Voltage drop is the phenomenon where electrical potential energy supplied by a source decreases as current flows through a circuit. This reduction occurs because all conductors possess electrical resistance, which converts a portion of the energy into heat. Consequently, the voltage available to operate the load (such as a motor or light) is less than the source voltage. Finding this drop is important for ensuring electrical systems operate with maximum efficiency and safety.
Calculating Expected Voltage Drop
The theoretical voltage drop within a circuit can be estimated using Ohm’s Law, which states that voltage (\(V\)) is equal to current (\(I\)) multiplied by resistance (\(R\)), expressed as \(V = I \times R\). In this context, \(V\) represents the voltage lost across the conductor, \(I\) is the current drawn by the load, and \(R\) is the total resistance of the wiring path.
To apply this, one must first determine the resistance of the wiring, which is influenced by the conductor’s material, gauge, and length. Resistance values are often available in reference tables, typically given as ohms per unit of length for a specific wire gauge. Since current must travel from the source to the load and back, the total length of the conductor must account for this round-trip distance. By multiplying the expected current draw by the total wire resistance, one can calculate the anticipated voltage drop.
Practical Measurement Using a Multimeter
Finding the actual voltage drop requires using a digital multimeter (DMM) set to the appropriate voltage function. For circuits powered by batteries, the meter should be set to measure Direct Current (DC) voltage. For household or industrial applications, the Alternating Current (AC) setting would be necessary.
The measurement technique involves placing the meter leads in parallel across the specific section of the circuit being tested. Crucially, the circuit must be energized and operating under its normal load for the test to be accurate, as the voltage drop only occurs when current is flowing. To measure the drop across a wire segment, the positive probe is placed at the beginning of the conductor, and the negative probe is placed at the end of that same conductor.
The reading displayed on the multimeter is the voltage lost across that exact section of the circuit, such as a specific length of wire, a connection point, or a switch. By testing segments sequentially, from the power source toward the load, one can pinpoint where the most significant voltage loss is occurring. This differential measurement across a component isolates its individual contribution to the total circuit resistance.
Diagnosing the Cause of Excessive Drop
While some voltage drop is unavoidable, electrical system efficiency requires that the total voltage drop not exceed a certain percentage of the source voltage. For many circuits, a maximum total drop of 5% is a common recommendation for ensuring reasonable equipment operation. If the multimeter measurement reveals a loss greater than this acceptable range, a physical defect is introducing excessive resistance into the system.
One frequent cause of an increased voltage drop is poor connections, often due to corrosion or oxidation at terminals and splices. This foreign material introduces high resistance at the contact point, which the multimeter test will isolate as a high reading across that connection alone. Loose connections, resulting from improper torquing or vibration, also create high-resistance junctions that significantly increase the measured voltage loss.
Another common culprit is undersized wiring, meaning the conductor’s gauge is too thin for the current load or the length of the run. A wire that is too small inherently has higher resistance, leading to a large drop, especially when carrying heavy current. In some cases, internal resistance breakdown within a component or faulty wiring can also contribute to an excessive drop, which is identified by systematically testing each part of the circuit under load.