Voltage drop is a term used to describe any reduction in the supply voltage in a complete electrical circuit. The term may be used to describe a voltage loss across a specific component in the circuit, the voltage loss measured across the entire circuit, or as a broad description of the phenomenon of voltage loss in a circuit in general. All electrical circuits, no matter how simple, present a certain amount of resistance to the flow of electrical current through them. This resistance effectively makes the electrical current work harder, and thus absorbs energy. This expenditure of energy is what causes the reduction in voltage described by the term voltage drop.
For example, a simple circuit can be made up of a 9-volt battery attached to a simple flash light bulb with a small switch. If one were to measure the voltage across the batteries terminals with the switch open, the multimeter reading would be approximately 9 volts. If one were to close the switch and illuminate the bulb, that reading would drop by approximately 1.5 volts. That reduction in voltage is what is known as a voltage drop, and it comes about as the result of the work the battery has to do to illuminate the bulb. Each and every component in a circuit, including the wiring, offers a certain amount of resistance to the flow of electrical current and will cause an associated voltage drop.
In applications that are extremely supply voltage sensitive, such as electronic devices, these voltage losses have to be carefully calculated and the supply voltage adjusted to make provision for them. A 12 volt direct current (DC) power supply, for instance, will typically produce an output of 13.8 volts to accommodate this voltage drop phenomenon. In applications that require very long cable runs, it is common practice to uses fairly heavy cables that present less resistance to the flow of electric current in an attempt to minimize the effects of voltage losses. The total potential loss of voltage in any circuit thus needs to be carefully calculated during the design and specification phase of a project to ensure that the final result meets all requirements.
Any voltage loss in a circuit can, fortunately, be calculated with great accuracy using a voltage drop formula. This makes it possible to achieve consistent and predictable results at the end of an installation. These calculations will differ according to the type of circuit, voltage supply, and components involved and can be extremely complex, often requiring the use of a voltage drop calculator. They do, however, take the guess work of accurately adjusting power supply specifications to accommodate circuit resistance.
Source / Courtesy : WiseGeek