What Is a Voltage Drop and How to Measure It

Have the lights in your home ever dimmed unexpectedly, only to return to normal in a minute or less?

If the answer is yes, then you’ve experienced a voltage drop.

Technically, a voltage drop occurs when electric potential decreases along the path of the current flowing in an electrical circuit.

It’s the drop in potential that causes lights to dim.

A good way to understand a voltage drop is to compare it to water pressure.

Electrical power, measured in watts, is the product of voltage (electrical pressure) multiplied by current (electrical flow).

In this analogy, the amount of water flow is equivalent to current (amperage), and the voltage is the pressure forcing water through the pipe.

The longer the distance water travels through a pipe, the more friction it meets along the way. 

Water pressure can be significantly lower at the destination than at the source.

The same concept applies to electrical power. 

Over long distances, the flow of current meets resistance (measured in ohms), causing a voltage drop.

Resistance results in lower output wattage at the same load at the destination than at the source.

Voltage drops are inherent to every real-world electrical system, even in the tiniest microcircuits, where the distance between a power source and destination is minuscule, such as semiconductors in a computer chip.

Now that we’ve got the technical explanation out of the way, let’s explore how voltage drops can affect you in a variety of situations — and what you can do about it.

What Causes Voltage Drops at Home?

Voltage fluctuations like drops and surges happen constantly in your home and in every electrical system.

It’s when you notice them that they become a problem.

Actually, that’s not accurate…

Even imperceptible variations in voltage can permanently damage sensitive electronic devices and appliances.

If you’re experiencing irregularities in your household electricity supply or have concerns about power quality, here are the most common culprits.

Brownouts

Brownouts are large-scale voltage drops that impact multiple end-users of electricity from the utility grid.

Brownouts can be planned by your utility provider or spontaneous events and are usually the result of demand for electricity exceeding the available supply that can be safely delivered through grid infrastructure.

Brownouts are similar to blackouts except that some electricity is still delivered by the grid.

Planned brownouts are more commonly used by utilities than scheduled rolling blackouts because having some power is less disruptive than having none.  

However, brownouts can actually be more damaging to sensitive electronics like computers and data storage devices than blackouts.

Utility providers use planned brownouts to manage peak demand, prevent grid overload and permanent infrastructure damage, and prevent uncontrolled blackouts.

Due to excessive electricity demand, brownouts occur most frequently during extreme weather events, particularly heat waves or cold snaps.

(Source: Climate Central)

Weather has been responsible for over 80% of US blackouts for decades. 

Most outages are still caused by physical damage to electricity transmission infrastructure, like downed power lines caused by severe storms.

But when everyone turns on their AC during a heat wave, it puts enormous pressure on the grid, leading to brownouts — intentional or not.

As Michael Webber, a professor in mechanical engineering at the University of Texas at Austin, told CNN, “[America’s] aging grid was built for the weather of the past, rather than the weather of the future.

If you experience infrequent or irregular voltage drops at home with no discernable pattern, check with your neighbors or utility company.

Chances are the drops in voltage are caused by brownouts rather than a fault in your home wiring or systems. 

Voltage Dips

If your lights dim when high-wattage appliances or systems  — such as central air conditioning — 

start, you’re almost experiencing a transient voltage dip, sometimes called a voltage sag.

Voltage dips occur when high-wattage appliances or HVAC systems draw a large surge of current — technically, an “inrush current” — causing a brief, localized voltage drop.

High-wattage appliances with motors, such as refrigerators, washing machines, and ACs, often require a surge in power that results in a brief and localized voltage dip.

That’s because all of the above — and many other home appliances — require much more power to start than to operate.

For example, the compressor motor for a central AC usually requires 5 to 7 times its operating wattage to start than to run.

Failing to account for the difference between starting and running watts can cause circuits to be overloaded by inrush current, resulting in transient voltage dips. 

It also frequently causes circuit breakers to trip due to current overload.

Vulnerability to current overload is one of the primary reasons electricians recommend installing a dedicated circuit for HVAC systems and exceptionally high-wattage appliances like electric water heaters. 

Localized Power Sags

The term “power sag” is often used interchangeably with brownout as it typically applies to widespread areas of disruption.

However, it is not uncommon for you to suffer voltage dips due to your neighbors’ electricity consumption patterns, especially in smaller communities.

Localized power sags can be extremely frustrating to diagnose — and to deal with.

For example, if your next-door neighbor’s expensive new central AC system is causing transient voltage dips at your home, they’re unlikely to stop using it — no matter how nicely you ask.

Plus, it’s not easy to determine whether a voltage dip is caused by an outside issue or a fault in your home. 

Localized power sags are caused when the electrical load exceeds the output capacity of a shared distribution transformer. 

Distribution transformers are part of the utility infrastructure that “steps down” the high-voltage electricity transmitted through power lines to 120/240V single-phase power used in virtually all US households.   

If you suspect that you may be experiencing a localized power sag, the first step is to check with your closest neighbors.

If the brownouts are caused by a shared transformer, it’s almost guaranteed that they’ll be experiencing voltage dips, too.

If you confirm that you’re not alone in experiencing power fluctuations, contact your utility company for assistance.

However, if the power sags are low intensity and short in duration, it’s unlikely your power company will treat it as anything other than a minor inconvenience — or replace the transformer.

If this is the case, take steps to protect the most sensitive electronic devices in your home, like computers and data storage devices.

Uninterruptible power supply (UPS) systems and portable power stations that offer <10ms auto-switchover provide significant protection against voltage dips, surges, brownouts, and blackouts.

Home Wiring

Whether you just moved into a new building or you’ve lived in the same place for years, your home’s internal wiring and circuitry are frequently the root cause of voltage drops.

The most common causes for voltage drops vary considerably based on whether it’s an older or newly constructed home.   

What Causes Voltage Drops in New Homes

When you move into a new home, the last thing you want to deal with is issues with essential building systems like electricity, HVAC, and plumbing (MEP).

Unfortunately, it’s a relatively common occurrence.

If you’re experiencing fluctuations in electricity leading to voltage dips, tripped breakers, or other issues, here are some of the most frequent causes.

1. Improper Wiring Practices

• Loose Connections: Home wiring consists of multiple electrical connections between the utility grid electricity, your main circuit board, junction boxes, switches, and outlets. Improper connections can increase resistance and cause voltage dips in part or all of your home.
• Damaged Wiring: Electricians and workers use multiple fasteners to secure the internal wiring in your home. Staples are the most common due to speed and ease of use. However, if a wire is stapled to the building’s foundation too tightly, it can cause additional resistance — similar to a kinked or pinched water hose. Multiple fastenings that are too tight will make resistance even worse. Unfortunately, you (or your contractor) may not become aware of this issue until the circuit is used under heavy to maximum load. At this point, the increased resistance can cause voltage dips. This issue can be difficult to diagnose and repair, as the damaged wiring is likely behind drywall or other interior materials.
• Incorrect Wire Gauging: No reputable electrician should make this mistake, but if the wrong wire gauge (thickness) is used in your home, it will likely lead to voltage dips and other issues when placed under high electrical load. 

2. Poor Design

• Undersized Circuits: Incorrectly sized home electrical circuits can malfunction even if they meet building code requirements. If a circuit is sized too close to the maximum potential load, it can trip a breaker or cause voltage dips in real-world use. A properly designed system will have plenty of “headroom” for circuits above the anticipated maximum load. 
• Lengthy Wire Runs: Voltage drops over distance in all electrical systems due to resistance. Excessively long wire runs from the main electrical panel to outlets and switches in distant parts of the building can lead to voltage dips, particularly if the wiring is incorrectly gauged. Thicker wires have less resistance, making them more suitable for long wire runs. In large homes, multiple distribution panels may be required, similar to apartment buildings where there’s typically an electrical panel in each unit.

3. Defective Components: Faulty circuit breakers, outlets, switches, and junction boxes can cause voltage dips and other issues. 

What Causes Voltage Dips in Older Homes

Voltage dips and electrical issues are frequently part of the growing pains of moving into a newly built home.

But what if you’ve lived in the same place for years or recently moved into an older home and started having voltage drops?

Like everything else, electrical systems degrade over time, and even previously reliable circuits can fail.

Voltage dips and tripped breakers are often early indicators that parts of your home electrical system need to be repaired, upgraded, or replaced.

1. Aging Wiring and Connections

• Oxidation and Corrosion: The metal in home wiring and connections is subject to oxidation and corrosion over time. Copper wiring is more durable, but aluminum wiring is common in older buildings and more susceptible to degradation. Oxidation and corrosion increase resistance, and voltage drops often result.
• Loose Connections: Previously reliable connections can loosen over time.
• Wiring Insulation Deterioration: Heat and aging can cause the insulation around metal wiring to crack, leading to increased resistance or ground faults, which can cause voltage drops and breaker trips.

2. Increased Electricity Demand

Surprisingly, average household electricity consumption in the US hasn’t increased that much since 1990, about 20% a year. Improvements in technology and energy efficiency of home appliances and systems have meant that even though we may be powering more devices than ever, overall consumption hasn’t dramatically increased. Nevertheless, older homes may be unable to handle increases in electrical load demand from electronic devices, appliances, and HVAC systems. Wiring and other electrical components can come under strain, resulting in voltage drops and tripped breakers.

3. Circuit Overload

• Extension Cords: Many homes don’t have enough electrical outlets to support all the devices and appliances in use. Extension cords and power stips frequently cause circuits to overload, leading to voltage drops.
• Too Many Appliances on One Circuit: Almost everyone has had the experience of turning on one too many appliances at once, resulting in a tripped breaker. Connecting multiple high-wattage appliances that overload a circuit can also lead to voltage dips.

4. Local Grid Infrastructure

• Utility Transformer Overload: A previously reliable distribution transformer that supplies electricity for your neighborhood can become overloaded if load demand increases. Voltage drops are often the first sign of a problem with local grid infrastructure. 

5. Hidden Damage: In addition to degradation from age-related issues like corrosion and oxidation, unseen moisture and rodent damage can also cause voltage drops and ground faults to your internal home wiring.

How Do You Test for Voltage Drops at Home?

It’s possible to test for voltage dips and drops at home, but even if you narrow down the potential causes, you’ll likely require a licensed electrician to fix them at some point.

Here’s a step-by-step guide to testing for voltage drops at home.

1. Safety First!
   By far the most crucial step. Working with high-wattage electricity is dangerous and potentially deadly. Don’t attempt any diagnostic testing or repairs that put your safety or your home at risk. ALWAYS turn power off at the circuit breaker before testing any outlets, switches, or wiring.
   
2. Carefully Observe the Issue (and Take Notes)

You can learn a lot about what’s potentially causing voltage dips by looking for patterns. 

• Does it happen when you turn on specific appliances or systems? 
• Do the dips frequently occur at a similar time of day?
• How long are the voltage drops? — a transient voltage dip of a few seconds or more prolonged power sags similar to brownouts?
• Does it only happen in one part of the house? Or a specific room? Kitchens, in particular, are prone to circuit overloads.

3. Rule Out the Obvious

If the voltage drops are confined to a specific part of the home, take the following steps.

• Reduce, Then Gradually Increase Circuit Electrical Load: Unplug or turn off devices and appliances on the same circuit. Turn them on one by one, starting with the high-wattage appliances first. If the voltage dip only occurs as the load increases, you’ve likely diagnosed the cause. A licensed electrician can easily add breakers to your electrical panel to permanently redistribute the load.
• Using Extension Cords or Power Strips? Long extension cords and overloaded or faulty power strips frequently cause voltage dips. Plug as many of your appliances as you can directly into wall outlets. If you’re using extension cords and power strips, you probably don’t have enough outlets. Discuss your options with a licensed electrician.

4. Isolate Problem Circuits

If you suspect that one circuit is causing voltage dips, but the issue is not isolated to one part of the home, try flipping off the breaker to see if the problem stops recurring. A single faulty or overloaded circuit can cause voltage fluctuations on multiple circuits, particularly in older homes.

If the voltage drops stop when you turn off one circuit, it’s likely due to an overloaded electrical panel or multiple circuits sharing a single neutral wire.

A licensed electrician is required to address both issues.

5. Inspect Outlets and Switches
   If a faulty outlet or switch is causing voltage drops, there are often visible or audible clues. Visually inspect outlets for signs of damage or degradation. Call an electrician immediately if you hear audible buzzing or crackling from an outlet or switch. It could be a sign of a dangerous condition. Flip off the circuit breaker the noisy switch or outlet is connected to, and don’t attempt testing or repair yourself.

6. Test Electrical Outlets and Sockets with a Multimeter

Using a digital multimeter to perform diagnostic testing on electrical receptacles — the technical term for light sockets and wall outlets — is usually done by licensed electricians. Multimeters are inexpensive and widely available — but that doesn’t mean you should do it yourself. If you’re comfortable working with high-voltage, it can be done safely, but always take the necessary precautions.

• NEVER WORK ON LIVE CIRCUITS: Before testing any electrical receptacle, it’s essential to flip off the circuit breaker it’s connected to on your home’s main electricity panel. Failure to do so can lead to serious injury or death. For example, never just turn off a light switch before checking a light socket and assume you’re safe. There are multiple reasons why a switch can fail to cut off high-voltage electricity, even if it turns off a light.
• Double-Check No Power is Present: Even if you’ve flipped the circuit breaker off, you must confirm that no electricity is flowing at the receptacle (outlet) before diagnostic testing. Confirm the power is off with a non-contact voltage tester or multimeter before proceeding to next steps. Once you’ve confirmed there’s no power present, you can proceed.
• Voltage Test (No Load): 
  • Set your multimeter to measure AC voltage. A 200V range is appropriate for household (120V) electrical outlets, or you can use a 20V range for increased precision.  
  • Connect the appropriate test cables of your multimeter to the corresponding hot and neutral terminals of the outlet, making sure not to touch the metal tips of the probes.
  • With the multimeter connected, turn the circuit breaker on. Read and record the voltage measurement from your multimeter. It measures the “source voltage” at the outlet supplied by your utility provider and home circuitry.     
• Voltage Test (Under Load)
  • Carefully plug an appliance (like a small fan known to operate normally) with an on/off switch into the same outlet the multimeter’s test leads are attached to. 
  • Switch the appliance on. Read and record the results from the multimeter. 

• Calculate the Voltage Drop 

Subtract the Voltage Test (Under Load) result from the Voltage Test (No Load) measurement. The difference between the readings is the voltage drop.

For example, if the Voltage Test (No Load) result is 120V and the Voltage Test (Under Load) is 115V, there will be a 5V voltage dip at the outlet when it’s in use.

A 5V drop at a 120V outlet equals a voltage drop by percentage of 4.17%.

Depending on the type of circuit (branch or branch/feeder), it’s above or at the higher end of what’s considered normal for receptacles (outlets) under NEC guidelines and warrants closer inspection (and possible repair) by a licensed electrician.  

7. Check With Your Neighbors

Ask your neighbors if they’re experiencing any similar issues with voltage drops or electricity fluctuations at home.

If so, your local distribution transformer is likely malfunctioning or overloaded. 

Contact your utility provider to report the problem. 

They will also inform you if there are any system-wide issues you’re unaware of that could cause voltage drops at your home.

When to Call an Electrician

If you’re confident working with electricity — and you’re careful — the above steps can help you diagnose voltage drops and other issues at home.

Even if you successfully track down the cause, many repairs and improvements must be undertaken by a licensed electrician—for safety reasons and likely by law or building code requirements.

Here’s when to give up on fixing or identifying the problem yourself and call a professional.

• You’re unfamiliar or uncomfortable with working with electrical safety guidelines. 
• You detect signs of damaged wiring or burnt connections.
• You suspect aluminum wiring may have been used when building your home. This is only likely if your home was built from the mid-1960s to mid-1970s.
• You can’t identify the source of the problem.
• Rewiring or electrical panel replacement/improvements are required.

Do Surge Protectors Protect Against Voltage Drops?

No. Surge protectors provide limited protection against surges in voltage but none against voltage drops and dips.

Voltage spikes happen less frequently than transient voltage dips but carry far more destructive power. 

Significant spikes typically originate from sources outside your home, like lightning strikes or issues with grid infrastructure.

Surge protectors should be considered the bare minimum protection for precision electronics.

Keep in mind that cheap “surge protectors” are frequently little more than glorified power strips that offer limited protection from spikes. 

At a minimum, ensure that any surge protectors you purchase are UL1449 certified for safety.

If you’re concerned about voltage drops and the long and short-term impact of electrical fluctuations on your sensitive electronics like computers and hard drives, an uninterruptible power supply (UPS) is a much better option.

A high-quality UPS provides ongoing protection from power surges, voltage drops, brownouts, and blackouts.

Many inexpensive UPS models only give you a few minutes to shut down your computer or other systems if a power outage occurs, but the built-in line conditioning will help preserve and protect your precision electronics and your valuable data.

EcoFlow’s RIVER 3 Pro portable power station provides data center level <10ms auto-switchover to battery backup power and offers double the runtime of a traditional UPS at a competitive price.

Frequently Asked Questions

Final Thoughts

It’s possible to diagnose some electrical faults that cause voltage drops, transient dips, and localized power sags at home by yourself…

If you feel comfortable following the guidelines above safely and carefully, you may be able to identify the likely causes.

However, most repairs and improvements that correct problematic fluctuations in voltage require a licensed electrician.

If you have a concern about an electrical issue in your home, it may be best to leave the diagnosis and treatment to a professional.

If you’re worried about the impact of power quality on sensitive electronics like computers, gaming consoles, and data storage, you can take affordable steps to protect them from fluctuations in voltage, like power surges and dips.

EcoFlow RIVER 3 Plus provides data-center-level line conditioning and uninterruptible backup battery power with <10ms UPS switchover. 

EcoFlow offers a wide variety of portable power stations and solar generators that reduce your dependence on the utility grid and protect you from blackouts, brownouts, and voltage drops.

Check out our selection today.