Voltage Drop Guide

Let's face it; you've probably never heard of the term "Voltage Drop", and if you have, it isn't something that crosses your mind every day. Did you realize that by taking just a few short minutes to do this calculation you could increase the operating life of your equipment? Misconceptions are common when it comes to voltage drop, but learning why it's important and how to calculate it could end up saving you time, money, and heartache in the long run.

When in the process of choosing a cable for a specific circuit, a common standard used to determine the cable type and size is ampacity. Ampacity refers to the greatest current a conductor can carry. In some situations, opting for a larger conductor size than required is essential. Usually, these instances occur with long runs of electrical wire when a larger conductor size is necessary to carry the existing current because of the excessive voltage drop. Voltage drop can become a major concern when installing long lengths of wire.

In long runs, the total voltage lost between the power supply and the device being powered can be considerable. In short circuits, the voltage drop will not be a major issue because the total voltage lost is not substantial. For example, with residential buildings wired circuits are typically not long enough for voltage drop to be a factor of consequence.

Some people are under the impression that the NEC (National Electric Code) requires you to size conductors to accommodate voltage drop. Although the NEC recommends that the maximum on the branch circuit should not exceed 3%, this is only a recommendation and is not mandatory. Having a voltage drop is essential since not doing so could be detrimental to the life of your equipment.

Because many factors need to be computerized, figuring out a precise voltage drop for a circuit is often a long and difficult process. There is a shorter, although less exact, method using charts provided by American Insulated Corp. In order to calculate the voltage drop, multiple the length of one conductor by the current and by the figure in the table for conductor size, circuit type, and power factor. Then, divide by 1,000,000. The number produced is the estimated voltage drop. If the projection is larger than 3%, you should consider selecting a larger conductor size.

Even though the NEC does not mandate a maximum voltage drop allowed in a branch circuit, the voltage drop between input and output connections can be important. Keeping the voltage drop to a minimum proves to be a wise, cost-effective resolve. Sizing your circuit conductors properly will limit voltage drop, help cut voltage waste, and ensure the competent operation of your equipment.

For more information, please see the voltage drop charts below by the American Insulated Wire Company.