Brownouts and Voltage Sags: What Low Voltage Does and How to Stop It

People use "brownout" for almost any low-voltage event, but the distinction between a short sag and a sustained brownout matters, because the causes, the risks and the fixes are different. Duration is the dividing line.

Two events, one dividing line

The term "brownout" originally referred to dimming incandescent lights during deliberate utility voltage reductions. Engineers prefer "undervoltage," but both mean the same sustained condition.

What "normal" voltage is: ANSI C84.1

In the US, ANSI C84.1 sets the voltage ranges that utilities design to and that equipment makers design for. For a 120 V nominal system it defines two ranges, measured at the service point (where the utility's wires meet yours):

The standard also defines somewhat lower minimums at the point of utilization (the outlet), roughly 108 V for Range A and 104 V for Range B, because branch wiring adds its own voltage drop. For the exact figures, including the different allowances for lighting circuits, consult the standard itself. For 240 V circuits the values double (228 to 252 V for Range A).

Practical reading: an outlet showing 118 V with nothing much running is healthy. An outlet that idles at 112 V is at the bottom of the comfortable range, and any added load or utility sag will push it lower.

Where sags come from

Faults elsewhere on the system

When a tree branch, animal or failed insulator causes a short circuit on a distribution line, fault current is enormous and voltage collapses across everything electrically nearby until protection operates. Customers on the faulted line see an interruption. Customers on neighboring feeders served from the same substation see a sag lasting a few cycles to a fraction of a second, which is why you can get flicker on a calm day with no outage.

Reclosers

Most overhead faults are temporary, so utilities use reclosers that open the circuit, wait, and try again, typically a few times within seconds before locking out. Each attempt into a still-present fault produces another sag or interruption. The "blink, blink, out" pattern at the start of a storm outage is this sequence.

Motor starting

Induction motors draw a starting current commonly several times their running current until they reach speed. Air conditioner and heat pump compressors, well pumps, and large shop tools are the usual residential culprits. The sag they cause depends on the motor size and the impedance of your service: a long service drop or a small, shared transformer makes it worse.

Loose or corroded connections

A high-resistance connection at a breaker, neutral lug, service entrance or meter base produces a voltage drop proportional to current. These sags get worse under load and often worsen over time as the connection heats. This is the one cause that can be a fire hazard.

Where brownouts come from

• Heavy demand: on very hot days, air conditioning load raises current on every feeder and transformer, and voltage at the end of long lines drops.
• Deliberate reduction: some utilities lower voltage slightly to reduce demand during emergencies or as a routine efficiency practice (often called conservation voltage reduction), normally staying inside the ANSI range.
• Overloaded or undersized transformer: a neighborhood that has added EV chargers and heat pumps can outgrow its transformer.
• Long runs: rural services, detached garages and outbuildings fed by long, thin conductors.

What low voltage does to equipment

Electronics

Switching power supplies regulate their own output and compensate for low input voltage by drawing more current. Many accept roughly 90 to 264 V. A computer running at 105 V is usually fine. Trouble comes from deep sags that drop below the supply's hold-up capability, or from rapid, repeated sags that confuse power-on logic.

Motors and compressors

Motors are the real victims. A loaded induction motor delivers roughly constant mechanical power, so at lower voltage it draws more current, and heating in the windings rises roughly with the square of current.

Worked example: a compressor designed for 115 V running at 104 V draws roughly 115 / 104 = 1.11 times its normal current, about 11% more. Winding heating rises by about 1.11 x 1.11 = 1.22, or 22% more. This is a simplified model (motor behavior is not perfectly constant-power, and efficiency changes too), but it shows why sustained brownouts shorten the life of refrigerator and air conditioner compressors while computers shrug them off. Starting is also harder: starting torque falls roughly with the square of voltage, so a compressor may stall and cycle on its overload protector.

Contactors and relays

Coils that hold contactors closed can drop out during deep sags, shutting down HVAC equipment, pumps or commercial machinery even though the sag lasted only a fraction of a second.

How each kind of protection responds

The key difference is the brownout column. A standby UPS can only fix low voltage by running on battery, and a battery that would last 10 minutes cannot cover a two hour brownout. AVR fixes it indefinitely by changing a transformer tap. See automatic voltage regulation for the mechanism.

Worked example: does AVR boost cover your brownout?

Many line-interactive units boost by a fixed ratio, often on the order of 10 to 15%, when input falls below a threshold. Suppose a unit boosts by 12% and transfers to battery if the boosted output would still be too low. With 104 V in, boosted output is about 104 x 1.12 = 116.5 V: good. With 92 V in, boosted output is about 103 V, which some units accept and others reject in favor of battery. The thresholds and boost ratio differ between brands and models, and some offer two boost steps, so read the spec sheet's "input voltage range on line" figure rather than assuming.

Diagnosing low voltage at home

1. Get a baseline. Measure outlet voltage with a plug-in meter or true RMS multimeter at a quiet time. Note it. See measuring power quality at home.
2. Load test one circuit. Plug a known resistive load (a hair dryer or small heater, around 1,200 to 1,500 W) into an outlet on the same circuit as the meter. A drop of a few volts is normal; a drop of 6 V or more on a 15 A circuit at that load suggests a long run, an undersized conductor or a poor connection.
3. Compare circuits and legs. Repeat on circuits fed from both sides of the panel. If one side rises while the other falls under load, suspect the neutral.
4. Watch during known motor starts. Note the dip when the air conditioner or well pump starts. Brief dips that recover within a second are typical.
5. Log over days. UPS logs or a logging meter reveal whether low voltage is time-of-day (utility demand) or event-driven (your loads, faults).
6. Escalate correctly. Low voltage at the main panel with nothing running is a utility issue. Low voltage only on some circuits, or only under your own loads, is a wiring issue for an electrician.

Fixes, from cheapest to most thorough

• Move big motor loads off circuits that share sensitive equipment.
• Repair connections identified by testing. This is often the biggest single improvement.
• Line-interactive UPS with AVR for computers and network gear: covers sags, brownouts and outages. Sizing in how to size a UPS.
• Online UPS where low voltage is chronic or deep, or where the load is very sensitive.
• Utility involvement for chronic service-level low voltage: a transformer tap change or upgrade is their fix, not yours.

Protecting a refrigerator or furnace has extra considerations (motor starting current, waveform), covered in UPS for a refrigerator and UPS for a gas furnace.