What is Voltage Drop?
One of the basic principles of electrical engineering is Ohm’s law, which states that the voltage drop across a conductor or load is equivalent to the product of current and resistance (V = I x R). The electric current is determined by the load on a circuit, while resistance is determined by the physical properties of the conductor.
How Can Voltage Drop Be Controlled?
Since there is no perfect conductor and all materials have electrical resistance, it is impossible to eliminate voltage drop completely. However, there are many ways to minimize it:
- Improving system efficiency
Assuming the load stays the same, increasing the efficiency of electrical equipment reduces power consumption. Since the supply voltage is constant, improved efficiency results in less current and a reduced voltage drop. - Troubleshooting
Some electrical issues cause an unnecessary increase in current or resistance, which leads to a higher voltage drop. Once these issues are solved, the voltage drop returns to normal. - Correcting conductor sizes
If the conductors in a circuit are not selected properly, they can experience a significant voltage drop. When selecting conductors, it is important to account for factors such as full-load current, ambient temperature, and the number of conductors in a raceway. - Centralized electrical distribution
If the main electrical shaft and distribution boards are located close to the center of a building, wiring must cross smaller distances to reach the different loads. This type of layout minimizes voltage drop. On the other hand, when the electrical shaft and panels are located at one end of the building, circuits must cross the entire construction to reach loads on the opposite side. - Balanced load distribution
Large commercial buildings typically use three-phase circuits, which have three live conductors as implied by their name. If one phase is too highly loaded, it will also experience a larger current and increased voltage drop compared with the other phases.
These are specific measures that can be deployed to reduce voltage drop. In general, any measure that accomplishes either of the following effects is viable, as long as it is allowed by the NYC Electrical Code:
- Decreasing load current
- Increasing conductor diameter
- Increasing the number of parallel conductors
- Decreasing conductor length
- Decreasing conductor temperature
Summary
The NEC recommends a maximum voltage drop of 5% across feeders and branch circuits, and 3% across the branch circuit alone. This voltage drop level is considered to offer the right conditions for optimal equipment performance. Note that the maximum allowable voltage drop level is not a safety measure, but a performance measure.
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