Minimising Severe Voltage Drop In Long Cable runs—The Cost-Efficient Way (Formulas, Calculations & Case Studies)



Electrical installations are bound to uncertainties that subject the connected loads to voltage variations. Amongst them are severe voltage drops that affect the equipment’s functionality, reliability, and safety. Hence, project managers should expect voltage drops in real-life applications and take mitigative actions to address the drawbacks. 


However, the question lies in choosing the age-old method of reducing conductor resistance or adopting modern voltage regulation technologies. 


In this article, we’ll explore why voltage drops happen, relevant calculation formulas, and ways to resolve the issue efficiently. 





Voltage drops describe the inability of an electrical conductor to sustain the same voltage level across its length. It’s an inevitable phenomenon that affects electrical cables. As a result, the voltage that electrical equipment receives falls below the value supplied by the electrical substation.





A voltage drop in an electrical circuit normally occurs when a current passes through the cable. It is related to the resistance or impedance to current flow with passive elements in the circuits including cables, contacts, and connectors affecting the level of voltage drop.



This parameter affects the overall resistance of the cable. Longer cable results in a more significant voltage drop compared to a shorter one as the current traverses a longer path. 


Cables with larger diameters exhibit a slighter voltage drop compared to thinner cables. 


When powering equipment that draws high current, the incoming supply will encounter a more considerable voltage drop, in compliance with Ohm’s Law.


Specific load equipment may cause an in-rush current when activated. The sudden surge in the electrical current will result in a momentary but significant voltage drop across the supply terminal. 


Copper cables, which are more conductive than aluminium, will experience lesser voltage drops across the same distance. 


The cable temperature varies according to the environment and affects its conductivity. 




Regulatory bodies define the allowable tolerance for voltage drops to ensure that electrical installations function optimally. However, the threshold differs according to countries and regions.




In Australia and New Zealand, industries must comply with the AS/NZS3000:2018, which stipulates the tolerance threshold for different installations. 

For example:

  • Clause 1.6.2 in AS/NZS 3000:2018 allows a 6% drop limit of the nominal voltage of 230V. 
  • 3.6.2 of AS/NZS 3000:2018 specifies a 5% drop between a 240V point of supply to any installation points. 


In the UK, the permissible voltage drop is defined by the BS7671 wiring regulations, and it limits voltage drop to 3% for lighting and 5% for other electrical loads.



The Energy Market Authority of Singapore requires all standard distribution networks for 22 kV, 6.6 kV, 400V and 230V to limit voltage variation between 6%.



According to the National Electrical Code, which regulates electrical installations in the US, the allowable permissible voltage drop is not more than 5% for both the feeder and branch circuit.



European countries are subjected to the IEC60364-5-52, which stipulates the following voltage drop tolerances:

  • 3% for lighting circuits and 5% for other uses; for public low-voltage distribution systems.
  • 6% for lighting circuits and 8% for other uses; for private low-voltage supplies. 




While voltage drop is natural, severe occurrences might increase safety risks. Prolonged undervoltage forces electrical equipment to operate continuously below the nominal rating, which causes energy inefficiency and underperformance. Some equipment, such as an AC motor, has a narrow input voltage tolerance. In severe voltage drops, the equipment might overheat and even damage the equipment as it draws more current. 





We list basic voltage drop formulas that help estimate the steady-state voltage received at installation points. They allow project managers to manage expectations when commissioning new installations and the first step is to get an Impedance of the copper cable table.


SINGLE PHASE AC Circuit / 3 Phase AC circuit / DC circuit

Cable Resistance = R = r L/A

Calculate the total current flowing in the circuit = I = V / R

Voltage drop in cable = Current x Cable Resistance = Vd = I x CR 

The above equations are defined by:

  • (L(ft) the resistance of a length of a conductor) 
  • (A = Cross-sectional area) 
  • (r = Specific Resistance )
  • Ohm’s laws V =I x R 

The above formulas provide a basic arithmetical understanding of voltage drop. In real-world installations, these factors are decisive in the voltage received. 

  • The number of conductors per conduit and conductor size.
  • Phase and circuit configurations. For example, voltage drop measurement across phase-to-neutral for a single phase 3-wire layout.
  • Conductor temperature. Voltage drop within 60°C and 90°C is reasonably similar but deviates substantially beyond the range. 





We use this table to help you calculate voltage drops with ease. The table allows you to estimate voltage drop with accuracy based on parameters like

  • Conductor size
  • Type of conduit (steel/aluminium/ nonmetallic)
  • Load power factor.
  • Conductor type (copper/aluminium)


Table: Voltage drop Volts/Amp per 100 Feet for 3 Phase/Phase To Phase




Here’s how to use the voltage drop table.

  1. Get the distance of the cable measured from the supply to the installation point. 
  2. Multiply the cable distance by the load current. 
  3. Divide the result, measured in Ampere-Feet, by 100.
  4. Look up the appropriate voltage drop factor in the table based on the parameters.
  5. Multiply the voltage drop factor with the calculated result to get the actual voltage drop.

For example, you had an installation for a 3-phase AC with these parameters

  • Supply Voltage: 400V
  • Load power factor 0.90
  • Max current: 150 A. 
  • Cable: 200 ft, 3/0 copper conductors, magnetic conduit.

To calculate the voltage drop, 

  1. Multiply the max load current with the cable distance = 150A x 200 ft = 30,000 Amp. Feet
  2. Divide the result by 100 = 30,000/ 100 = 300.
  3. From the table, the voltage drop factor is 0.0158.
  4. The actual voltage drop = 300 x 0.0158 = 4.74 Volt
  5. Expressed in percentage, the voltage drop = 4.74/400 x 100% = 1.185%, which is within the limit of standard electrical wiring regulations.



Besides calculating voltage drop, the above table allows you to estimate an appropriate conductor size to limit the voltage drop for a specific electrical load.

To do that,

  1. Divide the targetted voltage drop by the current across the cable length. The result is expressed in V/A.ft
  2. Then, multiply the result in step 1 by 100.
  3. Based on the conductor, conduit, and power factor, search for the nearest lower value in the table. 
  4. Use the matching conductor size as a reference to seek the appropriate cable. 

For example, you want to improve the above installation’s voltage drop to 3.5 Volt. Here’s the calculation to do so,

  1. 3.5V / (150A x 200ft ) = 0.0001167. V/A.ft
  2. 0.0001167 x 100 = 0.01167
  3. The table shows the closest voltage drop value is 0.0106 with a conductor size of 300 kcmil. 



Understanding voltage drop and its algebraic expression allow project managers to mitigate the issues appropriately. Often, they resort to the conventional method of increasing the cable diameter or using multiple cables to reduce the electrical resistance. There are pros and cons when resolving voltage drops with such an approach.


The obvious advantage is that adding conductors, or choosing a larger cable, will successfully resolve the issue. Fundamental electronic principles dictate that a larger conductive cross-reaction will reduce the resistance of the electrical path. Hence, the electron passes with less friction along the conductive material and reduces wasteful losses.


However, project managers face a couple of challenges when resorting to conventional methods. First, they face the daunting burden of increasing cable costs. The economics of cable price is simple – the more metal that goes into it, the pricier it will be. Second, replacing existing cables, or adding new ones, is complex and expensive. 


Despite the effectiveness, conventional methods are highly disruptive to existing installations. Also, certain industrial premises may discover or experience the issue after operating for months. Power cable replacement requires staggered planned shutdowns over an extensive period, which incur additional productivity losses.




The voltage drop compensator is a smarter solution to the field’s persistent voltage drop issues. Instead of stretching the budget for additional cables, the voltage drop compensator is an economical solution. For example, Singapore Power, the country’s national grid operator, saved $9 million in cable costs by using voltage drop compensators. 


Ashley Edison offers a series of voltage drop compensators capable of mitigating voltage drops of up to 125V. Engineered for reliability and precision, our voltage drop compensators ensure continuous stability regardless of load change, cable extension, and mains grid variation. These robust units are built with overload protection to ensure system stability amidst start-up inrush current.


Besides ensuring nominal steady-state voltage, Ashley Edison’s voltage drop compensators can suppress transient events. This protects electrical loads from abrupt electrical spikes, which might be damaging. Built with redundancy in mind, the voltage drop compensator features two parallel voltage regulation control modules to ensure 24/7 availability. 





Severe voltage drops are a common but manageable phenomenon in electrical installations. Working through formulas and calculations allows project managers to better understand the variables involved.  Without proactive actions, constant exposure to undervoltage might damage equipment and increase safety risks. 


We’ve compared a conventional solution to the one preferred by various industries. While increasing cable size or conductor counts are possible solutions, installing voltage drop compensators proves to be more financially viable and efficient.

Learn more about Ashley Edison’s range of voltage drop compensators here.




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