Voltage drop
Voltage drop is the reduction in voltage in an electrical circuit between the source and load. In electrical wiring national and local electrical codes may set guidelines for maximum voltage drop allowed in a circuit, to ensure reasonable efficiency of distribution and proper operation of electrical equipment (the maximum permitted voltage drop varies from one country to another).
Voltage drop  may be neglected when the impedance of the interconnecting conductors is small relative to the other components of the circuit.
For example, an electric space heater may very well have a resistance of ten ohms, and the wires which supply it may have a resistance of 0.2 ohms, about 2% of the total circuit resistance.  This means that 2% of the supplied voltage is actually being lost by the wire itself.
Excessive voltage drop will result in unsatisfactory operation of electrical equipment, and represents energy wasted in the wiring system. Voltage drop can also cause damage to electrical motors.
In electronic design and power transmission, various techniques are used to compensate for the effect of voltage drop on long circuits or where voltage levels must be accurately maintained.  The simplest way to reduce voltage drop is to increase the diameter of the cable between the source and the load which lowers the overall resistance.

# Voltage drop in direct current circuits
A current flowing through the non-zero resistance of a practical conductor necessarily produces a voltage across that conductor.  The dc resistance of the conductor depends upon  the conductor's length, cross-sectional area, type of material, and temperature.
If the voltage between the conductor and a fixed reference point is measured at many points along the conductor, the measured voltage will decrease gradually toward the load. As the current passes through a longer and longer conductor, more and more of the voltage is "lost" (unavailable to the load), due to the voltage drop developed across the resistance of the conductor.  In this diagram the voltage drop along the conductor is represented by the shaded area. The local voltages along the line decrease gradually from the source to the load. If the load current increases, the voltage drop in the supply conductor also increases. This arrangement reproduces the famous Ohm's experiment.
A principle known as Kirchoff's Law states that in any circuit, the sum of the voltage drops across each component of the circuit is equal to the supply voltage.

# Voltage drop in alternating current circuits
In alternating current circuits, additional opposition to current flow occurs due to the interaction between electric and magnetic fields and the current within the conductor; this opposition is called "impedance". The impedance in an alternating current circuit depends on the spacing and dimensions of the conductors, the frequency of the current, and the magnetic permeability of the conductor and its surroundings.
The voltage drop in an AC circuit is the product of the current and the impedance (Z) of the circuit.  Electrical impedance, like resistance, is expressed in ohms.  Electrical impedance is the vector sum of electrical resistance, capacitive reactance, and inductive reactance.  The voltage drop occurring in an alternating current circuit is the product of the current and impedance of the circuit. It is expressed by the formula E = I Z, analogous to Ohm's law for direct current circuits.

# Voltage drop in household wiring
Most circuits in a house do not have enough current or length to produce a high voltage drop. In the case of very long circuits, for example, connecting a home to a separate building on the same property, it may be necessary to increase the size of conductors over the minimum requirement for the circuit current rating. Heavily-loaded circuits may also require a cable size increase to meet voltage drop requirements in wiring regulations.
Wiring codes or regulations set an upper limit to the allowable voltage drop in a branch circuit. In the United States, the 2005 National Electrical Code (NEC) recommends no more than a 5% voltage drop at the outlet.. The Canadian electrical code requires no more than 5% drop between service entrance and point of use.  UK regulations limit voltage drop to 4% of supply voltage. Following changes to the BS7671:2008 on consumers' installation, the following will become in force on 1st July 2008:
Voltage drop of a branch circuit is readily calculated, or less accurately it can be measured by observing the voltage before and after applying a load to the circuit. Excessive voltage drop on a residential branch circuit may be a sign of insufficiently sized wiring or of other faults within the wiring system, such as high resistance connections.

# Using higher voltages
Over long distances, larger conductors become expensive, and it is preferable to redesign the circuit to operate at a higher voltage.  Doubling the voltage halves the current required to deliver the same amount of power, halving the voltage drop, and an additional doubling in efficiency is realized because that drop is a smaller fraction of the total voltage.
This is the motivation for commercial high voltage electrical power distribution, and for the use of the +12V power supply rail for high-power loads in modern personal computers.