Voltage drop
From Wikipedia, the free encyclopedia
Voltage drop is the reduction in voltage in an electrical circuit between the source and utilitization device. Voltage drop, which is present in all electrical circuits powering any device, must be considered to varying degrees in circuit design. 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.
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[edit] Fundamentals of voltage drop
A current flowing through the non-zero resistance of a practical conductor necessarily produces a voltage across that conductor. 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 dc resistance of the conductor depends upon the conductor's length, cross-sectional area, type of material, and temperature. 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.
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, and the intended load (the space heater) may be receiving an undervoltage. While electrical equipment is designed to operate over a range of voltages, excessive undervoltage may result in loss of performance, overload damage to electric motors and loss of operating efficiency.
[edit] Voltage drop in direct current circuits
A voltage drop is produced when a current flows through a conductor. The voltage measured from the supply end of a conductor (or one of many conductors in a circuit) to the load is called voltage drop. Voltage (and voltage drop) is typically measured in volts, current in amperes and impedence in ohms.
The relationship among these values in direct current circuits is expressed in the equation <math>V = I R</math> (Ohm's Law). The resistance of the conductors will vary with operating temperature.
[edit] Voltage drop in alternating current circuits
Alternating current is a current that continually reverses direction in a circuit in sinusoidal fashion. The current used in the distribution systems of the United States alternates at 60 cycles per second (60 Hertz), while those in other parts of the world may alternate at 50 cycles per second. The voltage drop in an alternating current (AC) circuit is the product of the current and the impedance (Z) of the circuit. Impedance is analogous to resistance; impedance takes into account, however, the additional electromagnetic properties involved with alternating current loads. Electrical impedance, like resistance, is expressed in ohms and opposes current flow in a circuit. 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 <math>E = I Z</math>, analogous to Ohm's law for direct current circuits.
[edit] Minimizing voltage drop in power transmission
Great distances often occur in electric power transmission. Any power generated, but not delivered to the customer, is a financial loss. Power is lost in the conductors throughout the entire length of the transmission lines. One (very expensive) way of reducing the lost power is to increase the conductor size and thus reduce the net resistance. Another way to minimize power lost because of voltage drop is to increase the voltage. This reduces the current for a given power transmission and hence the attendant voltage drop and power loss. However, the high voltages cause many problems; eg: corona losses, insulation breakdown of supports, magnetic influence to nearby objects, physical hazards. When we compare the two formulae for power(<math>W = I E</math> and <math>W = I^2 R</math>) it becomes evident that for a given amount of power transmitted, both the voltage drop and power loss are reduced when the current is decreased while the resistance remains constant. It is for this reason that utility supplied electrical mains are often at a very dangerous tens of thousands of volts; should the electricity have been transmitted to customers at the nominal end user voltage, the size, cost, and weight of the necessary conductors would be enormous.
[edit] Voltage drop in household wiring
In household wiring good design requires that wire size be sufficient to keep power dissipation within limits so that the wiring will not be overheated. In a given wire, the power dissipation is a function of the current. The maximum safe current for a given conductor is known as ampacity. Ampacity is independent of the length of the conductor and the supplied voltage; it has only to do with conductor composition (for example, copper or aluminum), the area of the conductor, and ambient conditions (such as insulating materials on the wire, hottest ambient temperature along the wire, the temperature and voltage rating of the insulation in the given environment, the geometery of the installation, and so on). The circuits are protected by overcurrent devices to prevent exceeding the rated ampacity. However, this is only the first consideration when selecting conductor sizes for applications.
The second consideration, often neglected by well-meaning homeowners installing their own electrical circuits, is the voltage drop for a given circuit and load. As already discussed in this article, voltage drop through a conductor depends in part upon the total net resistivity of the conductor, which in turn depends upon the total length of the conductor. Circuit loads (toasters, televisions, and so forth) have supplied to them a voltage equal to the originally induced voltage at the circuit panel (nominally 120V in North America or 220V in Europe) minus the voltage drop across the supplying conductor. The National Electric Code specifies that no more than 5% voltage drop shall be permitted in feeder + branch circuit wiring. While for a lightbulb a large voltage drop will result in a harmless condition of slightly less bright light being produced, incorrect voltages induced onto delicate circuitry (as for example in a DVD player, computer, and so forth) may quite easily result in an electrically damaging condition. It is quite easy to have a circuit well within the ampacity guidelines for its wiring, but whose voltage drop is too large.
For these reasons, it is wise to size wiring not only for the total current to be drawn, but also to insure that the net voltage drop on the branch | single circuit conductor shall not exceed 3%, or 5% total for a feeder and the branch. This is particularly the case when running long lengths of wire from one end of a house or to an outbuilding. An outbuilding/shed at twenty meters from your circuit panel to which you intend to draw ten amps should not, for example, be serviced by 14 AWG wire (whose ampacity is nonetheless well over your ten amp intention) due to the voltage drop. In such cases, it is wise, to use larger, more expensive wire.
To measure the voltage drop in a household circuit, remove all loads from the circuit, and measure the supplied voltage with a (preferably digital) volt meter. Then, plug in a household appliance which draws 10-15 amps of purely resistive load (such as a toaster, hair dryer, or space heater) and measure the voltage now present at the outlet while such a device runs. A voltage drop shall be clearly visible; if the drop exceeds 3%, your voltage drop is higher than the commonly accepted limit. For example, if an unloaded outlet shows 118V dropping to just under 117V when a space heater is applied, your voltage drop is within acceptable limits. If the same circuit shows 110V when loaded, your voltage drop is excessive.
[edit] The National Electric Code
In the United States the National Electric Code (NEC) specifies the wiring types and limitations for a variety of conditions.
[edit] See also
- Electrical distribution
- Electrical resistance
- Ohm's law
- Kirchhoff's voltage law
- Electrical conduction
- Ground loop (electricity)
- Power cable
