Voltage Drop Analysis and Calculation Essentials (MS Excel Sheet Bonus)

Problems Due To Voltage Drop

A voltage drop is the total amount of voltage loss that happens through all or part of an electric circuit due to impedance and other factors. The more the voltage drops in the system, the more negative impacts will likely occur. Voltage drop is considered a loss and an inefficiency in the system because it could lead to malfunction and poor performance of the entire system.

Since voltage and current are carried through cables and wires, resistance or impedance will occur depending on the size or length of the cable. Therefore, the final size of cables and conductors for any project must be performed following the voltage drop calculations.

For these reasons, voltage drop calculation is considered one of the major tasks the designer must perform for any project.

Electrical designers must follow international standards and regulations to achieve the permitted accepted percentage value. In this article, we will discuss in-depth details about all aspects of voltage drop causes, effects, calculations, and techniques to minimize the voltage drop to comply with international standards.

Figure 1 – Impedance Effect on Voltage

Table of Contents:

1. Voltage Drop Causes, Effects & Mitigation
   1. Causes of Voltage Drop
   2. Techniques of Reducing VD
   3. Permitted VD Percentage
2. Voltage Drop Calculation
   1. Example #1
   2. Example #2
3. Practical Techniques to Minimize Voltage Drop
4. Selection of Cable and Wire Types
5. Cable Size Calculation
   1. Example
6. BONUS! Download Excel Sheet
7. Conclusion

1. Voltage Drop Causes, Effects & Mitigation

When we say a system is experiencing a drop in the voltage, it means the rated voltage (e.g., 240V single-phase) is not fully delivered to the device or the appliance when it is under full load condition.

If the measured voltage is within the rated range while under full load, the voltage drop is considered acceptable, and the load can function efficiently. In contrast, the voltage’s rated value might be delivered to the load with a considerable drop (e.g. 216V instead of 240V) while it is running, which means 90% of the resistance in the circuit is in the load (216V), and 10% of the total circuit resistance is in the conductors (24V).

1.1 Causes of Voltage Drop

The main causes of the voltage drop include:

1. Conductors

1. Type of material from which the conductor is made of:
   Copper conducts electricity better than aluminum and will cause less voltage drop than aluminum for a given length and conductor size.
2. Diameter of the Conductor (size/ number of the conductors):
   Conductors with larger diameters will result in less voltage drop than conductors with smaller diameters of the same length.
3. Conductor Length: Shorter conductors will have less voltage drop than longer conductors for the same conductor size.
4. Temperature of the Conductor: As a general rule, most conductive materials will increase their resistance with an increase in temperature.

2. Load Current – Voltage drop increases on a conductor with an increase in the current flowing through the conductor.

3. Loose connections – Poor connections in splices or when connecting conductors to terminals contribute to a considerable amount of voltage drop.

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1.2 Techniques of Reducing Voltage Drop

The factors mentioned above are the major factors affecting increased voltage drop values. Therefore, considering these factors in the calculations is a very critical issue. So, how to mitigate the voltage drop in a system?

There are some techniques that, if followed, will indeed reduce the voltage drop values. These techniques include:

• Using lower resistive materials of conductors (e.g. copper),
• Increasing the size of a conductor,
• Increasing the number of conductors,
• Decreasing the conductor’s length, and
• Decreasing the load (Ampere).

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1.3 Permitted Voltage Drop Percentage

Electrical designers must know the permitted level of voltage drop that should be followed during performing voltage drop calculations. Wiring codes or regulations set an upper limit to the allowable voltage drop in a branch circuit. The National Electrical Code (NEC) recommends no more than a 5% voltage drop at the outlet in the United States.

UK (BS) regulations limit voltage drops to 4% of the supply voltage. Therefore, based on the rules the designer is following in the design, the voltage drop must be within the permitted percentage. Similarly, the Canadian electrical code requires no more than a 5% drop between service entrance and point of use.

Figure 2 – NEC Permitted Percentage of Voltage Drop

Generally, the percentage of voltage drop must be at most 5% of the branch load. The less drop that occurs in the calculations, the more efficient the system is.

After complying with the permitted voltage drop percentage regulations, the calculated value must be reflected in the design drawings of the project. In particular, the percentage of voltage drop of each cable connected between panels should be indicated in the single-line diagram.

For example, the VD value of 11.17 V may be converted to a percentage by dividing it by the system’s three-phase voltage, e.g., 415V, and multiplying it by 100 to equal 2.69%.

In the next section, we will reveal more about voltage drop calculations and how they should be performed.

Figure 3 – Voltage Drop indicated in yellow in a Single Line Diagram (SLD)

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2. Voltage Drop Calculation

In order to perform the voltage drop calculations accurately, it is essential to consider the permitted percentage of voltage drop according to the wiring code you are following. The following steps should be followed to calculate the voltage drop: