Nominal Voltage, Rated Voltage and Operating Voltage

In electrical engineering, we frequently come across the following three voltage ratings related to electrical equipment and power system −

• Nominal Voltage
• Rated Voltage
• Operating Voltage

In this article, we will discuss all these three terms related to the voltage rating of electrical systems.

What is Nominal Voltage?

The term nominal voltage means named voltage. The nominal voltage is defined as the voltage value assigned to an electric circuit or system to designate its voltage class. In simple words, the voltage level of an electrical power system is called the nominal voltage of the system. It is also called system voltage. The typical nominal voltages in a power system 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 any electric equipment is designed, the designer first considers the nominal voltage rating of the system on which the equipment will be operated. A tolerance of ± 10% or higher is also taken for safety regions.

Basically, the nominal voltage is not the precise operating voltage of the equipment. It is just a voltage value by which an electrical device has been named or is referred to. Thus the actual voltage at which the device operates may vary from the nominal voltage within a range of voltages that allows satisfactory operation of the device.

In practice, nominal voltage is mainly used as a voltage reference for describing ratings of electrical devices and systems. It is the voltage of the supply system to which the device may be connected. Hence, we may consider the nominal voltage as an approximate voltage level of a system.

What is Rated Voltage?

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

Hence, the rated voltage of electrical equipment is the maximum voltage at which the equipment can operate without being damaged and shows its expected performance. The voltage tolerance range is so chosen that it lies within the range of rated voltage.

The rated voltage is always mentioned on the nameplate of the equipment. For example, on the nameplate of a 1phase induction motor, the rated voltage is given as 240 V ± 10%, which means the motor can operate safely within the voltage range of 216 V to 264 V. Hence, if the motor is operated within this range of voltage, it will work reliably.

It should be noted that the rated voltage must be greater than the nominal voltage so that the device can operate safely. The difference between the rated voltage and nominal voltage must be large enough so that the variations in the nominal voltage in the power line can be easily analyzed.

Therefore, we may conclude that the maximum value of the voltage at which an electrical device can operate without being damaged is referred to as the rated voltage of the device.

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 of equipment is usually measured with help of voltage-measuring devices like voltmeters, multimeters, etc. It must be ensured that the operating voltage must not exceed the rated voltage for the safe, reliable, and economical operation of the equipment. If the operating voltage is not within the range of rated voltage, then the operation of the equipment gets adversely affected, or it may also damage the equipment.

Conclusion

• The voltage value of electrical equipment for which it is designed to be used is called nominal voltage.

• The maximum voltage that can be applied to equipment safely is called rated voltage.

• The voltage value at which equipment is being operated is called the operating voltage.

We can understand the nominal voltage, rated voltage, and operating voltage of electrical equipment by considering an example of an 11 kV power system. In this case, we have

• Nominal voltage = 11 kV
• Rated voltage = 11 kV ± 10% = 9.9 kV to 12.1 kV
• Operating voltage = It can be in the range of 9.9 kV to 12.1 kV.