What is Voltage Drop?


Voltage drop (VD) arises when the voltage decreases at the end of a cable run compared to the starting point. All wires, regardless of length or gauge, exhibit some resistance. Passing a current through this resistance results in a voltage drop. As the cable length increases, so does its resistance and reactance in direct proportion. Consequently, VD becomes a significant concern in lengthy cable installations, such as those in larger buildings or on expansive properties like farms. This approach is frequently employed to appropriately size conductors in single-phase, line-to-line electrical circuits and can be determined using a voltage drop calculator.

Electrical cables, while carrying current, inherently introduce resistance or impedance to current flow. VD is quantified as the voltage loss experienced across part or all of a circuit due to cable impedance, measured in volts.

Excessive VD within a cable’s cross-sectional area can result in flickering or dimming lights, suboptimal heater performance, and elevated motor temperatures, potentially leading to burnouts. This condition compels the load to operate harder with diminished voltage driving the current.

What Factors Determine Voltage Drop?

VD is determined by the following factors:

1. Conductor Material

Different materials have varying electrical conductivity. For example, copper is a better conductor than aluminum.

2. Conductor Diameter

A larger conductor diameter improves conductivity because it provides more material for current to flow through.

3. Conductor Length

Longer conductors have higher resistance because the current must travel a greater distance from the source to the load.

4. Conductor Temperature

Temperature affects material conductivity, with some materials becoming more or less conductive as temperature changes.

5. Current Carried by the Conductor:

Voltage drop is directly proportional to the current carried by the conductor. If the current doubles while resistance remains the same, the voltage drop also doubles.

6. Circuit Connections:

Connections in a circuit introduce contact resistance, and poor connections can lead to increased voltage drop.

Also See: What is Open-Circuit Voltage (Voc)?

How to Calculate Voltage Drop

It’s important to note that the VD formula varies based on the number of phases in the circuit, either single-phase or three-phase. In the equations below, we use the following variables:

  • Z: Conductor impedance (ohms per 1,000 feet or ohms/kft)
  • I: Load current (amperes)
  • L: Length (feet)

Single-phase system: V Drop = 2 ×— Z ×— I ×— L / 1000

Three-phase system: V Drop = 1.73 ×— Z ×— I ×— L / 1000

We divide these formulas by 1,000 because standard impedance values are provided for every 1,000 feet, converting them to ohms per foot. Chapter 9 of the NEC offers conductor properties based on a 75°C temperature rating.

To illustrate the process, let’s assume a single-phase 120-V circuit with a current of 22 A, a conductor impedance of 1.29 ohms per 1,000 ft, and a circuit length of 50 ft. The voltage drop would be:

Voltage drop = (2 ×— 1.29 ohms/kft ×— 22 A ×— 50 ft) / 1,000 = 2.84 V

Percentage VD = 2.84 V / 120 V = 0.0237 = 2.37%

If there are multiple conductors per phase, divide the above calculation by the number of conductors per phase since resistance is reduced. For instance, if there are two conductors per phase in the example above, resistance is halved, resulting in a voltage drop of 1.42 V (1.18%).

Also Read: How to Calculate Voc of Solar Panel

How to Control Voltage Drop

Achieving complete elimination of VD is unattainable because all materials possess inherent electrical resistance. However, several strategies can effectively minimize it:

  • Enhancing Efficiency: Improve equipment efficiency to reduce power consumption and subsequently lower voltage drop.
  • Troubleshooting: Identify and resolve electrical issues causing unnecessary current or resistance increase.
  • Correcting Conductor Sizes: Properly select conductors based on factors like current, temperature, and raceway capacity.
  • Centralized Distribution: Place main electrical components centrally to minimize wiring distances within buildings.
  • Balanced Loads: In three-phase systems, ensure balanced load distribution to prevent uneven voltage drop.

Must Read: What is Nominal Voltage?

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