Voltage drop

Voltage drop is the reduction in voltage in the passive elements (not containing sources) of an electrical circuit. Voltage drops across conductors, contacts, connectors and source internal resistances are undesired as they reduce the supplied voltage while voltage drops across loads and other electrical and electronic elements are useful and desired.

In electrical wiring, national and local electrical codes may set guidelines for maximum voltage drop allowed in a circuit conductors, to ensure reasonable efficiency of distribution and proper operation of electrical equipment (the maximum permitted voltage drop varies from one country to another)[1]. 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 conductor between the source and the load which lowers the overall resistance. The more sophisticated techniques use active elements to compensate the undesired voltage drop.

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.

The local voltages along a long line decrease gradually from the source to the load

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. Voltage drop exists in both supply and return wires of a circuit.

A principle known as Kirchhoff's circuit laws 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 $E=IZ$, analogous to Ohm's law for direct current circuits.

  ==Voltage drop in building 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 has become in force since 1 July 2008:

Type of Supply	Voltage drop lighting	Voltage drop other
DNO	3%	5%
Private	6%	8%

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.

Note: voltage drops for installations are specified from the point of common coupling (i.e. where the utility supply is connected to the premises) to the point at which the electrical load is connected

How to Calculate Voltage Drop

In situations where the circuit conductors span large distances, it is necessary to calculate the voltage drop in order to determine if the circuit's voltage will be maintained over the long distance. If the voltage drop is too great, the circuit conductor must be increased (up-sized) in order to maintain the current between the points. The calculations for a single phase circuit and a three phase circuit differ slightly. Listed below are the voltage drop calculations for the two:
Single Phase Voltage Drop Calculation:

File:Voltage Drop Calc - Single Phase.png


Three Phase Voltage Drop Calculation:

File:Voltage Drop Calculation - 3 phase.png


VD = The Voltage Drop (conductor temp of 75°C) in volts

VD% = The percentage of voltage drop (VD ÷ source voltage x 100). It is this value that is commonly called "voltage drop" and is cited in the NEC 215.2(A)(4) and throughout the NEC.

L = One way length of the circuit's feeder (in feet)

R = Resistance Factor per NEC Chapter 9, Table 8, in Ohm/Ft

I = Load Current (in Amperes) Source Voltage = The voltage of the branch circuit at the source of power. Typically the source voltage is either 120, 208, 240, 277, or 480.

Important Note: According to NEC 215.2(A)(4) fine print note No. 2, the voltage drop for feeders cannot exceed 3% and the voltage drop for branch circuits cannot exceed 5%.

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.

References

• Electrical Principles for the Electrical Trades (Jim Jennesson) 5th edition

See also

• Electrical distribution
• Electrical resistance
• Kirchhoff's voltage law
• Electrical conduction
• Ground loop (electricity)
• Power cable
• Mesh analysis

External links

• Freeware NEC Tables, CEC Tables and Electrical Calculation for your PC
• Voltage Drop Calculator
• EPRI Power Quality - Voltage Drop Calculator
• Voltage drop of British mains cables
• BS 7671:2008 Cable Sizing Tool