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How to Calculate Voltage Drop

Voltage drop describes how the 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).

The voltage drop of any insulated cable is dependent upon the cable length (in meters), the required current rating (in amperes) and the relevant total impedance per unit length of the cable. The maximum impedance and voltage drop applicable to each cable at maximum conductor temperature and under a.c. conditions is given in the tables. For cables operating under dc conditions, the appropriate voltage drops may be calculated using the formula.

Calculating voltage drop for a single phase circuit:

Formula for this is given as:

VD = (2 × L × K × I)/CM

In the above formula:

  • VD is the voltage drop across the circuit
  • L is the length of the run
  • K is the resistivity of the wire
  • I is the current across the circuit
  • CM is the measure of diameter of the wire

The values for resistivity and diameter of the wire are available from NEC tables.

How to calculate the voltage drop

Example:

If you want to find the voltage drop across the across a single phase circuit whose length is 200 foot and having a load of 50 A. It is also given that the wire is a copper wire, 3 wire and 120/240 volt.

VD = (2 × 200 × 12 × 50)/41740

(Resistivity of copper is 12)

VD = 5.75 Volts

Therefore, the average voltage drop across the circuit is 5.75 volts.

Now, to calculate the percentage of voltage drop

Percentage of VD

(VD/ V) × 100 = 5.75 volts/240 volts

= 0.0239 %

Therefore, the voltage drop across the circuit having the above dimensions is 0.0239 %. This is less than 3% so you can use it.

Calculating VD for a three-phase conductor:

To calculate the voltage across a three-phase circuit we use the same formula, but multiply the above formula by 0.866. Basically to calculate the voltage drop between any two phase conductors we multiply it by 1.732/2.

Therefore, the formula for calculating for a three phase conductor is given as:

VD  = 0.866 (2 × L × R × I/ CM)

The above formula can also be given as:

VD = 1.732 (L × R × I /CM)

You can calculate the voltage drop across the circuit using the above formula, if you know the values for length of run, resistivity of the conductor, current across the circuit and measure of the diameter and change the dimensions of the conductor if necessary to change the VD across the circuit.