What is voltage drop and why is it important?

• A. Voltage drop is the loss of voltage along a conductor; excessive drop can cause dim lighting and poor performance, so it should meet code limits.
• B. Voltage rise along a conductor under load.
• C. The voltage difference between two phases in a three-phase system.
• D. The change in voltage when a switch opens, which may cause nuisance tripping but is not code-related.

Answer: (A)

Voltage drop is the loss of voltage that occurs as current flows through a conductor because the conductor has resistance (and reactance in AC). As electricity travels from the source to a load, some of the supply voltage is used up fighting that resistance, so the voltage available at the far end is lower than at the source. The longer the run and the smaller the conductor, the greater the drop, especially when the load current is high.

Why this matters: electrical devices and lighting need a certain minimum voltage to operate properly. If the drop is too large, lights can appear dim, motors may run slowly or overheat, and circuits can trip or equipment can misbehave. Codes and good practice require controlling voltage drop so the load still receives adequate voltage at normal operating currents.

In practice, designers look at the total voltage drop along feeders and branch circuits and size conductors or adjust routing to keep it within acceptable limits. Remedies include using larger wires, shortening the run, splitting loads, or supplying at a higher voltage when appropriate. The other descriptions—voltage rise along a conductor, the voltage difference between phases, or a switch opening causing a transient—describe different concepts and do not define voltage drop.