Difference between Rated,Nominal and Operating Voltage

In this post, we will discuss the difference between rated,nominal and operating voltage.The equipment life enhances with operating at designed operating voltage.

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There are three voltage rating terms widely used for the electrical equipment.

  • Rated Voltage
  • Nominal Voltage
  • Operating Voltage
Difference between Rated,Nominal and Operating Voltage

Rated Voltage

The rated voltage is the voltage for which equipment is designed to operate reliably. Thus the rated voltage of any electrical equipment is the maximum voltage at which equipment can be operated within its thermal limit without jeopardizing the life of the equipment.The equipment designer also take voltage safety margin in designing for operation of equipment within the range of rated voltage.

Read More : What is Voltage?

On the nameplate of the motor, the rated voltage is engraved as 440+/- 10 %. It means the equipment can be operated safely in the voltage range of 396 to 484 volts. The lower limit of the voltage is 396 volts and higher limit of the voltage is 484 volts. If the equipment is operated within this range the equipment will work reliably. The +/10 % margin is the voltage safety margin. 

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Nominal Voltage

The nominal voltage is the voltage level of the electrical power system. The system voltage is  440 V, 690 V, 3.3 kV, 6.6 kV,  11kV, 33 kV, 66 kV, 132 kV, 220 kV, 400 kV, 765 kV. The voltage level of the electrical power system is known as nominal voltage or system voltage. The equipment designer first consider the nominal voltage for which equipment is operated. The designer takes the voltage safety margin of +/- 10 % or higher while equipment designing. 

Operating Voltage

The operating voltage of the equipment is the voltage at which equipment is being operated. The equipment must be operated within the range of rated voltage for reliable operation. For example, an induction motor rated at 440 V +/- 10 % can be operated within the voltage range of 396 to 484 V for the nominal or system voltage 440 Volts. The operating voltage is the actual voltage applied at equipment terminal.

The actual measurement of voltage applied to equipment terminal can be measured with multi-meter. The voltage at which equipment being operated is called the operating voltage. The operating voltage of the equipment should be in the range of rated voltage for safe,economic and reliable operation.

Read More : Effect of Unbalance Voltage on Induction Motor

In some of the equipment like an induction motor, if the applied voltage is higher or lower in magnitude than its rated voltage, the performance of the equipment gets badly affected.

For example, a circuit breaker installed for 132 KV power system has the following specifications.

Nominal Voltage  – 132 KV
Rated Voltage       – 132 KV +/- 10 % [118.8 – 145.2 KV ]
Operating Voltage – Can be in the range of 118.8 to 145.2 KV.

The equipment designer keeps the maximum rated voltage of the equipment above the nominal voltage or system voltage.The equipment designed at the higher voltage than its maximum rated voltage have better factor of safety.

Read More : What is voltage unbalance & How to Calculate it?

If the operating voltage is not within the range of rated voltage, equipment operation gets adversely affected. The utility company maintains the system  voltage within +/- 10 % range.

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