Nominal Voltage, Rated Voltage and Operating Voltage

This post explains the concepts of nominal voltage, rated voltage, and operating voltage, highlighting their significance in electrical engineering:

In electrical engineering, we often talk about these three voltage ratings for electrical equipment and power systems:

  • Nominal Voltage
  • Rated Voltage
  • Operating Voltage

Let’s discuss these three voltage terms and how they relate to the voltage ratings of different electrical systems.

What is Nominal Voltage?

The term nominal voltage refers to the named voltage of an electrical system. Nominal voltage is the voltage assigned to a circuit or system to represent its voltage class. Simply put, it is the voltage level at which a power system operates, often called the system voltage. Nominal voltage is important because it ensures the safety and compatibility of electrical systems and devices.

The nominal voltage of a three-phase system is the designated voltage level at which the system is designed to operate efficiently and safely. Common nominal voltages in 3-phase power systems are 440 V, 3.3 kV, 6.6 kV, 11 kV, 33 kV, 66 kV, 132 kV, 220 kV, 400 kV, and 765 kV.

When designing electrical equipment, the first consideration is the nominal voltage rating of the system on equipment operates. A tolerance of ±10% or more is usually included to ensure safe operation for system voltage variations.

Nominal voltage is not the exact operating voltage of the equipment. Instead, it is the voltage used to identify or refer to the device. The actual operating voltage may differ from the nominal voltage but will stay within a range that ensures the device works properly.

In practice, nominal voltage serves as a reference voltage for describing the voltage ratings of electrical devices and systems. It represents the supply system voltage to which a device can be connected. Therefore, the nominal voltage in an electrical system can be considered an approximate voltage level.

The “nominal voltage” for single-phase electricity is 230 volts in most parts of the world. However, in North America, the standard nominal voltage is usually 120 volts.

Thus, in electrical terms, “nominal voltage” refers to the specified voltage assigned to a system or circuit, representing its standard operating level for design purposes. While the actual voltage may vary slightly above or below this value during operation, the nominal voltage serves as a reference point for defining the circuit or system’s voltage level.

What is Rated Voltage?

The rated voltage of electrical equipment is the voltage value for which the equipment is specifically designed to operate safely and reliably.

Therefore, the rated voltage of electrical equipment represents the maximum voltage at which it can function safely without damage while delivering its expected performance. The voltage tolerance is carefully selected to stay within this rated voltage range.

The rated voltage is always specified and written on the equipment’s nameplate. For instance, the nameplate of a three-phase induction motor might state a rated voltage of 440V ± 10%, indicating that the motor can operate safely within a voltage range of 396 V to 484 V. Operating the motor within this range ensures reliable performance.

The rated voltage should be higher than the nominal voltage to ensure the device operates safely. The difference between the rated and nominal voltage should be more to account for changes in the nominal voltage on the power line.

In conclusion, the rated voltage of an electrical device is the highest voltage at which it can operate safely without damage.

What is Operating Voltage?

The actual value of the supply voltage applied at the equipment terminals is called the operating voltage. In other words, the voltage at which equipment is being operated is called the operating voltage of the equipment.

The operating voltage is the actual supply voltage applied to the equipment terminals. In other words, it is the voltage at which the equipment operates.

The operating voltage of equipment is typically measured using devices like voltmeters or multimeters. It is important to ensure that the operating voltage does not exceed the rated voltage for safe, reliable, and economical operation. If the operating voltage exceeds the rated range, it can harm the equipment’s performance or even lead to damage.

Nominal Voltage v/s Rated Voltage

Nominal voltage refers to the voltage of an electrical power system, also known as system voltage. In 3-phase systems, it is the standard voltage for an electrical power system is initially designed. On the other hand, the rated voltage of equipment is the range within which it operates reliably and stably.

To ensure safe operation, the rated voltage must always be higher than the nominal voltage, and designers must consider a safety margin when determining the rated voltage. A significant difference between nominal and rated voltage helps analyze variations in the power line and maintain system safety.

For example, a circuit breaker is a device used to control and protect electrical systems. Its rated voltage is determined by its insulating system and represents the highest RMS voltage it can handle, expressed in kV RMS. This value is also its maximum operating voltage. The rated voltage of a circuit breaker defines the highest voltage it can safely interrupt without causing arcing, ensuring reliable performance and protection of the system.

Nominal Voltage v/s Operating Voltage

Operating voltage refers to the actual voltage at which a device or equipment operates. For the equipment to function reliably, it must be operated within its specified rated voltage range. The operating voltage is the voltage applied to the equipment terminals during its operation.

A multimeter is used to measure the voltage at the equipment terminal. If the voltage applied is higher or lower than the equipment’s rated voltage, the equipment may not perform properly or it can be damaged.

Example of Nominal Voltage, Rated Voltage, and Operating Voltage

For a 132kV power system, the following information is provided on the installation of a circuit breaker. The functioning of the equipment might be negatively impacted if the operating voltage is beyond the range of the rated voltage.

For a 33 kV power system, the circuit breaker is designed to operate within a specific voltage range. If the operating voltage is not within the rated voltage range, the circuit breaker may not function properly. This can negatively impact the electrical system’s performance and even lead to equipment damage or failure. Therefore, the voltage supplied to the system must remain within the specified rated voltage for safe and reliable operation.

Nominal Voltage–Ranges 33 kV

Rated Voltage –Rated 33 kV +/- 10 % [29.7 kV– 36.3 kV ]

Operating Voltage –Operation Range of 29.7 kV– 36.3 kV.

Nominal voltage, rated voltage and operating voltage of the different voltage ratings are listed in the table below.

System VoltageNominal Voltage Rated VoltageOperating Voltage
440 V440 V396 V- 484 V396 V- 484 V
6.6 kV6.6 kV6.0 kV-7.2 kV6.0 kV-7.2 kV
11 kV11 kV9.9 kV -12.1 kV9.9 kV -12.1 kV
33 kV33 kV29.7 kV– 36.3 kV29.7 kV– 36.3 kV
66 kV66 kV59.4 kV-72.6 kV 59.4 kV-72.6 kV
132 kV132 kV118.8 kV– 145.2 kV118.8 kV– 145.2 kV

Nominal Voltage of a battery

A battery’s nominal voltage is the average voltage it provides when fully charged. However, the actual voltage can vary. When a battery is fully charged, the actual voltage is typically higher than its nominal value, but it decreases as the battery discharges. The nominal voltage is a simplified term to represent the energy capacity per charge.

For instance, a 12V car battery has a nominal voltage of 12 volts, but its actual voltage may fluctuate during operation. Batteries are made up of multiple cells connected in either series or parallel configurations. In a series connection, the voltages of individual cells add up to determine the total nominal voltage, while in a parallel connection, the nominal voltage remains the same as that of a single cell, although the capacity increases.

A battery with an actual voltage of 1.62 V may commonly be referred to as a “1.5-volt battery”, indicating its nominal voltage of 1.5 V. This highlights how nominal voltage serves as a general reference rather than the precise operating voltage.

Another example is the term “DC 12V,” which refers to a 12V lead acid battery. In reality, the actual voltage of such a battery can vary depending on its charge state, ranging from 13.7 Vdc when fully charged to around 10 Vdc when discharged. These examples emphasize how nominal voltage provides a convenient standard for labeling, even though actual operating voltages may differ.

Nominal Voltage of an Electric Motor

Although a motor can operate within the provided voltage range, it is best to run it at its nominal voltage, as this is the recommended voltage for optimal performance. Operating the motor at its nominal voltage ensures efficiency, reliability, and longevity, reducing the risk of potential issues caused by voltage deviations.

For DC motors, the nominal voltage is a critical parameter as it directly influences the motor’s performance. Specifically, the torque-speed curve of a DC motor is determined by its nominal voltage. Operating the motor at this voltage ensures the desired relationship between torque and speed, enabling the motor to deliver optimal performance for its intended application.

Difference between Nominal voltage, Rated Voltage, and Operating Voltage

SpecificationNominal VoltageRated VoltageOperating Voltage
DefinitionNominal voltage is the standard voltage for which electrical equipment is designed to operate.Rated voltage is the maximum voltage that can be safely applied to equipment.Operational voltage is the voltage at which a device is operated.
Voltage ToleranceFor safety purposes, a tolerance of ±10% or higher is also used. The voltage tolerance range is set within the rated voltage range.For safe operation, the operating voltage tolerance must remain within the rated voltage range.
PerformanceNominal voltage is used as a voltage reference and is considered an “approximate” voltage level.For safe operation, the rated voltage must be higher than the nominal voltage.The operational voltage is the voltage that is actually applied to the equipment’s terminal.
CapabilityThe nominal voltage denotes the total amount of voltage that the equipment can handle.The rated voltage is characterized by the resistance rate, while the resilience (ability to withstand) is expressed as a percentage rate.Operating voltage refers to the voltage at which the equipment is currently running.
Typical Representation 132 kVRated 132 kV +/- 10 % [118.8 kV– 145.2 kV ]Operation Range of 118.8 kV to 145.2 kV

Conclusion

To summarize the article, the following key points can be made:

  • Nominal Voltage refers to the voltage value for which electrical equipment is designed to operate.
  • Rated Voltage is the maximum voltage that can be safely applied to the equipment.
  • Operating Voltage is the actual voltage at which the equipment is running.

For example, consider an 11 kV power system:

  • Nominal Voltage = 11 kV
  • Rated Voltage = 11 kV ± 10%, which is in the range of 9.9 kV to 12.1 kV.
  • Operating Voltage = It can fall within the range of 9.9 kV to 12.1 kV.

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