Voltage Drop

PICKING THE RIGHT CABLE FOR THE LONG RUN

Reliability may not be a tangible item that’s installed alongside a new furnace or wired into a dock-side crane, but it is nonetheless an essential “accessory” that can mean the difference between overtime and lost time; in-stock and out-of-stock; perfect fits and refits. Being labeled “unreliable” can mean ruin for a business, regardless of what you’re making, installing or servicing. That’s why it is vital to understand simple, yet often overlooked problems, like voltage drop in product applications.

What is Voltage Drop?

Voltage drop is the reduction in voltage in an electrical circuit between the source and the load. Wires carrying electricity have inherent resistance, or impedance, to current flow. Voltage drop is the amount of voltage loss that occurs through a circuit due to this impedance.

For equipment to operate properly, it must be supplied with the right amount of power, which is measured in watts which is calculated by multiplying current (amps) x voltage (volts). Motors, generators, tools — anything that runs on electricity — is rated for power. The correct amount of power enables equipment to meet its designed power rating and operate efficiently. Too much or insufficient amounts of power can result in inefficient operation, wasteful power usage, and even equipment damage. That is why understanding voltage drop calculations and selecting the correct cable for each application is so important.

The National Electrical Code (NEC) catalogs the requirements for safe electrical installations and represents the primary document for guidance in the United States. Providing direction for both trained professionals and end users, these codes set the foundation for the design and inspection of electrical installations. So how does the Code treat voltage drop issues? For branch circuits, look to NEC (NFPA 70) Section 215.2(A)(3) footnote 2 and Section 210.19(A)(1) footnote 4. Both advise that conductors for feeders to dwelling units should be sized to prevent voltage drop exceeding 3% and maximum total voltage drop on both feeders and branch circuits should not exceed 5% for “reasonable efficiency of operation.”

In addition, look to NEC (NFPA 70) Section 647.4 (D) when dealing with sensitive electronic equipment. It states that voltage drop on any branch circuit shall not exceed 1.5% and the combined voltage drop on branch-circuit and feeder conductors shall not exceed 2.5%. It is important to note that much of the equipment manufactured today contains electronics that are particularly sensitive to excessive voltage drop.

Ampacity, a cable’s electric current-carrying capacity, is also connected to voltage drop. The Code stresses the importance of accounting for voltage drop when considering a cable’s ampacity rating and the need to satisfy both requirements. NEC Section 310.15 (A)(1) states that ampacity tables do not take voltage drop into consideration.

How is Voltage Drop Calculated?

For DC current, voltage drop is proportional to the amount of current flow and wire resistance. In AC circuits, total impedance and power factor (power loss ratio) also need to be considered. Since wire resistance is a factor of wire size, material and length of run, it is important to choose the proper wire size for the length of run to keep voltage drop at the desired level.

Use the following voltage drop calculation table to simplify your voltage drop calculations.

This table makes calculating project voltage drop straightforward and easy. For example, let’s say your project involves a 100-foot run of 12/3 SOOW wire, 12 amps line current for equipment, line circuit of 120 volts AC, 3 phase, 100% power factor. According to the calculation table, the factor is 3190. Next, multiply current x distance (feet) x factor: 12 x 100 x 3190 = 3,828,000. Finally, place a decimal in front of the last six figures, and the result is the volts lost, or voltage drop, which equals3.8 volts in this example (3.2% of overall voltage).

So to ensure the reliability of your products, installations, or service calls, be sure to account for voltage drop when selecting cable. While it is primarily a nuisance issue, voltage drop can affect equipment efficiency, power consumption and cause potential damage to sensitive electronics and other systems. Fortunately, these issues are easily avoided, especially when you rely on the NEC codes and standards that relate to voltage drop: each of which provides useful guidance in ensuring the success of your application.

By selecting a cable with the correct voltage drop characteristics, you will optimize the operation of your connected equipment, increase your efficiencies and prevent equipment damage. And that’s a pretty good payoff, in the short term or the long run.