What is Voltage Drop?
Voltage Drop is the reduction in electrical voltage that occurs as current travels through a wire, cable, connection, or other circuit component. Wire resistance, circuit length, and load current affect the amount of reduction. Excessive Voltage Drop can cause dim lights, motor problems, heat, poor equipment performance, and nuisance shutdowns.
Quick Facts About Voltage Drop
Category
Electrical measurement and circuit performance
Formula
Voltage Drop equals current multiplied by resistance
Measured by
Digital multimeter under operating load
Common confusion
Voltage Drop is not the same as low utility voltage
Also called
Voltage Loss, Circuit Voltage Drop
Often discussed with
Electrical Code Corrections, Circuit Breaker Replacement
Key Takeaways About Voltage Drop
- Longer circuits and higher current loads usually produce more Voltage Drop.
- Undersized wire can make equipment run poorly and create unwanted heat.
- Electricians measure voltage at the source and load while the equipment operates.
- Loose or damaged connections can create localized Voltage Drop and fire hazards.
- Circuit design should account for conductor size, length, current, and load type.
Understanding Voltage Drop
Voltage Drop describes the voltage lost as electricity moves through a circuit. Every conductor has resistance. Some electrical energy changes into heat during the trip from source to load. The amount mainly depends on conductor material, wire size, circuit length, and current flow.
Related glossary terms: American Wire Gauge, Ampacity, Amperage.
A small loss is normal. But excessive loss can affect performance. A light may appear dim. A heating appliance may take longer to operate. A motor may struggle to start. A loose terminal, corroded splice, damaged receptacle, or undersized conductor can create unusual loss. This loss may occur at one point in the circuit.
How Voltage Drop Works, Is Measured, or Is Used?
The basic relationship follows Ohm's law. Voltage Drop equals current multiplied by resistance, written as V = I × R. A 10-amp load travels through a circuit with 0.5 ohms of resistance. This load would produce a 5-volt drop. Resistance rises with smaller conductors or longer wire runs.
An electrician measures the circuit while it operates. The electrician doesn't measure it only while idle.digital multimeter can measure voltage at the source and then at the load. The difference shows the circuit loss. For a complete check, the electrician also measures current. The electrician inspects wire size and checks terminal connections. The electrician also considers the full round-trip length of hot and neutral conductors.
Voltage Drop calculations help select conductors and plan circuits before installation.National Electrical Code includes guidance on feeder and branch-circuit voltage drop. Project specifications may set stricter limits. A designer may choose a larger conductor for a long run. This choice reduces resistance and keeps load voltage closer to source voltage.
Why Voltage Drop Matters?
Excessive Voltage Drop reduces the voltage available to equipment. Motors may draw more current while trying to produce normal torque. This can increase heat and shorten motor life. Electronic equipment may reset. Lighting may flicker. Heating devices may deliver less output.
The location of the loss also matters. A long wire run creates distributed resistance. A loose connection can create concentrated resistance and heat at one terminal. A circuit may show a sudden voltage difference across a switch, splice, breaker, or receptacle. This situation deserves careful inspection because the connection may be damaged or unsafe.
When Voltage Drop Matters Most?
Voltage Drop becomes more important on long circuits and high-current circuits. It also matters on circuits that supply motors or sensitive electronics. Common examples include detached garages, well pumps, air-conditioning equipment, electric heaters, compressors, and large kitchen appliances. Temporary loads and extension cords can also develop significant loss. This happens when the cord is long or too small for the equipment.
Use the following checks when evaluating a suspected problem.
- Compare voltage at the panel with voltage at the operating load.
- Record current while the equipment starts and while it runs.
- Confirm conductor size, circuit length, breaker rating, and load requirements.
- Inspect terminals, splices, receptacles, and disconnects for heat or discoloration.
- don't treat a larger breaker as a remedy for undersized wiring.
How to Evaluate Voltage Drop?
- Measure source and load voltage while the equipment is running, then calculate the difference.
- Check the conductor size against the circuit length and the equipment nameplate current.
- Look for flickering lights, hard-starting motors, warm terminals, damaged insulation, or discolored receptacles.
- Measure voltage across individual connections to find a high-resistance splice or terminal.
- Compare the design with National Electrical Code guidance and any equipment manufacturer requirements.
Related Concepts Compared
Voltage Drop vs. Resistance
Resistance is the opposition to current flow in a material or component. Voltage Drop is the resulting reduction in voltage when current passes through that resistance.
Voltage Drop vs. Low Utility Voltage
Low utility voltage affects the incoming supply to a building or service area. Voltage Drop occurs within a circuit because of wire resistance, connections, or load current.
Voltage Drop vs. Overloaded Circuit
An overloaded circuit carries more current than its wiring or protective device is designed to handle. An overloaded circuit can cause Voltage Drop, but Voltage Drop can also occur on a properly loaded long circuit.
Voltage Drop vs. Power Loss
Power loss describes energy dissipated in a circuit, often as heat. Voltage Drop describes the change in electrical potential that produces or accompanies that loss.
Expert Note
A voltage reading taken with no load can look normal even when a circuit fails under demand. Testing must include the equipment running, because current flow reveals resistance that an idle circuit may hide.
Common Mistakes or Myths About Voltage Drop
- Assuming a larger circuit breaker will correct Voltage Drop from undersized wiring.
- Testing voltage with the equipment off and concluding that the circuit is healthy.
- Measuring only the hot conductor while ignoring the neutral path and connection points.
- Blaming the utility before checking long wire runs, extension cords, and loose terminals.
Voltage Drop in Practice: A Real-World Example
A detached garage sits 180 feet from a home's panel and supplies a 12-amp compressor. The compressor receives noticeably less voltage during startup, so it struggles to start and the lights dim. A larger conductor and sound connections can reduce the circuit's resistance and improve performance.
Sources & Further Reading on Voltage Drop
- National Fire Protection Association, NFPA 70 National Electrical Code
- Fluke, What Is Voltage Drop?
Related Terms
American Wire Gauge
American Wire Gauge is a standardized system that identifies electrical wire diameter with numbered sizes. Smaller…
Ampacity
Ampacity is the maximum current, measured in amperes, that a conductor can carry continuously under specified…
Amperage
Amperage is the amount of electric current flowing through a circuit, measured in amperes, also called…
Load Calculation
Load Calculation is a method for estimating the electrical demand a building or circuIt'll place on…
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