Under voltage is a condition where the electrical supply voltage drops below the normal operating range for a sustained period. In a standard 120-volt residential system in the U.S., voltage is expected to stay within about 5% above and 10% below that nominal value. When it drops further, equipment starts to malfunction, overheat, or suffer permanent damage.
How Under Voltage Is Defined
The main U.S. standard for acceptable voltage levels, ANSI C84.1, defines two ranges. Range A covers the voltages your utility should deliver under normal conditions: for systems at 600 volts and below, that’s +5% to -5% at the service entrance (where power enters your building). At the actual outlet or equipment terminals, the acceptable window widens to +5% to -10%, accounting for losses in your building’s wiring.
Range B is the emergency zone. Service voltage can dip as low as -8.3% before corrective action is required. At the equipment terminals, the standard allows down to -13.3%, but voltages in this range should be rare and short-lived. Anything beyond Range B demands immediate attention from the utility or building owner. For a nominal 120-volt circuit, Range B’s lower limit at the outlet works out to roughly 104 volts. Drop below that, and you’re firmly in under voltage territory with real risk of equipment damage.
Under Voltage vs. Sags vs. Brownouts
These terms overlap, but they describe different things. A voltage sag is a brief dip lasting fractions of a second to a few seconds, often triggered by a large motor starting up or a fault clearing on the grid. A brownout is a sustained, partial voltage reduction that can last minutes to hours, sometimes imposed deliberately by the utility during peak demand to avoid a full blackout. Under voltage is the broader term covering any condition where voltage stays below the acceptable range, whether it lasts a few minutes or becomes a chronic problem at your location.
The practical difference matters because your equipment responds differently to each one. A brief sag might cause lights to flicker or a computer to reboot. A sustained under voltage condition causes motors to overheat and electronics to behave unpredictably over time.
What Causes Under Voltage
The most common cause is an overloaded or undersized transformer, either on the utility’s side or within a building’s electrical system. During peak demand periods, when everyone in a neighborhood is running air conditioning simultaneously, the demand for power exceeds the transformer’s capacity and the voltage sags. This is why under voltage problems tend to be worst on the hottest and coldest days of the year.
Other causes include wiring that’s too small for the load it carries (common in older buildings that have added appliances and circuits over the decades), long cable runs between the transformer and the building, loose or corroded connections at the service panel, and utility grid problems like a downed line or failed voltage regulator upstream. In rural areas, where homes sit at the end of long distribution lines, chronic low voltage is a well-known issue.
How Under Voltage Damages Motors
Electric motors are the most vulnerable equipment in your home or facility, and they’re everywhere: in your refrigerator, air conditioner, furnace blower, washing machine, and garage door opener. When voltage drops, a motor tries to maintain its speed by drawing more current. According to the Electric Power Research Institute, even a 10% voltage reduction causes a 19% loss in torque (the motor’s turning force), while full-load current increases by up to 10%. That extra current generates heat, potentially raising the motor’s internal temperature by 15%.
The torque loss is especially problematic during startup. Starting torque varies with the square of the voltage change, meaning a modest voltage drop causes a disproportionately large reduction in the motor’s ability to get spinning. A compressor that starts easily at 240 volts may stall or struggle at 215 volts, leading to repeated failed start attempts that stress the windings and starting components.
Effects on HVAC and Refrigeration
Air conditioners and refrigerators rely on compressor motors that work against significant pressure. Under low-voltage conditions, the compressor may not generate enough torque to overcome that pressure, leading to hard starts and increased wear. Some newer systems have built-in low-voltage protection that shuts the compressor down when voltage drops below a safe threshold. While this prevents immediate damage, frequent on-off cycling from unstable power still wears out contactors, capacitors, and starting components.
Low voltage also causes contactors and relays (the switches that turn the compressor on and off) to “chatter,” pulling in weakly rather than making clean, firm contact. This chattering creates arcing that pits the contact surfaces and eventually causes failure. Capacitors, which help single-phase motors start and run efficiently, can also degrade in a chain of events triggered by sustained low voltage. The symptoms often look like unrelated failures: a motor that won’t start, a control board that shorts out, a compressor that cycles every few minutes.
What makes diagnosis tricky is that the voltage problem may not be present when a technician arrives to inspect. The spike that occurs when normal voltage returns after a brownout can itself destroy control boards and sensitive electronics, and by then the voltage reads normal on a meter.
How to Check Your Voltage
You can measure voltage at any outlet using a digital multimeter set to AC voltage (the ṽ symbol). Insert the black lead into the COM jack, the red lead into the V jack (never the A jack, which would create a dangerous short circuit), and connect the probes to the outlet. A standard 120-volt outlet should read between roughly 108 and 126 volts under normal conditions.
A single reading only tells you what’s happening at that moment. Since under voltage often occurs during peak demand, the more useful approach is to check voltage during the times you notice problems: the hottest part of a summer afternoon, or when large appliances are running simultaneously. Many multimeters have a MIN/MAX function that records the lowest and highest readings over time, which helps capture dips you’d otherwise miss. If you consistently see readings below 108 volts on a 120-volt circuit (or below 216 volts on a 240-volt circuit), the under voltage is outside the normal tolerance range and worth investigating.
For 240-volt circuits powering air conditioners, dryers, or electric ranges, the same approach applies but you’re measuring at the dedicated outlet or disconnect box. Check voltage between both hot legs and between each hot leg and neutral to identify whether the drop is on one leg or both, which helps narrow down whether the issue is inside your panel or on the utility’s side.
How to Fix or Prevent Under Voltage
The right solution depends on the cause. If the problem originates with the utility, contacting them with documented voltage readings often prompts an investigation. Utilities may need to upgrade a transformer, adjust tap settings, or balance loads across phases. These are their responsibility, not yours.
For building-side issues, an electrician may find undersized wiring, overloaded circuits, or poor connections at the panel. Corroded or loose connections are a surprisingly common cause of voltage drop and are relatively inexpensive to fix.
When the root cause can’t be easily resolved, two types of equipment can compensate. An automatic voltage regulator (AVR) continuously monitors incoming voltage and adjusts it using a variable transformer, delivering stable output voltage regardless of what’s coming in from the grid. AVRs handle under voltage, over voltage, and transient fluctuations, making them a good choice for protecting sensitive equipment like computer systems, medical devices, or manufacturing controls.
An uninterruptible power supply (UPS) goes further by including a battery backup. Online UPS systems convert incoming power to DC, store it in a battery, and convert it back to clean AC for the load. This provides seamless protection against both voltage problems and complete outages. For most homes, an AVR or a line-interactive UPS on critical equipment is sufficient. For facilities where any power interruption is unacceptable, such as data centers or hospitals, an online UPS is the standard solution.