Voltage drop is defined as the amount of voltage loss that occurs through all or part of a circuit due to impedance. Understanding voltage drop is the key to a successful circuit design.

A common analogy used to explain voltage, current and voltage drop is a garden hose. Voltage is like the water pressure supplied to the hose. Current is like the water flowing through the hose.

And the inherent resistance of the hose is determined by the type and size of the hose – just like the type and size of an electrical wire determines its resistance.

Voltage drop (VD) occurs when the voltage at the end of a run of cable is lower than at the beginning. Any length or size of wires will have some resistance and running a current through this resistance will cause the voltage to drop.

As the length of the cable increases, so does its resistance increase in proportion. Hence, VD is particularly a problem with long cables runs, for example in larger buildings or on larger properties such as oil/gas plants.

Electrical cables carrying current always present inherent resistance, or impedance, to the flow of current. VD is measured as the amount of voltage loss which occurs through all or part of a circuit due to what is called cable “impedance” in volts.

Excessive voltage drop in a circuit can cause lights to flicker or burn dimly, heaters to heat poorly, and motors to run hotter than normal and burn out. This condition causes the load to work harder with less voltage pushing the current.

Most Electrical Codes recommend limiting the voltage drop from the breaker box to the farthest outlet for power, heating, or lighting to 3 percent of the circuit voltage.

You can calculate the voltage drop by using any of several accepted voltage drop formulas. Subtract the resulting number from the measured voltage decrease, and you have voltage loss.

Calculations are done by selecting the right size of wire using “Voltage Drop Tables” supplied by various cable manufactures or by computer programs designed for calculating the voltage drop automatically, some can even size the cable for you as well, programs like Electrical Design Management.

The longer your runs get, the lower the voltage is at the point of utilization. But not all the voltage difference may be due to voltage drop.

Voltage drop is not caused by poor connections, bad contacts, insulation problems, or damaged conductors; those are causes of voltage loss.

It’s important to distinguish voltage drop from voltage loss. You can have both voltage drop and voltage loss in any circuit.

Understanding voltage drop and the effects it has on an electrical circuit will help in the correct cable selection for your electrical equipment.