February 27, 2024

Picture this: energy coursing through cables, only to dissipate along the way, leaving behind a diminished voltage. This phenomenon, aptly termed voltage drop, is more than just a technicality—it's a pivotal aspect of electrical engineering with profound implications.

In this article, we’ll be covering why it matters, the main factors influencing voltage drop (such as types of line losses), the mathematical formula for calculating it, and some strategies to minimize voltage drop. Let’s get into it.

Voltage drop (VD) is the lowering of voltage magnitude by the end of a cable run length (or conductor, more generally), and it’s the result of energy wasted along a cable. To check voltage drop, you can use a voltmeter at the beginning and end of a cable. There are three major causes of energy waste along cables, and these are the three types of line losses, which we’ve covered in a separate blog. For your reference, the three types of line losses in an electrical system are:

- Resistive

- Capacitive

- Inductive

Read the blog: Can we eliminate these three types of line losses in transmission systems?

Besides line losses, length of the cable, and its gauge size, also influence voltage drop. As a rule of thumb, the longer the cable and the smaller the gauge, the more voltage drop will occur. Hence, voltage drop is particularly a problem with long cable runs found in larger buildings, farms and, of course, in transmission lines, for example.

Additionally, the type of power travelling along cables influences voltage drop. Alternating current (AC) power suffers from all three types of line losses, while direct current (DC) power only suffers from certain types of resistive line losses. So, if DC power is flowing through a cable, less voltage drop will occur when compared to a cable carrying the same wattage and voltage of AC power. If you’re wondering how much voltage drop occurs in typical AC electrical systems in buildings, the answer is about 2% for branch circuits plus 1% to 2% for feeders, so the maximum branch-circuit voltage drop is typically 2.4 V in a 120 V system and 4.8 V in a 240 V system (according to EEPower). This is why buildings that implement DC power distribution systems suffer from less voltage drop and save energy (and operating costs). We’ll discuss DC power distribution later in this article.

The amount of current travelling in an electrical conductor is another factor that influences voltage drop. To understand why, it’s important to look at the relationship between current, resistance, power loss, and voltage drop. Essentially, the power loss (measured in Watts) is calculated by multiplying the square of current with the total resistance of the conductor. See the image below. Having more current in a cable greatly increases the power lost present in that cable, which increases voltage drop. Thus, it’s beneficial to minimize the amount of current flowing through a cable, more so than decreasing the resistance. This is done by increasing the amount of voltage flowing through it (to maintain the same wattage), which explains why cables carrying higher voltages suffer from less voltage drop and waste less energy than cables carrying lower voltages. Additionally, cables carrying lower voltages generally must be thicker (have a larger gauge) because the extra current needs space to pass through cables with less resistance.

The more voltage drop that occurs in electrical systems in buildings, the higher the cost of electricity in those buildings.

In summary, the primary influences on voltage drop are:

- Cable gauges (measured in AWG and/or mm²)
- Cable lengths
- The distribution of AC power (as opposed to DC power)
- Current at the power source

To understand voltage drop better it’s helpful to understand how it’s calculated, and we can use Ohm’s Law to do this.

The calculation for voltage drop holistically takes the above factors into consideration, including power lost from the three types of line losses, cable gauge size (AWG and/or mm²), cable length, type of power (AC or DC), and the voltage and current at the power source.

The Ohm’s law method to compute voltage drop and may employ the following formula (according to EEPower):

VD = 2 x I x R x L

*Variables Defined: *

VD = Voltage drop (V) in conductors

I = Current (A)

R = Conductor resistance (or impedance in AC circuits) in Ω/km

L = Conductor length, in meters, from the source to the load divided by 1 000

The number 2 reflects the return conductor from the load to the source

If you’d like a further breakdown of how to calculate variables required to calculate voltage drop (such as conductor resistance), feel free to check out this voltage drop calculator on the Cence website. This tool calculates voltage drop along cables carrying DC power, just fill out the cable specifications.

Line losses are the source of voltage drop along power lines and, as we mentioned earlier, there are three types of line losses (resistive, capacitive and inductive). AC power suffers from all three types of lines losses, while DC power only suffers from certain types of resistive line losses. Because they generally suffer from less line losses, cables carrying DC power do not experience as much voltage drop as cables carrying AC power at the same voltage.

Learn more about line losses in the video below.

Maybe you’re wondering why resistive power losses affect both AC and DC power, while DC power evades other types. Resistive power loss occurs when power is lost due to the resistance of a conductor, like a power cable. The truth of the matter is that there are no perfect conductors (except for super conductors). They all have a little bit of electrical resistance, and when electricity meets any resistance, electrical power is converted into thermal power (or heat). This loss of energy in the form of heat is what causes voltages to drop along conductors, especially over long distances. AC power alternates at a specific frequency, while DC power does not. This frequency makes it so that AC power also meets resistance (and voltage drop) from a cable's capacitive and inductive properties.

It’s important to minimize voltage drop as much as possible because of its potential to contribute to carbon emissions, and climate change. For example, even making the switch from high-voltage AC power transmission to DC power transmission can make a big difference in reducing energy waste. EE Power points out that the increased efficiency of high-voltage DC over high-voltage AC reduces losses from 5 - 10% in an AC transmission system to around 2 - 3% for the same application in a DC transmission system. The more voltage drop that an electrical system suffers from, the more electricity must be generated to compensate for it, and generating electricity is a huge contributor to carbon emissions. According to the Government of Canada’s website, “In 2022, about 35% of the world’s electricity came from burning coal”, and coal produces approximately 20% of the world’s greenhouse gas emissions. This is why minimizing voltage drop can make a big difference, and we’ll cover methods of doing so in the next section.

Calculate carbon emissions from electrical usage: kWh to CO2 Calculator

Voltage drop can be minimized either by compensating for it, or by reducing overall energy wasted along cables. Reducing voltage drop comes down to mitigating its main causes, here’s a list of those causes as a refresher:

- Small cable gauges (measured in AWG and/or mm²)
- Longer cable lengths
- The distribution of AC power (as opposed to DC power)
- Lower voltages, and higher currents, distributed along cables

One effective (but expensive) method for compensating for voltage drop is by increasing the thickness of conductors (cable gauge) used in the electrical system. Larger cable gauges have lower resistance, which helps mitigate the effects of resistive line losses, reducing voltage drop in low-voltage applications (as we touched on earlier). When current flows through a larger conductor, there is less opposition to its flow, resulting in reduced resistive line losses and less voltage drop. This approach is particularly useful in situations where the current flow is high, such as in low-voltage applications. By selecting appropriately sized conductors based on the expected current load, the voltage drop can be minimized for more efficient power transmission.

Another practical strategy to compensate for voltage drop is to reduce the distance between the power source and the load. As electricity travels through a conductor, it encounters resistance, leading to voltage drop over longer distances. By minimizing the length of conductors between the power source and the load, the overall voltage drop can be decreased. This can be achieved through careful planning of the layout of electrical systems, locating power sources closer to the loads they supply. Shorter distances translate to lower resistance and subsequently reduced voltage drop, contributing to more efficient energy distribution.

Voltage regulation devices offer another avenue for compensating for voltage drop and maintaining stable voltage levels in electrical systems. Constant current drivers are examples of such devices that help regulate and stabilize voltage levels, thereby reducing the impact of voltage drop. Constant current drivers ensure a consistent current flow, by precisely varying voltage levels. You can read more about constant current drivers, and their benefits for LED lighting, on our blog. Integrating constant current drivers into power systems can effectively mitigate the effects of voltage drop, ensuring reliable and steady voltages, especially in lighting systems.

High voltage power distribution offers a compelling solution to mitigate resistance along cables and minimize voltage drop in electrical systems. By transmitting electricity at higher voltages (whether AC or DC), the current flowing through cables can be reduced to reach the same wattage, as per the formula for power, P = I*V. Lower currents translate to decreased power loss along cables. Moreover, with less current, the effects of voltage drop are significantly diminished, allowing for more efficient energy transmission over longer distances. This approach enables power grids, for example, to deliver electricity to distant locations with minimal loss, ensuring their cost-effective operation. The benefit of high-voltage power distribution lies in its ability to optimize energy transmission, maximizing the efficiency of electrical systems while minimizing environmental impact and operational costs associated with voltage drop. Power grids typically employ high-voltage AC transmission for all these reasons and more. However, as we know, AC power suffers from resistive line losses, as well as capacitive and inductive line losses. This revelation has (in part) led to high-voltage DC power transmission grids becoming more common, as DC power only suffers from certain types of resistive line losses. We’ll cover how DC power reduces voltage drop next.

If you’re looking for the best electrical system to reduce voltage drop, the solution is a DC power system that distributes high-voltages. Not only does DC power suffer from less line losses than AC power when compared at the same voltage, a high-voltage DC system also suffers from less resistive line losses. High-voltage DC power grids have been in use since the 1950s, and now they are implemented whenever their efficiency makes up for their expensive upfront cost (at the break-even distance).

More recently, high-voltage DC power distribution has become available for use in commercial buildings, data centers, telecom equipment and more. This is thanks to the National Electrical Code (NEC) standardizing fault-managed power systems in their 2023 edition, and the UL publishing supporting standards (under UL 1400-1). A fault-managed power system is a DC distribution system that can safely supply up to 450V DC along intelligently monitored cables, and shut power off almost instantaneously if a fault is detected. This intelligent fault-management technology prevents electrical shock, fire, and damage to equipment, as well as allows this high-voltage system to send power along cables that make use of low-voltage wiring practices that don’t require mechanical protection or conduit. The development of fault-managed power systems (or Class 4 rated power systems in the NEC), allows many more applications to take advantage of high-voltage DC, and significantly reduce the occurrence of voltage drop.

Fault-managed power systems have many more benefits, and you can read more about them on our blog: The Next Big Thing in Energy Efficiency: Fault-Managed Power Systems

In conclusion, understanding voltage drop is crucial for optimizing energy efficiency in electrical systems. Voltage drop occurs when energy is wasted along cables, resulting in decreased voltage magnitude at the end of a cable run. Factors influencing voltage drop include cable gauge, length, current at the power source, and the type of power distributed. By recognizing the primary causes of voltage drop and employing effective strategies to compensate for it, such as increasing conductor size, shortening cable distances, distributing DC power, using higher voltages, and utilizing voltage regulation devices, the detrimental effects of voltage drop can be minimized.

High-voltage DC power distribution emerges as the optimal solution for reduced voltage drop, as it significantly reduces power lost along cables. With recent developments like this, there is hope to significantly minimize voltage drop and energy loss from electrical distribution and transmission.

Cence offers a high-voltage DC (Class 4) Fault-Managed power system that’s perfect for telecom, datacenter, and lighting applications. Learn more about it on our website: cencepower.com

We improve the value of commercial and multifamily buildings with an intelligent DC power distribution system that's pain-free to install. It combines the benefits of low-voltage wiring practices with voltage capabilities of up to 450 Volts DC.