Table of Contents
Voltage Drop in Electrical Design
Voltage drop is a key constraint in electrical cable design. Excessive voltage drop can cause equipment to operate outside its rated voltage range, resulting in poor performance, overheating, or motor failure. Electrical standards, therefore, specify maximum voltage drop limits that must be met when selecting conductor sizes.
Accurate voltage drop calculations must consider conductor resistance at operating temperature, cable reactance, load current, and power factor. Simplified methods often ignore these factors, leading to oversized cables.
The engineering considerations in this article are based on over 20 years of experience performing advanced cable calculations for practical installations, including motor circuits, long cable runs, solar systems, and fire-survival circuits.
If you prefer to perform voltage drop and cable sizing calculations automatically, you can use our free cable sizing calculators. These tools calculate current-carrying capacity, voltage drop, and short-circuit performance, and automatically select compliant active, neutral, and earth conductor sizes in accordance with applicable standards and installation conditions.
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1. Accurate Voltage Drop Formula
Voltage drop calculations should use accurate formulas that account for both resistance and reactance, rather than simplified approaches based solely on resistance.
In AC systems, voltage drop depends on the cable impedance and the load power factor. The resistance and reactance components contribute differently depending on the load’s power factor.
Accurate calculations, therefore, require impedance values appropriate to the cable construction and installation conditions.
Refer to this article for examples of voltage drop calculations, including how to calculate conductor temperature and how to use it for determining conductor resistance.
2. Long Cable Runs to Small Loads
For long cable runs supplying relatively small loads, voltage drop often dictates the cable size.
In these situations, the voltage drop limit specified by standards becomes the controlling factor in cable selection.
For short cable runs with low load current, cable size is more commonly dictated by short-circuit withstand requirements than by voltage drop limits.
3. Poor Power Factor
The voltage drop of large cables increases significantly when the load power factor is poor.
Power factor affects large cables more than small cables because large cables typically have higher reactance relative to resistance.
Equipment that commonly produces a poor power factor includes:
Induction motors
Computer equipment
Fluorescent and LED lighting
Transformers
Welding machines
Electrical standards typically require installations to maintain a lagging power factor of approximately 0.9 or higher.
Example – Power Factor Effect on Cable Size
A three-phase load of 300 A is supplied over 200 m on a 400 V system, with a 2.5% voltage drop limit, using 3C+E V-75 copper cable installed unenclosed in air and spaced from surfaces.
The table shows that the power factor significantly affects the minimum cable size required to meet the voltage drop limit:
At a power factor = 1, a 240 mm² cable satisfies the limit with 2.37% VD.
At a power factor = 0.95, the cable size must increase to 300 mm².
At a power factor = 0.9, the required size increases to 400 mm².
As the power factor decreases, the reactive component of cable impedance increases the voltage drop, requiring larger, more expensive cables to remain within the allowable limit.
4. Heavily Loaded Cables
Hotter-running cables have higher resistance and therefore greater voltage drop.
As the conductor temperature increases, its resistance increases. This results in a greater voltage drop along the cable.
It is therefore important to consider the temperature dependence of conductor resistance when performing voltage drop calculations.
5. Unbalanced Three-Phase Circuits
Unbalanced three-phase circuits produce higher voltage drops.
Two situations should be considered.
Out-of-balance conditions that are intermittent
Voltage drop may be calculated under balanced three-phase load conditions using the current of the heaviest-loaded phase.
Out-of-balance conditions that are consistent
Voltage drop should be calculated on a single-phase basis by summing the voltage drop in the heaviest-loaded phase and the neutral conductor.
6. Multicore Cables
Generally, the voltage drop in a multicore cable is lower than that in a single-core cable.
For large cable sizes, single-core cables usually exhibit higher voltage drop due to their higher reactance.
For small cables, multicore cables may have slightly higher voltage drop.
Multicore cables typically have lower current-carrying capacity compared with equivalent single-core cables.
7. Motor Circuits
Voltage drop calculations for motor circuits are based on the motor’s full-load current (FLC).
During start-up, the voltage at the motor terminals should remain above 80% of the rated voltage. If the voltage falls below this level, the motor may fail to start.
For direct-on-line motors subject to frequent starting and stopping, a practical design approach is to multiply the full-load current by 1.4 when assessing voltage drop.
Motor rated current, voltage, and power factor should be obtained from the motor nameplate.
8. Voltage Rise Calculations
Voltage-rise calculations use the same method as voltage-drop calculations.
In solar photovoltaic installations, current flows from the inverter toward the point of connection, causing voltage to rise along the cable.
The voltage rise must be calculated for each circuit section, and the total voltage rise along the connection path must be within the limits specified by the network operator.
9. Flexible Cables
Flexible cables often have higher resistance than non-flexible cables because they contain more strands.
This higher resistance generally results in greater voltage drop.
However, for large cable sizes, flexible cables may, in some cases, have up to 10 % lower voltage drop than non-flexible cables.
For accurate calculations, resistance values should be obtained from the cable manufacturer or from engineering software that includes flexible cable data.
10. Fire Conditions
Critical power and control circuits may be required to operate for 30, 60, or 120 minutes during a fire.
Voltage drop can be calculated for cables exposed to fire by assuming:
The cable temperature during a fire, and
The length of the cable affected.
Under fire conditions, the conductor’s resistance increases significantly, resulting in a higher voltage drop.
In the absence of detailed calculations, a practical design approach is to increase the conductor size by two sizes for circuits required to operate during fire conditions.
A fire temperature of approximately 842 °C is commonly assumed for these calculations.
References
AS/NZS 3008.1.1 – Electrical Installations – Selection of Cables.
IEC 60287 – Electric Cables – Calculation of the Current Rating.
IEC 60364 – Electrical Installations of Buildings.
BS 8519:2020 – Selection and installation of fire-resistant power and control cable systems for life safety and fire-fighting applications – Code of practice.
