Voltage drop

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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.

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.

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.

1 Voltage drop in direct current circuits
2 Voltage drop in alternating current circuits
3 Voltage drop in household wiring
4 See also
5 External links

[edit] 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.

Fig. 1: The local voltages along the long line decrease gradually from the source to the load

The local voltages along the long line having a resistance R_l may be measured, in order to build the voltage distribution (the voltage diagram). In this diagram the voltage drop along the conductor is represented by the shaded area. The local voltages along the line decrease gradually (linearly) from the source to the load. Actually, this arrangement reproduces the famous Ohm's experiment [1].

If the load resistance R_L varies, the current I varies as well. As a result, all the local voltages along the line vary proportionally. The right end of the voltage diagram moves vertically while the left end is fixed at V_IN level; the output voltage decreases - V_OUT = V_IN - V_Rl.

[edit] 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.

[edit] Voltage drop in household wiring

Circuits wired within a residential building usually are not long enough or heavily-loaded enough to make voltage drop a factor in selection of wiring. 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. Wiring codes or regulations may set an upper limit to the allowable voltage drop in a branch circuit.

Voltage drop in a branch circuit 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.