Voltage Drop For Cable takes place due to current flow through it in presence of resistance. We shall understand the phenomenon in detail and the process of calculating the voltage drop in this article.
When a difference in voltage is caused between two points, voltage “drops”. Potential drops in an electrical wire when the potential at the end of the cable is less than that at the start of the cable. The resistance or impedance of the cable and physical characteristics accounts for the voltage drop for cable.
What is voltage drop in cable?
Voltage drop in a cable is a result of the two end’s potential difference. The measure of this quantity varies from DC to AC. In DC, simply the product of current and resistance gives the voltage drop value.
Following is the step by step method to calculate voltage drop of the cable:
- Take the value of the current flowing through the cable.
- Multiply twice the length of the cable with the quantity obtained as the voltage drop happens for the return path as well
- Divide the result by 100 for millivolts to volt conversion
Voltage drop calculation formula for cables:
The base formula for calculating the voltage drop for cables is ohm’s law. Further, we need to consider some related points as only ohm’s law is not enough for the accurate voltage drop calculation.
The voltage drop calculation formula for single phase and three phase cables are given in the table below-
|DC||Voltage drop = Current × Resistance (for DC)|
|Single phase AC||Voltage drop = Current × (2 × Length of the wire × Resistance / 1000)|
|Three phase AC||Voltage drop = √3 × Current × (2 × Length of the wire × Resistance / 1000)|
Voltage drop for armoured cable:
Armoured cable or steel/Aluminum wire armoured (SWA/AWA) cable is a 3-core highly protective electrical cable. The armour is designed to provide added protection against mechanical stress, high loads etc.
Armoured cables are used in underground purposes, power networks. These are available in 11 kV and 33 kV cables. The process of calculating voltage drop for steel or aluminum armoured cable is the same as the general cables. Maximum permissible voltage drop is 3% for lighting circuits and 5% for other circuits.
Voltage drop for control cable:
The control cables are often called by their application, such as the supply cable, car cable and robot cable. They are of three types- CY, YY and SY. The voltage drop percentage for a control cable should not exceed 1.5%.
Control cables are utilized for a wide range of applications such as automatic industrial processes like the transmission, calibration and control of signals. As they are another special form of electrical cables, the voltage drop calculation is the same for them.
Permissible voltage drop for cable:
Permissible voltage drop for cable in India varies with the nature of the place. The percentage is higher in urban or semi-urban areas than rural areas due to rural areas requiring long distance transmission.
We know, voltage drop increases with cable length. So, the maximum allowed voltage drop in rural sector cables is 3% of the supply. This percentage is 5% for suburbs and 6% for cities as they are nearer to power stations. Acceptable limit of voltage drop is 2% for a lighting circuit and 5% for an industrial circuit.
Voltage Drop For Cable- Frequently Asked Questions (FAQs)
What are the effects of voltage drop for cable?
The resistance or impedance triggers the voltage drop for cable and the other passive elements such as contacts, cable connectors affect the voltage drop. Longer the cable, more significant the voltage drop.
Effects of voltage drop for cable are-
- Heaters stop heating properly
- Motors attached to the circuit run slowly and sometimes get burnt
- Lights become dimmer
We can simply lower the amount of voltage drop, increasing the diameter of the cable between the supply and the load as it puts down the net resistance.
Why is there less voltage drop in higher size cables?
The cross-sectional area of a higher size cable is higher with respect to a small size cable. For this reason, the resistance of the higher size cable decreases and the voltage drop also comes down.
Therefore higher size cables have less resistance than the small size cables. But, the voltage drop increases with the length of the electrical cable. In power distribution systems, if the systems use a very high voltage, it is possible to transmit a certain amount of power by a small voltage drop.