Nominal Voltage of an Electrical Installation – Asutpp

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: U_n.

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.

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 %.

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.