Part 7 – Voltage Drop Calculation
Introduction
After selecting a preliminary cable size and verifying its corrected current carrying capacity, the next engineering verification is Voltage Drop. This is one of the most critical design checks because a cable that safely carries the required current may still be unacceptable if it causes excessive voltage loss between the inverter and the Medium Voltage (MV) panel.
Voltage drop directly affects:
- Inverter performance
- Equipment operating voltage
- Plant efficiency
- Energy yield
- Cable losses
- Grid code compliance
- Long-term operational reliability
In modern utility-scale solar power plants, engineers do not select cables based solely on ampacity. They also ensure that the voltage delivered to downstream equipment remains within the acceptable design limits specified by the project, client specification and applicable electrical standards.
A properly designed AC cable should not only carry the inverter output current safely but also maintain the required voltage throughout the entire cable route.
Before finalizing the cable size, professional engineers always verify that the calculated voltage drop is within the allowable limit.
The OneCalcApp.com AC Cable Sizing Calculator automatically performs voltage drop verification after evaluating the current carrying capacity. However, every engineer should understand the engineering principles behind the calculation before relying on any software or calculator.
What Is Voltage Drop?
Voltage Drop is the reduction in electrical voltage that occurs as current flows through a conductor.
No conductor has zero resistance.
Whenever current flows through a cable, a small portion of electrical energy is consumed in overcoming the conductor resistance and reactance.
As a result:
The voltage measured at the receiving end of the cable is always slightly lower than the sending-end voltage.
This reduction is known as Voltage Drop.
Simple Illustration
Inverter
800 V
↓
AC Cable
↓
Cable Resistance + Cable Reactance
↓
MV Panel
795 V
In this example:
Input Voltage
↓
800 V
Output Voltage
↓
795 V
Voltage Drop
↓
5 V
Why Does Voltage Drop Occur?
Every conductor possesses two important electrical properties:
- Resistance (R)
- Reactance (X)
Resistance converts electrical energy into heat.
Reactance opposes alternating current because of the cable's inductive characteristics.
Since AC systems contain both resistance and reactance, both parameters influence the total voltage drop.
As cable length increases:
Resistance increases.
Reactance also increases.
Consequently:
Voltage Drop increases.
This is why long cable routes generally require larger conductors even when the cable already satisfies the ampacity requirement.
Understanding Conductor Resistance
Resistance is the natural opposition offered by a conductor to the flow of electric current.
Every metallic conductor—including copper and aluminium—has electrical resistance.
The resistance of a conductor depends upon:
- Material
- Cross-sectional Area
- Length
- Temperature
The relationship is:
Where:
R
Conductor Resistance
ρ
Electrical Resistivity of the Material
L
Length of the Conductor
A
Cross-sectional Area
Physical Meaning of the Formula
The above equation explains why larger conductors have lower resistance.
Increasing conductor area
↓
Lower Resistance
↓
Lower Voltage Drop
↓
Lower Power Loss
Similarly:
Increasing cable length
↓
Higher Resistance
↓
Higher Voltage Drop
This simple relationship forms the basis of electrical cable design.
Understanding Cable Reactance
Unlike DC systems, AC systems contain another electrical parameter called Reactance.
Reactance is produced because alternating current continuously creates a changing magnetic field around the conductor.
This changing magnetic field produces inductive effects that oppose current flow.
Although cable reactance is usually much smaller than conductor resistance in low-voltage systems, it becomes increasingly important for:
- Long cable routes
- High-current systems
- Utility-scale solar plants
- Medium Voltage cable circuits
Therefore, professional voltage drop calculations include both resistance and reactance.
Why Resistance and Reactance Both Matter
Many beginner engineers ask:
"Why doesn't the voltage drop equation use only resistance?"
The answer lies in the nature of alternating current.
In AC systems:
Total impedance consists of:
Resistance
+
Reactance
Current flowing through resistance creates a voltage drop in phase with the current.
Current flowing through reactance creates another voltage drop shifted by 90°.
Therefore:
Both effects must be included in the engineering calculation.
Ignoring reactance may underestimate the actual voltage drop, especially for longer AC cable routes.
Three-Phase Voltage Drop Formula
The standard three-phase voltage drop equation is:
Where:
Vd
Voltage Drop
I
Load Current
R
AC Resistance of the Conductor
X
Cable Reactance
cos φ
Power Factor
sin φ
Reactive Component of the Load
L
One-Way Cable Length
WHY √3 APPEARS AGAIN
Just like the Full Load Current equation, Voltage Drop is also calculated using line-to-line voltage.
In a balanced three-phase electrical system:
Therefore:
The √3 factor naturally appears in the voltage drop equation.
This is not an arbitrary multiplier.
It originates from the mathematical relationship between phase quantities and line quantities in a balanced three-phase system.
Understanding Every Variable
Load Current (I)
The current flowing through the cable.
Higher current
↓
Higher Voltage Drop
Resistance (R)
Represents conductor losses.
Higher resistance
↓
Higher Voltage Drop
Reactance (X)
Represents inductive opposition.
Higher reactance
↓
Higher Voltage Drop
Cable Length (L)
Voltage drop increases directly with cable length.
If cable length doubles:
Voltage drop approximately doubles.
This is why accurate cable routing is essential during detailed engineering.
Power Factor
Power Factor influences the contribution of resistance and reactance to the total voltage drop.
Loads operating closer to unity Power Factor generally experience lower voltage drops than heavily reactive loads under similar conditions.
Percentage Voltage Drop
Voltage Drop is usually expressed as a percentage of the operating voltage.
The relationship is:
Where:
Vd
Calculated Voltage Drop
V
Nominal System Voltage
Expressing voltage drop as a percentage makes it easier to compare different systems operating at different voltage levels.
Why Percentage Voltage Drop Is Preferred
Suppose:
Project A
Operating Voltage
400 V
Voltage Drop
5 V
Project B
Operating Voltage
800 V
Voltage Drop
5 V
Although both systems have the same voltage loss, their percentage voltage drops are different.
Therefore:
Professional engineers generally specify voltage drop limits as percentages rather than absolute voltage values.
Engineering Design Philosophy
Voltage drop should not be evaluated independently from the rest of the cable sizing process.
A cable may pass the current carrying capacity requirement but fail the voltage drop requirement.
Similarly, a cable may achieve a very low voltage drop but be unnecessarily oversized from an ampacity perspective.
Therefore, the final cable selection should balance:
- Current carrying capacity
- Derating
- Voltage drop
- Power loss
- Short-circuit withstand capability
- Installation requirements
- Project specifications
- Applicable electrical standards
The objective is to select the most technically suitable and economically practical cable rather than simply selecting the largest conductor.
Factors That Increase Voltage Drop
Voltage drop generally increases when:
- Load current increases.
- Cable length increases.
- Conductor resistance increases.
- Cable reactance increases.
- Conductor cross-sectional area decreases.
- Power factor conditions increase the reactive contribution.
Understanding these relationships helps engineers optimize cable size during detailed design.
Engineering Best Practices
Professional engineers generally follow these recommendations:
- Use the actual one-way cable route length.
- Verify conductor resistance at the appropriate operating temperature.
- Obtain cable reactance from approved manufacturer data where required.
- Use the actual design Power Factor.
- Confirm the nominal operating voltage.
- Compare calculated voltage drop with the project allowable limit.
- Document all assumptions used in the calculation.
- Recheck voltage drop whenever cable size or route length changes.
How Onecalcapp Helps Engineers
During detailed engineering, voltage drop calculations are often repeated several times while optimizing cable sizes, route lengths and conductor materials.
Instead of manually recalculating every design iteration, the OneCalcApp.com AC Cable Sizing Calculator allows engineers to compare different scenarios quickly while still following the engineering methodology described in this guide.
This helps reduce repetitive calculations and supports more efficient cable selection without replacing engineering judgment.
Summary
By the end of this chapter, we have learned that:
- Voltage drop occurs because of conductor resistance and AC reactance.
- Cable length directly influences voltage drop.
- Larger conductor sizes generally reduce resistance and voltage drop.
- Three-phase voltage drop calculations include the √3 factor.
- Both resistance and reactance should be considered for AC cable circuits.
- Percentage voltage drop provides a practical basis for comparing different systems.
- A cable must satisfy both ampacity and voltage drop requirements before final selection.
Engineering Takeaway
A cable that passes ampacity and derating checks is not automatically suitable for the project.
The calculated voltage drop must also remain within the allowable design limit.
Therefore, the cable sizing process should continue through:
Full Load Current
↓
Preliminary Cable Selection
↓
Derating Verification
↓
Voltage Drop Verification
↓
Final Cable Selection
Calculate. Verify. Design with confidence.