Nominal Voltage of an Electrical Installation

The term ‘nominal voltage of an electrical installation’ is officially defined within the IEC 60050-826-2022 as:

value of the voltage by which the electrical installation or part of the electrical installation is designated and identified.

IEC 60050-826-2022

According to IEC 62497-1-2013, the term in question has the following short symbol: Un.

Note 1 to entry: Voltage transients, such as those due to switching, or temporary voltage variations due to abnormal operation, such as those due to fault conditions in the system supplying the installation, are not taken into consideration.

Note 2 to entry: The actual value of the voltage in the installation may differ from the nominal voltage by a quantity within normal tolerances.

The British Standard BS 7671 defines the term “voltage, nominal” as follows:

Voltage by which an installation (or part of an installation) is designated. The following ranges of nominal voltage (RMS values for AC) are defined:

  • Extra-low. Not exceeding 50 V AC or 120 V ripple-free DC, whether between conductors or to Earth.
  • Low. Exceeding extra-low voltage but not exceeding 1000 V AC or 1500 V DC between conductors, or 600 V AC or 900 V DC between conductors and Earth.
  • High. Normally exceeding low voltage.

Features

The electrical installation of a building is normally connected to the low-voltage electrical distribution network. The electrical installation of a building is itself a collection of interconnected electrical equipment which performs specific functions. It is therefore necessary to match the characteristics of all electrical equipment used in both the electrical distribution network and the electrical installation of a building to ensure that it functions correctly, including the nominal voltage.

The nominal voltage values for the electrical installations of buildings, and other low-voltage and high-voltage electrical installations, are laid down in IEC 60038, which applies to:

  • AC transmission, distribution and utilization systems and equipment for use in such systems with standard frequencies 50 Hz and 60 Hz having a nominal voltage above 100 V;
  • AC and DC traction systems;
  • AC and DC equipment having nominal voltages below 120 V AC or below 750 V DC, the AC voltages being intended (but not exclusively) for 50 Hz and 60 Hz applications; such equipment covers batteries (from primary or secondary cells), other power supply devices (AC or DC), electrical equipment (including industrial and communication), and appliances.

Value Ranges

IEC 60038 specifies the standard voltage values that are intended to be used:

  • as preferential values for the nominal voltage of electrical supply and utilization systems, and
  • as maximum, nominal and minimum reference values for both equipment and power supply in both electricity supply and utilization systems so that product and power system committees can co-ordinate their documents.

NOTE 1 – Two main reasons have led to the values specified in this standard:

  1. The values of nominal voltage (or highest voltage for equipment) specified in this standard are mainly based on the historical development of electrical supply systems throughout the world, since these values turned out to be the most common ones, and have achieved worldwide recognition;
  2. The voltage ranges mentioned in this standard have been recognized to be the most appropriate ones as a basis
    for design and testing of electrical equipment and systems.

However, as specified in IEC 60038, the determination of appropriate test values, test conditions and acceptance criteria is the task of systems and product standards.

Table 1 of subclause 4.1 “AC systems having a nominal voltage between 100 V and 1 000 V inclusive and associated equipment” of IEC 60038 specifies the nominal voltages of AC systems in the range of 100 V to 1000 V, which must be consulted for the selection of nominal voltages in electrical distribution systems and the electrical installations of buildings connected to them.

Table 1 – AC systems having a nominal voltage between 100 V and 1 000 V inclusive and related equipment [2]
Three-phase four-wire or three-wire systems Single-phase three-wire systems
Nominal voltage, V Nominal voltage, V
50 Hz 60 Hz 60 Hz
120/208 120/240d
230c 240c
230/400a 230/400a
277/480
480
347/600
600
400/690b
1000

a) The value of 230/400 V is the result of the evolution of 220/380 V and 240/415 V systems which has been completed in Europe and many other countries. However, 220/380 V and 240/415 V systems still exist.

b) The value of 400/690 V is the result of the evolution of 380/660 V systems which has been completed in Europe and many other countries. However, 380/660 V systems still exist.

c) The value of 200 V or 220 V is also used in some countries.

d) The values of 100/200 V are also used in some countries on 50 Hz or 60 Hz systems.

In Table 1, the three-phase four-wire systems and single-phase three-wire systems include single-phase circuits (extensions, services, etc.) connected to these systems.
The lower values in the first and second columns are voltages to neutral and the higher values are voltages between phases. When one value only is indicated, it refers to three-wire systems and specifies the voltage between phases. The lower value in the third column is the voltage to neutral and the higher value is the voltage between lines.

Concerning supply voltage range, under normal operating conditions, the supply voltage should not differ from the nominal voltage of the system by more than ±10 %.

For example, the nominal voltage 230/400 V means the following: 230V is the voltage between phase and neutral, and 400V is the voltage between phases. The voltage at the connection point of a single-phase electrical installation of a building to the low-voltage distribution network must be 230 V ± 10 % and of a three-phase electrical installation of a building 400 V ± 10 %.

Voltages in excess of 230/400 V are intended for heavy industrial applications and large commercial premises.

For the utilization voltage range, in addition to the voltage variations at the supply terminals, voltage drops may occur within the consumer’s installations. For more information, see IEC 60364-5-52. This utilization voltage range should be taken into account by product committees.

Table A.1 of IEC 60038 below gives the highest, nominal and lowest reference voltage values at supply terminals and utilization terminals for 50 Hz and 60 Hz AC systems having a nominal voltage between 100 V and 1 000 V.

The values are derived from Table 1 in Clause 4, and IEC 60364-5-52.

NOTE – Values in Table A.1 are based on 3 % (lighting) and 5 % (other) voltage drops in the consumer’s premises (IEC 60364- 5-52) and a ±10 % supply voltage range at 50 Hz on three-phase systems calculations. 60 Hz three-phase values are based on a supply voltage range of +6 % to −14 %, except for 230/400 V which is ±10 %.

Table A.1 – Highest and lowest voltage values at supply and utilization terminals for AC systems having a nominal voltage between 100 V and 1 000 V [2]
Systems Rated frequency, Hz Voltage, V
Highest voltage for equipment (supply or utilization voltage2) Nominal voltage of system Lowest supply voltage Lowest utilization voltage
Lighting Other
Three-phase four-wire, threewire or singlephase two-wire systems 50 254 230c 207 200 196
254/440 230/400a 207/360 200/348 196/340
440/759 400/690b 360/621 348/600 340/587
1100 1000 900 870 850
60 127/220 120/208 103/179 100/173 97/168
254 240c 206 199 194
254/440 230/400a 207/360 200/348 196/340
294/509 277/480 238/413 230/398 224/391
509 480 413 398 389
368/636 347/600 298/516 288/498 281/486
636 600 516 498 486
Single-phase three-wire systems 60 127/254 120/240d 103/206 100/199 97/194

a) The value of 230/400 V is the result of the evolution of 220/380 V and 240/415 V systems which has been completed in Europe and many other countries. In some countries, 220/380 V and 240/415 V systems still exist.

b) The value of 400/690 V is the result of the evolution of 380/660 V systems which has been completed in Europe and many other countries. In some countries, 380/660 V systems still exist.

c) The value of 200 V or 220 V is also used in some countries.

d) The values of 100/200 V are also used in some countries on 50 Hz or 60 Hz systems.

2) The term “utilization voltage” is defined in IEC 60038 as follows: the phase-to-phase or phase-to-neutral voltage at the outlets or at the points where utilisation equipment is intended to be connected to the fixed installation.

The nominal voltage of three-phase electrical installations of residential and public buildings, medical facilities and commercial premises is usually 400 V, single-phase – 230 V.

IEC standard 60449 specifies two AC and DC voltage ranges for electrical installations in buildings. Table 1 of Section 3 “AC voltage bands” shows the two AC voltage ranges and Table 2 of Section 4 “DC voltage bands” shows the two DC voltage ranges. These voltage ranges classify electrical installations according to their nominal voltage.

Table 1: AC voltage bands.

Bands Earthed systems Isolated or not effectively earthed systems*
Phase to earth1, V Between phases, V Between poles, V
I U≤ 50 U≤ 50 U≤ 50
II 50<U≤600 50<U≤1 000 120<U≤1 000

* If the neutral is distributed2, electrical equipment supplied between phase and neutral is to be chosen so that its insulation corresponds to the voltage between phases.

U = nominal voltage of the installation.

Note 1: Phase-to-earth voltage is the voltage between a phase conductor and a reference earth at a given point in an electrical circuit.

Note 2: There is a neutral conductor in the electrical system.

Table 2: DC voltage bands.

Bands Earthed systems Isolated or not effectively earthed systems*
Pole to earth3, V Between poles 4, V Between poles, V
I U≤ 120 U≤ 120 U≤ 120
II 120<U≤900 120<U≤1 500 120<U≤1 500

* If a middle wire is distributed5, electrical equipment supplied between poles and middle wire is to be chosen so that its insulation corresponds to the voltage between poles.

U = nominal voltage of the installation.

Note 3: The pole-to-earth voltage is the voltage between the pole conductor and the reference earth at a given point in an electrical circuit.

Note 4: Pole-to-pole voltage is the voltage between two pole conductors at a given point in an electrical circuit.

Note 5: There is a mid-point conductor in the electrical system.

Band II in Table 1 of IEC 60449 includes all nominal voltages specified in Table 1 of IEC 60038. Band I sets the upper limit for extra-low-voltage AC power used in electrical installations of buildings in such electric shock protection measures as “extra-low-voltage provided by SELV and PELV”.

Table 1 of IEC 60038 specifies nominal AC voltages from 100 V to 1000 V, and low-voltage installations use electrical equipment and electrical circuits operating at voltages less than 100 V. In addition, IEC 60038 does not specify nominal DC voltages for low-voltage electrical installations. Therefore, table 6 of IEC 60038, subclause 4.6, “Equipment having a nominal voltage below 120 V AC or below 750 V DC”, lists the nominal voltages of AC electrical equipment falling within voltage band I and that of DC electrical equipment falling within both voltage bands (I and II).

Table 6 – Equipment having a nominal voltage below 120 V AC or below 750 V DC [2]
DC AC
Nominal values Nominal values
Preferred, V Supplementary, V Preferred, V Supplementary, V
2,4
3
4
4,5
5 5
6 6
7,5
9
12 12
15 15
24 24
30
36 36
40
48 48
60 60
72
80
96
100
110 110
125
220
250
440
600

NOTE 1. Because the voltage of the primary and secondary cells is below 2,4 V, and the choice of the type of cell to be used in various applications will be based on properties other than the voltage, these values are not included in the table. The relevant IEC technical committees may specify types of cells and related voltages for specific applications.

NOTE 2. It is recognized that for technical and economic reasons, additional voltages may be required for certain specific fields of application.

According to Table 6 of IEC 60038, electrical equipment with nominal voltages of 6, 12, 24 and 48 V is usually used in the AC circuits of electrical installations of buildings operating at extra-low voltages. It is also possible to use electrical equipment with nominal voltages of 5, 15 and 36V. If DC electrical equipment is used in the electrical installation of a building, it will generally have values of nominal voltage shown in the first two columns of Table 6 of IEC 60038.

References

  1. IEC 60050-826-2022
  2. IEC 60038-2021
  3. IEC 60449

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