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. It is also good to keep in mind that the return conductor will also have a voltage drop associated with it, which is usually the same as the supply conductor. This arrangement reproduces the famous Ohm’s experiment.
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 underground cable also depends on external pressure.
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.
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
^ Walking along the resistive film visualizes voltage drops along various conductive materials
^ National Fire Protection Association, National Electrical Code, 2005 edition Quincy MA, FPN 4 to rule 210.19
^ Rick Gilmour et al., editor, Canadian Electrical Code Part I, Nineteenth Edition, C22.1-02 Safety Standard for Electrical Installations, Canadian Standards Association, Toronto, Ontario Canada (2002) ISBN 1-553246-00-X, rule 8-102
Electrical Principles for the Electrical Trades (Jim Jennesson) 5th edition
See also
Electronics portal
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
Categories: Electricity | Electrical parameters | Cables
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