electrical panels for loose connections using a thermal imager, visual inspection, or by de-energizing and checking proper torque. Correct any you find.
- 3 Verify voltages at panels and subpanels with an appropriately rated multimeter.
- 4 If voltage looks normal and problems persist, connect a power analyzer with recording capability and let it run for at least 1 week. 1 week
- 5 Read the recording. Apply the internal-versus-external test at every sag event captured.
- 6 After mitigation is in place, ask operators to keep a log of continuing problems, and leave the analyzer connected to confirm stability.
What current does tells you who owns the problemOne reading of current during a sag decides whether you are fixing your panel or calling the utility. When to act on this now, and when to hold
✓ Act now- Equipment is resetting on its own and nobody has a definitive explanation.
- Motor-driven processes are failing or producing errors more than they used to.
- Workers are reporting flickering lights or lighting levels dropping.
- Out-of-warranty repairs are climbing and eating into capital budgets.
- You have never run a power quality study and cannot answer whether disturbances are internal or external.
✗ Hold or defer- You are already inside a full power quality remediation project with a current analyzer recording.
- A recent study returned clean and equipment behavior is stable.
- The affected loads are non-critical, tolerant, and cheap to reset by hand.
- You have no data yet and are being pushed toward a UPS purchase before any diagnostic work has been done.
- Decision matrix
When to act on this now, and when to hold
✓ Act now- Equipment is resetting on its own and nobody has a definitive explanation.
- Motor-driven processes are failing or producing errors more than they used to.
- Workers are reporting flickering lights or lighting levels dropping.
- Out-of-warranty repairs are climbing and eating into capital budgets.
- You have never run a power quality study and cannot answer whether disturbances are internal or external.
✗ Hold or defer- You are already inside a full power quality remediation project with a current analyzer recording.
- A recent study returned clean and equipment behavior is stable.
- The affected loads are non-critical, tolerant, and cheap to reset by hand.
- You have no data yet and are being pushed toward a UPS purchase before any diagnostic work has been done.
Questions for your morning huddle- Have we ever conducted a stakeholder interview to map where electrical symptoms are showing up across our facility, and whether they are isolated to one zone or spread throughout?
- When we log equipment resets or process failures, is anyone checking whether a voltage event preceded the problem, or are those being attributed to the equipment itself?
- Have we ever deployed a power analyzer for a week-long recording to see what is actually happening on our line at every hour of the day?
- For each mitigation option on our list, from loose-connection fixes through SPDs, regulators, and UPS, do we know which disturbance type each critical load is actually exposed to?
The one thing to rememberThe most valuable single tool in this topic is the internal-versus-external test. It decides whether you fix your own panel or engage the utility, and it costs you one power analyzer recording to run.
Get a power analyzer connected to your service and record for at least a week. When a sag hits, look at what current did. Rising current means the problem is inside. Flat or dropping current means the problem is upstream. Direct your budget accordingly.
Know if the numbers actually pencil out before you sign anything.
A written second opinion on the project in front of you, whether that is a rate change, new equipment, or a renewable installation.
- 01A short call, to figure out quickly whether we can actually be helpful. If we can't, we'll say so on the spot.
- 02We pull the data, your bills, your rate structure, vendor proposals, project specs.
- 03You get the verdict in writing: whether the payback will materialize, and the opportunities or risks nobody has raised.
Get a Blueprint at blueprint.tac-nrg.com Free for Indiana-based operations spending five figures or more a month on electricity. No obligation. You keep the write-up either way. The most valuable single tool in this topic is the internal-versus-external test. It decides whether you fix your own panel or engage the utility, and it costs you one power analyzer recording to run.
Get a power analyzer connected to your service and record for at least a week. When a sag hits, look at what current did. Rising current means the problem is inside. Flat or dropping current means the problem is upstream. Direct your budget accordingly.
The Energy Decision BlueprintKnow if the numbers actually pencil out before you sign anything.
A written second opinion on the project in front of you, whether that is a rate change, new equipment, or a renewable installation.
- 01A short call, to figure out quickly whether we can actually be helpful. If we can't, we'll say so on the spot.
- 02We pull the data, your bills, your rate structure, vendor proposals, project specs.
- 03You get the verdict in writing: whether the payback will materialize, and the opportunities or risks nobody has raised.
Get a Blueprint at blueprint.tac-nrg.com Free for Indiana-based operations spending five figures or more a month on electricity. No obligation. You keep the write-up either way.why does my equipment keep resetting for no reason
Electronic power supplies need peak voltage inside a tight range. A sag of 10% or more below nominal, or a swell of 10% or more above nominal, is enough to make equipment shut down, restart, or run degraded. Before you replace the machine, connect a power analyzer and see whether a voltage event preceded the reset. If it did, the problem is your power, not your equipment.
how do I tell if a voltage problem is from the utility or inside my facility
Use a power analyzer and watch current during a sag. If current rises as voltage drops, a load inside your building is pulling the line down and the cause is internal. If current stays flat or drops while voltage drops, the disturbance is coming from upstream on the utility side. That one reading separates your problem from the utility's problem and tells you where to spend.
how long should I record with a power analyzer
At least 1 week. You need to capture the full day-of-week pattern, including shift changes, weekend behavior, and any time-of-day when large loads start or stop. Anything shorter risks missing the event that is actually causing your problem.
do I need a UPS or a voltage regulator
It depends on the disturbance type. A UPS is built for interruptions and short sags on loads that cannot tolerate any drop, and it carries the highest capital cost of any option. A voltage regulator is the right answer for chronic sag and swell conditions where you need to hold voltage steady but the load can tolerate a brief transfer. Match the device to the problem you actually measured, not to the one you assumed.
how do I tell if a voltage problem is from the utility or inside my facility
Use a power analyzer and watch current during a sag. If current rises as voltage drops, a load inside your building is pulling the line down and the cause is internal. If current stays flat or drops while voltage drops, the disturbance is coming from upstream on the utility side. That one reading separates your problem from the utility's problem and tells you where to spend.
how long should I record with a power analyzer
At least 1 week. You need to capture the full day-of-week pattern, including shift changes, weekend behavior, and any time-of-day when large loads start or stop. Anything shorter risks missing the event that is actually causing your problem.
do I need a UPS or a voltage regulator
It depends on the disturbance type. A UPS is built for interruptions and short sags on loads that cannot tolerate any drop, and it carries the highest capital cost of any option. A voltage regulator is the right answer for chronic sag and swell conditions where you need to hold voltage steady but the load can tolerate a brief transfer. Match the device to the problem you actually measured, not to the one you assumed.
what causes a voltage swell in an industrial plant
The most common cause is a large load, like a big motor, turning off, which lets voltage on the line jump briefly. Swells can also come from switches being activated or from a line-to-ground fault on one phase, which pushes voltage up on the other two non-faulted phases. When a swell lasts longer than 1 minute, it is reclassified as overvoltage, and the causes shift to transformer taps set wrong or generation feeding back into the network.
should I do a power quality study if my facility is not showing symptoms
Yes, periodically. A study establishes a baseline and catches developing problems before they turn into failures. It can also identify corrective actions that improve equipment performance and reduce energy consumption. Chronic low-grade sags degrade motors and drive up utility costs quietly, so waiting for visible symptoms means paying for the damage first.
- Voltage sag (dip)
- A momentary drop in RMS voltage of ten percent or more below nominal. Most common cause is a load turning on.
- Voltage swell
- A momentary rise in RMS voltage of ten percent or more above nominal. Often caused by a large load shutting off or a line-to-ground fault on another phase.
- Undervoltage
- A sag that persists longer than one minute. Typically caused by overloaded transformers or undersized conductors.
- Overvoltage
- A swell that persists longer than one minute. Caused by incorrect transformer taps, loads near the start of a distribution system, or generation feeding back into the network.
- Transient
- A temporary unwanted voltage, from a few volts to several thousand, lasting microseconds to milliseconds. Sources include lightning, capacitor bank switching, and faulty contactors.
- Power analyzer
- An instrument that records voltage and current continuously and timestamps disturbances. The core diagnostic tool for sags and swells.
- SPD
- Surge protective device. Sized to divert transient energy away from sensitive loads. Not a substitute for a regulator or a UPS.
- Isolation transformer
- A transformer that magnetically decouples a load from the supply, blocking certain disturbances and providing a clean local ground reference.
- CAT III / CAT IV
- Safety ratings for measurement tools used on live industrial panels. CAT IV applies where the likelihood of transients is highest, such as service entrances.