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Voltage Drop Calculations

Voltage drop is defined as when supplied energy of a voltage source is reduced as electric current moves through the passive elements (elements that do not supply voltage) of an electrical circuit. Voltage drops across internal resistances of the source, across conductors, across contacts, and across connectors are undesired; supplied energy is lost (dissipated). Voltage drops across loads and across other active circuit elements are desired; supplied energy performs useful work. Recall that voltage represents energy per unit charge. For example, an electric space heater may 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 approximately 2% of the supplied voltage is lost in the wire itself. Excessive voltage drop may result in unsatisfactory operation of, and damage to, electrical and electronic equipments.

National and local electrical codes may set guidelines for the maximum voltage drop allowed in electrical wiring, to ensure efficiency of distribution and proper operation of electrical equipment. The maximum permitted voltage drop varies from one country to another. In electronic design and power transmission, various techniques are employed 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. More sophisticated techniques use active elements to compensate for the undesired voltage drop.

Voltage drop in alternating-current circuits: impedance

In alternating-current circuits, opposition to current flow does occur because of resistance (just as in direct-current circuits). Alternating current circuits also present a second kind of opposition to current flow: reactance. This "total" opposition (resistance "plus" reactance) is called impedance. The impedance in an alternating-current circuit depends on the spacing and dimensions of the elements and conductors, the frequency of the alternating current, and the magnetic permeability of the elements, the conductors, and their 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. 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 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.

Calculating Voltage Drop

In situations where the circuit conductors span large distances, the voltage drop is calculated. If the voltage drop is too great, the circuit conductor must be increased to maintain the current between the points. The calculations for a single-phase circuit and a three-phase circuit differ slightly.

Single-phase voltage drop calculation:

VD = [ 2 x L x R x I ]/1,000
VD% = [ VD/Source Voltage] x 100

Three-phase voltage drop calculation:

VD = [( 2 x L x R x I)/1,000] x .866
VD% = [ VD/Source Voltage] x 100


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

VD% = 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/kft

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 V.

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User Reviews/Comments:

By: JD - Jan 6, 2014
can u explain the logic of multiplying 0.866 in 3phase ckts

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