VOLTAGE DROP TEST: WHAT IT IS AND HOW TO DO IT

Ever flipped a switch and noticed lights dim, or watched a motor lag when it should run smoothly? 

That might be a sign of excessive voltage drop. Safe, efficient electrical systems depend on knowing how much energy is lost as current flows through a circuit. A voltage drop test helps technicians catch those issues before they cause bigger problems. 

This skill is one of many that electrical pros use every day. At Universal Technical Institute (UTI), aspiring electrical professionals can train hands-on in any of our four electrical training programs. While enrolled, students can build skills needed to troubleshoot circuits, read meters and apply the same diagnostics used by working professionals today.¹³⁰ 

Read on to learn more about voltage drops and when to test electrical systems. 

What Is Voltage Drop? 

Voltage drop happens when electrical energy is lost as current flows through a conductor. The longer the run and the smaller the wire, the more resistance builds up, lowering the voltage that reaches the load. This loss can cause lights to dim, motors to run hot or equipment to perform less efficiently. 

Resistance, current and conductor length 

When current flows through a conductor, resistance naturally builds. The size and material of the wire determine how much energy is lost along the way. 

Longer wire runs and smaller diameters increase that resistance, which means less voltage reaches the load. This can cause lights to dim, motors to struggle and circuits to work less efficiently. 

Symptoms of excessive voltage drop  

Recognizing the signs early is one of the best ways to understand what is voltage drop and how it affects system performance. 

• Dim or flickering lights: When the voltage is too low, lights may not shine at full brightness or may flicker, especially under heavy load. 
• Motors running hot: Reduced voltage forces motors to draw more current, which can cause overheating and shorten their lifespan. 
• Slow motor performance: Equipment may start sluggishly or fail to reach full speed, making it less efficient and less reliable. 
• Nuisance breaker tripping: Protective devices can trip unexpectedly when circuits operate outside of safe voltage ranges. 
• Premature equipment wear: Prolonged exposure to low voltage can stress components, leading to frequent repairs or early replacement. 

Read: Grounding vs. Bonding: What’s the Difference? 

When Should You Perform a Voltage Drop Test? 

A voltage drop test isn’t limited to advanced or industrial systems — it’s a useful check for everyday circuits as well. Technicians use it to verify that wiring, loads and connections are all working within safe limits. Performing the test regularly can help catch hidden issues before they lead to failures or hazards. 

Dim lighting, slow motors and nuisance trips 

One of the most common warning signs of voltage issues is lighting that looks dim or inconsistent. When voltage falls below what fixtures are designed to use, bulbs and lamps can’t operate at full brightness. This symptom often shows up first on longer runs or heavily loaded circuits. 

Motors are also sensitive to low voltage. They may start slowly, overheat or trip protective devices more often than expected. Running motors under these conditions can cause extra wear, shorten their service life and lead to costly downtime. 

Long runs, undersized conductors and loose terminations 

The longer a conductor is, the more resistance it introduces into the circuit. If the wire is also too small for the load, the combined effect can cause a noticeable voltage drop that limits performance and efficiency. Testing helps confirm whether upsizing is needed to keep circuits safe. 

Loose or poorly made connections add another layer of resistance. Terminals, splices and lugs that aren’t tight enough can heat up, waste energy and make systems unreliable. Regular checks ensure these weak points don’t create larger failures. 

Read: Wire Gauge Chart: AWG, Ampacity and Wire Size Basics 

Tools You Need for a Voltage Drop Test 

Digital multimeter and clamp meter 

A digital multimeter is the primary tool for a voltage drop test. It allows technicians to measure voltage at different points in the circuit and compare those readings against the source. Using it correctly provides accurate insight into how well the system is performing. 

A clamp meter adds another layer of detail by measuring current flow while equipment is under load. This helps confirm whether the wire and connections can handle demand without excess loss. Together, these tools can give a complete picture of circuit health. 

PPE and lockout/tagout basics 

Personal protective equipment (PPE) is just as important as the testing tools themselves. Insulated gloves, safety glasses and protective clothing help shield technicians from accidental contact or arc flashes. Using the right gear reduces the risk of injury during testing. 

Lockout/tagout procedures add another level of safety by ensuring circuits remain de-energized while being set up for a voltage drop test. By securing breakers and labeling equipment, technicians prevent others from turning power back on unexpectedly. These steps create a safer environment for accurate testing. 

Read: Types of Electrical Circuits Explained  

How To Perform a Voltage Drop Test Step by Step 

Safe setup and reference measurements 

Before any testing begins, technicians establish a safe environment. This means confirming that the circuit is properly isolated, equipment is grounded and all protective gear is in place. Careful setup reduces the chance of errors and ensures accurate readings. 

Once conditions are safe, the first step is to measure voltage directly at the source. This creates a baseline reference point for comparison with downstream measurements. Without this reference, it’s difficult to know how much energy is being lost across the circuit. 

Testing under load and interpreting results 

After the reference is set, the next step is to measure voltage while the circuit is under load. This means keeping lights, motors or other equipment running so the test reflects real operating conditions. Without a load, the results won’t show how the system performs during actual use.   

Technicians then compare the load measurement with the source reading. A small difference is normal, but a large gap signals excessive voltage drop that needs attention. Interpreting results this way helps identify whether the issue is wiring, connections or conductor size. 

Read: Top Electrical Safety Tips for Beginners 

Voltage Drop Calculation Basics 

Simple rules of thumb and when to upsize wire 

A quick way to estimate performance is by using general rules of thumb. For example, if a run is longer than 100 feet, many technicians automatically check for loss and consider a larger conductor. These guidelines can help spot issues before they become serious. 

Still, rules of thumb only go so far. A precise voltage drop calculation is needed to confirm whether the wire size is truly adequate. Running the numbers ensures that conductors can carry the load safely without wasting energy or damaging equipment. 

Copper vs. aluminum and temperature rating effects 

Copper is the most common conductor material because it offers low resistance and strong durability. It can handle more current in a smaller size compared to aluminum, which is why it’s often preferred in critical circuits.   

Aluminum, on the other hand, is lighter and usually less expensive, but it requires larger sizes to carry the same load. Both materials are also affected by temperature ratings, meaning insulation type and operating conditions can change their performance. 

Read: Series vs. Parallel Circuits: What Future Electrical Technicians Need to Know 

Troubleshooting Excessive Voltage Drop 

Tighten connections, shorten runs and increase conductor size 

One of the simplest fixes for excessive voltage drop is tightening loose connections. Even a slightly loose lug or splice can create extra resistance that wastes energy and causes overheating. Regular checks keep these weak points from turning into failures. 

If the issue comes from the circuit design itself, shortening the wire run or upsizing the conductor may be necessary. Larger wires reduce resistance, while shorter runs minimize the distance current has to travel. Both adjustments can bring the voltage back within safe limits. 

Check terminations, splices and breaker sizing 

Terminations and splices are common points where hidden resistance builds up. Corrosion, improper crimping or worn fittings can all limit conductivity and raise the chance of failure. Inspecting and repairing these spots is a key step in reliable circuit maintenance.   

Breaker sizing also plays a role. If a breaker is rated too high for the conductor, the wire may overheat before the breaker trips. Matching breaker size to the conductor’s ampacity helps ensure the protective device does its job and keeps the circuit safe. 

Read: Electrical Technician Career Pathways: What Comes After Graduation 

Learn Voltage Drop Troubleshooting at Universal Technical Institute 

Through training in wind energy, robotics and automation, industrial maintenance or electronics, Universal Technical Institute helps students build career-ready skills.¹ Labs cover essentials like the voltage drop test, equipment setup and troubleshooting. 

Hands-on practice with meters and real wiring scenarios 

At Universal Technical Institute, students get hands-on training in labs that simulate real conditions. They use test meters, wiring tools and standard equipment to measure and evaluate circuits. This training is designed to teach them the same skills technicians rely on every day. 

In addition to practicing with equipment, students learn the right way to set up test points and ensure safe connections. These steps keep results accurate and help prevent system damage. By repeating them in the lab, students can build habits that carry into future careers. 

One of the key skills covered is the voltage drop test. Rather than only learning theory, students run the test under load, record readings and analyze outcomes. These exercises connect classroom instruction with practical application. 

Building job-ready skills in one year or less 

Within one year or less, students train on actual systems and develop the in-demand skills needed for entry-level roles after graduating. No matter which program they pursue, they finish with practical skills that support career readiness. 

Electrical Training for Voltage Drop Test Skills! 

Testing and troubleshooting circuits isn’t just for experienced pros. With the right training, you can learn in-demand electrical skills — like how to conduct a voltage drop test and more — in a year or less.