Voltage drop is the loss of electrical potential energy between the power source and the load, such as a motor or light bulb. This reduction in voltage occurs as the electrical current travels through the circuit’s conductors and components, encountering resistance. Calculating this drop is necessary to ensure electrical devices receive the proper voltage for optimal performance and to maintain system safety. An excessive drop can result in inefficient operation, overheating of wires, or equipment damage.
Core Electrical Concepts Causing Drop
The fundamental relationship governing voltage drop is defined by Ohm’s Law, which connects voltage, current, and resistance within a circuit. This law states that the voltage drop (\(V_d\)) across any part of a circuit is the product of the current (\(I\)) flowing through it and the resistance (\(R\)) of that part, expressed as \(V_d = I \times R\). Resistance is the inherent opposition to the flow of electrical current and is the root cause of all voltage loss within a circuit.
The amount of resistance determines how much electrical energy is converted into heat as current passes through it. Current is the flow rate of electric charge, measured in amperes, and is drawn by the load connected to the circuit. Therefore, any increase in the circuit’s resistance or the load’s current demand will directly and proportionally increase the voltage drop. The remaining voltage is the original source voltage minus the voltage drop.
Calculating Voltage Drop Using Known Resistance
The most straightforward method for finding the voltage drop across a specific component is by applying Ohm’s Law when the component’s resistance is known. This approach is typically used to analyze the voltage consumed by a load, such as a heating element or a fixed resistor. For example, if a device has a resistance of 10 Ohms and a current of 12 Amperes flows through it, the voltage drop across that specific device is 120 Volts (12 A x 10 Ω).
This calculation, \(V_d = I \times R\), provides the amount of electrical potential energy dissipated by the component. The resistance value used in this formula must be an accurate measurement of the specific element being examined, not the total circuit resistance. This technique is also employed to determine the voltage lost across circuit elements like fuses, switches, or terminals if their specific resistance and the circuit current are known.
Calculating Voltage Drop in Wiring Runs
Calculating the voltage drop in a wiring run is necessary because the conductors themselves contribute resistance, which increases with length. Wire resistance must first be determined based on the physical characteristics of the cable. The total resistance of a wire is directly proportional to its length and the material’s inherent resistivity (\(\rho\)), while being inversely proportional to its cross-sectional area. Copper has a lower resistivity than aluminum, meaning it offers less resistance for the same size and length of wire.
The American Wire Gauge (AWG) system classifies wire size; a lower AWG number indicates a larger cross-sectional area and lower resistance. The overall length (\(L\)) of the conductor must account for the current’s round trip (distance from the source to the load and back). Once the total resistance of the two-way wire run (\(R_{\text{wire}}\)) is calculated, the voltage drop is found by multiplying this total wire resistance by the circuit current (\(V_d = I \times R_{\text{wire}}\)).
Interpreting Results and Addressing Excessive Drop
The calculated voltage drop value represents the voltage lost in the circuit conductors before the power reaches the load. This value is often expressed as a percentage of the original source voltage to determine if the loss is acceptable for the application. Industry standards, such as those recommended by the National Electrical Code, suggest that the combined voltage drop for the feeder and branch circuits should not exceed five percent. For a standard 120-Volt circuit, a five percent drop means the load receives 114 Volts.
If the calculated percentage exceeds the recommended limit, the voltage drop is considered excessive and requires correction to prevent poor device performance or equipment failure. The most common solution is to increase the conductor’s cross-sectional area, which means using a larger wire gauge (a smaller AWG number). A thicker wire has less resistance, directly reducing the voltage drop. Other options include reducing the length of the wire run or increasing the supply voltage to lower the current needed to deliver the required power.