Knowledge Base
Everything you need to know about voltage drop limits, calculations, and cable sizing for safe electrical installations.
Voltage drop is one of the most important factors to consider when selecting an electrical cable, especially for long cable runs and high-current loads.
A cable has electrical resistance. When current flows through the cable, some voltage is lost along the way, so the voltage available at the equipment end can be lower than the voltage at the supply end.
If the voltage drop becomes too high, equipment may not perform properly, motors may have difficulty starting, and the system can become less efficient.
This guide explains what voltage drop is, how to calculate voltage drop in a cable, what factors affect it, and how to select a cable while keeping voltage drop under control.
Voltage drop is the reduction in voltage that occurs as current flows through an electrical cable.
For example, if you have 230 V at the supply end and only 222 V reaches the equipment, the voltage drop is:
230 V − 222 V = 8 V
Voltage drop is a normal electrical phenomenon. The objective during cable selection is to keep it within an acceptable level for the particular installation.
A cable may have sufficient current-carrying capacity and still have excessive voltage drop if the cable run is very long.
High voltage drop can result in:
This is why cable selection should not be based only on current capacity. Cable size, cable length, load current, conductor resistance and the type of load all need to be considered.
The main factors are:
For a simple calculation where conductor resistance is the main factor, the voltage drop can be estimated using:
Single-phase circuit
Voltage Drop (V) = 2 × I × L × R / 1000
Where:
The factor 2 accounts for the outgoing and return conductors.
Three-phase circuit
For a balanced three-phase system:
Voltage Drop (V) = √3 × I × L × R / 1000
The √3 factor comes from the relationship between line and phase quantities in a balanced three-phase system.
These formulas are useful for a straightforward resistance-based calculation. For more detailed AC design work, the effect of power factor and reactance should also be considered.
For AC circuits, voltage drop can be calculated using both conductor resistance and reactance.
For a single-phase circuit:
ΔV = 2 × I × L × (R cosφ + X sinφ)
For a balanced three-phase circuit:
ΔV = √3 × I × L × (R cosφ + X sinφ)
Where:
This approach is more appropriate where the load has a significant power-factor component or where cable reactance cannot be ignored.
Once you know the voltage drop in volts, calculate the percentage as:
Voltage Drop (%) = (Voltage Drop / Supply Voltage) × 100
Example
Suppose:
Then:
Voltage Drop % = (6 / 230) × 100
= 2.61%
So the voltage drop is approximately 2.6%.
Yes, significantly.
If all other conditions remain the same, increasing the cable length increases the voltage drop.
For example, a cable carrying the same current over 20 metres will normally have much less voltage drop than the same cable carrying the same current over 100 metres.
This is why a cable size that works perfectly well for a short connection may not be suitable for a long-distance installation.
Yes.
A larger conductor generally has lower resistance, so it produces less voltage drop for the same current and cable length.
For example, if a particular cable size results in excessive voltage drop, increasing the conductor cross-sectional area is one of the common ways to reduce it.
However, don’t select a cable only by voltage drop. The cable must also satisfy current-carrying capacity, short-circuit, installation and applicable standard requirements.
No.
These are two different considerations.
Current-carrying capacity tells you how much current a cable can carry under specified installation conditions without exceeding its permissible operating temperature.
Voltage drop tells you how much voltage is lost between the supply and the load.
A cable selection should satisfy both requirements.
This is one reason simply saying “this cable can carry 20 A, so it is suitable” is not always enough.
There is no single percentage that applies to every installation and every application.
As a commonly referenced guideline, IEC 60364-5-52 gives maximum voltage-drop values of 3% for lighting and 5% for other uses for low-voltage installations supplied directly from a public LV distribution system. Different limits can apply depending on the type of supply and installation.
In India, IS 732:2019 – Code of Practice for Electrical Wiring Installations is the relevant Indian installation standard and includes voltage-drop requirements.
Always check the applicable project specification, installation standard and equipment manufacturer’s requirements before finalizing a design.
Motors can be particularly sensitive to voltage drop.
During starting, a motor can draw substantially higher current than during normal operation. This temporary high current can create a larger voltage drop.
If the voltage at the motor terminals becomes too low, the motor may have difficulty starting or may not develop the required torque.
For motor circuits, voltage drop during both normal operation and starting conditions may need to be considered.
Not accurately.
SQMM tells you the conductor’s nominal cross-sectional area, but voltage drop also depends on:
IEC 60228 specifies nominal conductor cross-sectional areas along with requirements relating to conductor construction and resistance.
So, “bigger SQMM = less voltage drop” is generally true, but SQMM alone is not enough to calculate the actual voltage drop.
For a proper calculation, use the applicable conductor resistance value for the cable being considered.
Conductor resistance is commonly expressed in:
Ω/km (Ohms per kilometre)
For standard cable conductors, resistance values are specified in relevant standards. IEC 60228, for example, specifies conductor resistance requirements for standardized conductor sizes and constructions.
For an actual cable design, it is better to use the manufacturer’s published electrical characteristics rather than relying only on a theoretical resistance calculation.
Yes.
Conductor resistance increases as conductor temperature rises.
Therefore, the resistance of a conductor under operating conditions can be higher than its resistance at 20°C.
This matters particularly when calculating voltage drop for cables carrying high continuous loads.
For this reason, professional cable calculations should use appropriate resistance data and temperature considerations rather than treating resistance as a fixed number in every situation.
There are several options:
In most practical installations, increasing the conductor size is the simplest way to reduce voltage drop when the cable route and load cannot be changed.
Voltage drop itself is not the same thing as cable overheating.
However, both are related to current flowing through the resistance of the conductor.
The power loss in a resistive conductor is related to I²R, so higher current and higher resistance can produce greater heat.
A cable therefore needs to be selected considering both current-carrying capacity and voltage drop.
Before calculating voltage drop, collect these basic details:
Once these values are available, the voltage drop can be calculated and compared with the applicable design limit.
The calculation is different because the electrical relationships are different.
For a simplified resistance-based calculation:
Single Phase:
ΔV = 2 × I × L × R
Three Phase:
ΔV = √3 × I × L × R
When using R in Ω/km and L in metres, divide the result by 1000.
For accurate AC calculations, the R/X and power-factor terms should also be included.
Yes, a voltage-drop calculator can be very useful for quick design checks.
However, you should understand what information the calculator is using.
A good calculator should account for factors such as:
The calculator is a tool; it does not replace proper engineering judgement or checking against the applicable standards.
The most important point is this:
Don’t select an electrical cable based only on SQMM or current rating.
A proper cable selection should consider:
Current capacity + Voltage Drop + Short-Circuit Requirement + Installation Conditions + Applicable Standards
For long cable runs, voltage drop can become one of the deciding factors in selecting the conductor size.
A slightly larger cable may cost more initially, but it can provide better voltage performance and reduce losses over the life of the installation.
For a simple resistance-based calculation:
Single Phase
ΔV = 2 × I × L × R / 1000
Three Phase
ΔV = √3 × I × L × R / 1000
Then:
Voltage Drop % = (ΔV / Supply Voltage) × 100
For detailed AC calculations, include conductor reactance and power factor.
The formulas on this page are intended to help explain the basic method of voltage-drop calculation. Actual cable selection should be carried out using the relevant cable standard, installation requirements, manufacturer’s electrical data and project specifications.
For Indian electrical installations, refer to the applicable requirements of IS 732:2019 and other relevant standards. IS 8130:2013 covers conductors for insulated electric cables and flexible cords and includes conductor resistance testing requirements.
For international applications, the applicable IEC or local national standards should be followed.