In the process of wiring cables and conductors within a low-voltage switchboard, it is critical to follow specific dos and don'ts, as well as safety precautions. This involves choosing the correct conductor cross-sections, using the right connection techniques, and putting in place measures to protect against short circuits and magnetic interference.
It should be noted from the outset that there is no universal model for wiring low-voltage switchboards and panelboards. Nevertheless, given the diverse array of installations and power rating ranges, local work practices, regulations, and international standards do apply. The LV switchboard incorporates various conductor types, with the optimal selection being contingent upon their intended application, as specified for each installation.
Nevertheless, the selection of distribution switchboards is not solely based on current-carrying capacity. It also depends on various factors such as the panel requirements, rated voltage, installation method, insulation type, and application types.
Proper "wiring hygiene" in a switchboard is something I always appreciate. When wiring is done neatly and correctly, it greatly simplifies future troubleshooting and maintenance.
This technical article provides guidance on selecting the cross-sections of wiring conductors within switchboards, their methods of connection, and various dos and don'ts, along with precautions to safeguard against short circuits and magnetic effects.
Table of Contents:
1. Cross-sections of Wiring Conductors within Switchboards
The selection of conductors for use within a switchboard, as well as their cross-sectional areas, falls to the original manufacturer. These conductors are required to have a minimum cross-section that complies with IEC 60364-5-52. The table below, extracted from IEC 61439-1, provides examples of adapting this standard to the conditions present within the switchboard.
There are two types of conductors:
PVC is commonly used for insulating conductors with either PVC or rubber insulation, typically applied in wiring conductors up to 35 mm².
PR is used for conductors with polyethylene or elastomer insulation, typically reserved for cross-sections exceeding 35 mm².
The naming of installation and ambient temperature conditions has been determined empirically:
IP rating of less than 30 is suitable for conductors installed under optimal cooling conditions such as an open or naturally ventilated enclosure, with low to medium wiring density, and an internal temperature of the enclosure close to the ambient temperature, up to 35°C.
IP rating greater than 30 is recommended for conductors installed in suboptimal cooling conditions—such as a sealed enclosure, high wiring density, multi-core cables, and an internal temperature that may reach 50°C.
Table 1 – Guidance on minimum cross-sectional values (in mm²)
Where:
Imaxc refers to the current-carrying capacity I30 for a three-phase circuit as specified in IEC 60364-5-52, Table B.52.10, Column 5, with the installation method being point F in Table B.52.1. For cross-sections smaller than 25 mm², the values are calculated according to IEC 60364-5-52 Annex D, with a k2 factor of 0.88, as indicated in point 4 of Table B.52.17 for two circuits.
Imaxd refers to the current-carrying capacity I30 for a three-phase circuit as specified in IEC 60364-5-52, Table B.52.10, Column 7, with the installation method being point G in Table B.52.1. For cross-sections smaller than 25 mm², the values are calculated in accordance with IEC 60364-5-52 Annex D, assuming a k2 factor of 1.
Column 1 is applicable when conductors from various circuits are installed in contact with each other and are bundled together, such as when placed in trunking or bundled into strands.
Figure 1 – Multiple circuits are housed within the same trunking, with wiring arranged in both vertical and horizontal orientations, referred to as Column 1 installation.
Column 2 is applicable when conductors or cables are spaced apart in open air (refer to Figure 2 below).
Figure 2 – Conductors are installed without contact, secured by guide rings in a Column 2 configuration.
Figure 3 – In open-air horizontal circulation, only the vertical conductors are bundled in trunking, as seen in column 2 installation. Should the packing ratio of the vertical trunking be high, a column 1 installation is used.
Typical cross-sections for protective conductors (PE) in switchboards are outlined below.
Table 2 – Determining the cross-sections of protective conductors in switchboards is based on the current requirements.
I (A) | SPE (mm2) |
10 | 1.5 |
16 | 2.5 |
20 | 4 |
25 | 4 |
32 | 6 |
40 | 10 |
63 | 16 |
80 | 16 |
100 | 16 |
125 | 25 |
160 | 35 |
200 | 50 |
250 | 70 |
315 | 95 |
400 | 120 |
500 | 150 |
630 | 185 |
800 | 240 |
1000 | 185 or 2×150 |
1250 | 240 or 2×165 |
1600 | 240 or 2×240 |
> 1600 | SPE/4 |
The cross-sections of conductors for wiring within switchboards are not encompassed by a singular standard document. However, IEC 60364-5-52 provides guidelines for minimum cross-sections, and adaptations for switchboard conditions are detailed in IEC 61439-1.
Standard IEC 60364
Standard IEC 60364 advises determining the cross-sections based on installation methods 31 and 32.
Indeed, the application of the method is challenging because it necessitates information for the correction factors that is only available post-installation.
Parts which run vertically,
Parts which run horizontally,
Groups,
Number of layers,
Separate conductors or cables, as well as
Monitoring the ambient temperature within an enclosure is consistently challenging.
Standard IEC 61439-1
The IEC 61439-1 standard does not prescribe specific cross-sections but defines a "current range" for temperature rise tests. It considers conductors with PVC insulation, without specifying the ambient temperature, which means not all application scenarios are addressed.
2. Connecting Conductors and Recommendations
2.1 Conductors Featuring A Rigid Copper Core
This conductor type, extensively used in permanent installations, does not necessitate special precautions as the terminal it connects to is designed for the necessary cross-section and current. The connections' quality and longevity are guaranteed through the use of a suitable tool and adherence to the advised tightening torques.
When connecting small conductors in direct pressure screw terminals, certain precautions are necessary:
Avoid tinning the core at the stripped sections as it may lead to the conductor breaking.
Avoid over-tightening to prevent shear.
The conductor's end may be folded back to ensure improved contact.
Figure 4 – Connecting conductors that have a rigid copper core
2.2 Conductors Featuring A Flexible Copper Core
Given the fragile nature of the strands that constitute the core, connecting flexible conductors necessitates certain precautions. Over-tightening can result in shearing of the strands, while an incorrect cross-section can lead to strand dispersion and inadequate contact.
Figure 5 – Connecting conductors with flexible copper core
When connecting conductors that have a flexible copper core, it is important to ensure proper handling to avoid loosening and risk of strand dispersion. It is recommended to bend the core back on itself in the initial direction, often to the left, and not to tin flexible conductors before connection to prevent fretting corrosion.
To prevent the strands from loosening and dispersing, it is advised to bend the core back upon itself in its initial direction, usually to the left.
Avoid tinning flexible conductors before connection; the application of tin can lead to a disintegration process called "fretting corrosion" over time. The danger of dielectric breakdown renders the use of conductive contact grease unwise in moist or conductive environments. In challenging operating conditions, it is advisable to use cable ends, sleeves, or lugs.
The risk of strand shear and dispersion, particularly prevalent in direct screw terminals, can be mitigated by employing a crimping tool for ferrules.
2.3 Branching Conductors
Connecting two rigid conductors with identical cross-sections simultaneously is generally not feasible upstream. Connecting two conductors with different core types or cross-sections is strongly discouraged. However, downstream branching is permissible. In such instances, the capacities, conductor types, and combinations are specified on the products or in the accompanying documentation.
2.4 PE Conductors
Branching or connecting in the same terminal is prohibited on protective circuits. It is also not allowed on the terminals of operating devices, with the exception of socket outlets, luminaires, lighting units, etc., provided that suitable terminals are available.
Branching, necessitated by the multitude of circuits, must be executed with suitable, secure devices.
Figure 6 – An additional terminal block is needed for the neutral conductor on the distribution block.
2.5 Conductors With Aluminium Core
Aluminum is an outstanding conductor and provides a favorable weight-to-conductance ratio for large cross-sections. It is extensively used in power systems and its application is expanding to power distribution. Understanding the unique challenges associated with connecting this metal is essential to prevent inevitable issues that may arise.
Problem #1 – When exposed to the open air, aluminum rapidly forms a thin, extremely hard insulating layer of alumina. As a result, it should be connected promptly after being stripped and, if needed, following an abrasive surface treatment.
Problem #2 – Aluminum expands significantly more than other commonly used metals such as iron, copper, and brass, which can lead to the loosening of connections. Therefore, the connection terminals for aluminum should also be made of aluminum or an alloy, or include elastic components like washers or strips to accommodate the differences in expansion.
Problem #3 – Aluminum possesses a notably negative electrochemical potential (-1.67 V), making it prone to corrosion upon contact with numerous other metals. This "sacrificial anode" characteristic is exacerbated in moist or conductive settings. Consequently, it is crucial to prevent direct contact between aluminum and metals such as stainless steel, silver, or copper.
However, metals like zinc, steel, and tin are compatible with aluminum. It is advisable to re-tighten connections to the specified torque after a few days in all instances.
When appropriate metals are selected and the environment is dry, the likelihood of electrolytic corrosion is minimal. However, this risk escalates in moist conditions where water serves as an electrolyte in the resultant battery. Applying a neutral grease, typically silicone-based, can mitigate this risk.
Figure 7 – Applying a neutral grease can mitigate electrolytic corrosion.
Table 3 – When comparing the cross-sections of aluminium and copper conductors, the aluminium conductor's cross-section must be approximately 1.6 times that of the copper conductor to be used in the same system.
Copper cross-section (mm2) | Aluminium cross-section (mm2) | Aluminium cross-section (mm2) |
At the same temperature rise | At the same voltage drop | |
6 | 10 | 10 |
10 | 16 | 16 |
16 | 25 | 25 |
25 | 35 | 35 |
35 | 50 | 50 |
50 | 70 | 70 |
70 | 95 | 95 |
95 | 150 | 150 |
120 | 185 | 185 |
150 | 240 | 240 |
185 | 300 | 400 |
3. Wiring Precautions
Wiring components should remain undamaged from mechanical or thermal stress.
This damage can be caused by:
Electrodynamic effects caused by short-circuits
Expansions and contractions caused by temperature rises
Magnetic effects caused by the current flowing through them
Movement of the moving parts of the switchboard, etc.
It is also important to ensure compliance with the following points:
Avoid the cables coming into contact with sharp edges and the moving parts of the switchboard
Comply with the bend radius of the cables (values provided by the cable manufacturers)
Check that the cables are not subjected to any pulling or twisting
Check that the connections of the devices mounted on removable parts of the switchboard (doors, pivoting faceplates, etc.) are made using flexible cables and that these conductors are held in place by attachments other than the electrical connections.
3.1 Protection Against The Effects of Short Circuits
Two harmful effects that can impact conductors during a short-circuit include overheating, which can lead to fires, and damage to electrical components, which can cause equipment failure.
Thermal stress is typically countered by protective devices such as fuses and circuit breakers.
Electrodynamic stresses involve forces between conductors.
In the event of a short-circuit between two live conductors, which is a likely scenario, the conductors carrying the short-circuit current will experience a repulsive force. This force is directly proportional to the square of the current flowing through them.
Incorrectly attached conductors may whip around, risking detachment from their connections and contact with another conductor or an exposed conductive part, leading to a short-circuit and potentially destructive arcing.
Multicore cables are engineered to endure the forces exerted among conductors. Nonetheless, employing single-core cables necessitates specific precautionary measures.
The table below is designed to highlight the significance of properly attaching conductors. However, this information alone does not ensure resistance to short-circuit conditions, which require simulation through testing.
Table 4 – Wiring Precautions
Prospective short circuit value (Ik) | Attachment of conductors | |
Ik < 10 kA | The standard IEC 61439-1 does not specify any particular precautions or tests | |
10 kA < Ik < 25 kA | Conductors should be secured using clamps and may be assembled into stranded cables for the same circuit. |  |
25 kA < Ik < 35 kA | Conductors within the same circuit should be separated and individually attached. When bundled in stranded cables, the quantity of clamps should be augmented to one every 50 mm. |  |
35 kA < Ik < 50 kA | In a circuit, conductors should be individually mounted on a rigid support, such as a crosspiece or profile, that has no sharp edges. They need to be physically separated, and each point of attachment should include two clamps arranged in a cross configuration. |  |
Ik > 50 kA | When dealing with such high short-circuit values, the forces necessitate specially designed attachment devices, such as machined crosspieces and threaded rods. In some extreme scenarios, stainless steel profiles and flanges may be utilized. |  |
While the installation conditions for busbars are systematically and precisely determined concerning short circuits (such as the distances between supports), this is often not the case for the conductors inside switchboard panels. These conductors frequently become sources of damage, and this risk ought to be clearly considered.
3.2 Protection Against Magnetic Effects
High currents flowing through conductors generate magnetic effects in the nearby metallic conductive components. These effects can lead to an unacceptable increase in the temperature of the materials. Consequently, it is crucial to implement certain wiring precautions. Hysteresis loss, which is related to the saturation of magnetic materials, occurs in the frameworks formed by structural elements (such as enclosure structures, chassis, and support frames) that are situated around the conductors.
To diminish the induction generated, conductors should be positioned to ensure the field is as feeble as possible. To curtail the induction in magnetic loops, it is recommended to house all live conductors of a circuit (phases and neutral) within the same metallic (steel) enclosures.
When the vector sum of the currents is zero, the resultant fields generated are also zero.
Figure 8 – All the live conductors in the same circuit (phases and neutral) in the same metal (steel) frames
To minimize induction in magnetic loops, it is advisable to have all live conductors of the same circuit (phases and neutral) within the same metal (steel) frames, as this ensures the vectorial sum of the currents is zero, thereby nullifying the fields created.
If it's not feasible for all conductors in the same circuit to pass through simultaneously without incorporating ferromagnetic components (as might occur with device supports, cable entry plates, or dividers), they should be housed in non-magnetic material supports such as aluminum, copper, stainless steel, or plastic. This practice is advised for currents of 400 A per conductor and becomes critical for currents exceeding 630 A.
To minimize induced fields, it is advisable to arrange conductors in a trefoil formation whenever possible.
When inserting and attaching separate conductors to cable ladders, certain precautions are necessary. To prevent substantial temperature increases in the cable ladder components, it is recommended to remove the parts that form frames around a conductor.
It is possible to disrupt the magnetic frame by removing components. However, in all instances, ensure that the mechanical integrity of the support is still satisfactory.
Figure 9 – Removing parts that create frames around a conductor is advisable.
4. Wiring Ahead of Protective Devices
Protection against the consequences of a potential fault (between phases and metal conductive parts) is not guaranteed upstream of overcurrent protection devices in TN and IT neutral earthing systems, or residual current devices in the TT system.
Avoiding the risk of indirect contact is crucial; in practice, double insulation is the sole measure to counter this risk. It can be achieved through the use of devices designed with double insulation or by adding supplementary insulation to the installation.
The implementation of Class II ahead of protective devices adheres to four fundamental rules:
Rule #1 – The utilization of conductors or cables that possess double insulation due to their composition is a common practice.
Rule #2 – The installation of supplementary insulation around conductors lacking double insulation, such as in insulated ducting, conduits, or enclosures, is essential.
Rule #3 – The application of insulating components that secure the bare conductive elements (busbars) with a clearance that is twice the conventional value.
Rule #4 – Secure the conductors by clamping them to prevent any contact with nearby exposed conductive parts in case they become accidentally detached or disconnected.
4.1 The Placement of Conductive Components Should Ensure Electrical Continuity and Conductivity with Metal Conductive Parts
Example of 500 V Insulation Voltage
An example of a 500 V insulation voltage is the requirement that insulation resistance should be approximately one megohm for every 1,000 volts of operating voltage, with a minimum value of one megohm.
Figure 10 – The placement of conductive components in proximity to metallic conductive parts is crucial for ensuring electrical continuity and safety.
These regulations presuppose that minimum distances are consistently upheld, even in the event of faults (electrodynamic forces), through the use of suitable clamping. Clearances may be substituted with slimmer insulating elements (such as screens, supports, separators) that possess adequate mechanical robustness and a minimum dielectric strength of 2500 volts or 4000 volts.
Table 5 – Selection of Conductors and Installation Requirements (IEC 61439-1)
Type of conductor | Requirements |
Bare conductors or single-core conductors with basic insulation, such as cables that comply with IEC 60227-3, are examples of this type of electrical component. | It is essential to avoid mutual contact or contact with conductive parts, which can be achieved, for instance, by employing separators. |
Single-core conductors with basic insulation, capable of withstanding a maximum usage temperature of at least 90°C, such as cables meeting IEC 60245-3 standards, or thermoplastic cables with PVC insulation that are heat-resistant as per IEC 60227-3, are available. | Mutual contact or contact with conductive parts is allowed provided that no external pressure is exerted. It is essential to avoid contact with sharp edges. The conductors should be loaded in a manner that ensures the operating temperature does not exceed 80% of the conductor's maximum allowable temperature for usage. |
Conductors with primary insulation, such as cables meeting the IEC 60227-3 standard, and additional secondary insulation, such as cables individually sheathed with retractable sleeves or installed separately in plastic conduits, are examples of this. | No additional requirements |
Conductors are insulated with materials that possess high mechanical strength, such as ethylene tetrafluoroethylene (ETFE), or they may have double insulation with a reinforced external sheath suitable for use up to 3 kV, like cables that comply with the IEC 60502 standard. | |
Single-core or multicore cables encased in sheaths, such as those complying with IEC 60245-4 or IEC 60227-4 standards, are available options. |
Flexible bars are rated for an insulation voltage of 1000 V. They may be categorized as double-insulated conductors if the operating voltage is limited to U0: 500 V, which is then deemed reinforced insulation. Alternatively, and more preferably, by securing the insulation on the bars mechanically (using clamps, supports, or their inherent rigidity) and maintaining a proper distance from any metallic parts (10 mm), they can be safely used.
Table 6 – Cables are considered to have double insulation.
Type of conductor | Requirements |
U0: 500 V | U0: 250 V |
U-1000 R12N | H05 RN-F |
U-1000 R2V | H05 RR-F |
U-1000 RVFV ** | H05 VV-F |
H07 RN-F | H05 VVH2-F |
A07 RN-F | FR-N05 VV5-F |
FR-N1 X1 X2 | A05 VVH2-F ** |
FR-N1 X1 G1 | |
H07 VVH2-F |
** According to the conditions of use.
5. Permanently Energized Wiring Circuits
Certain measurement, signaling, and detection circuits must be connected upstream of the main protective device in a switchboard. Besides their protection against indirect contact, these circuits require special precautions:
Precaution #1 – To protect against the risks of short circuits
Precaution #2 – To mitigate the risks associated with remaining energized after the main protective device is disconnected, the double insulation specification should be implemented. This will limit the risk of contact with exposed conductive parts, and precautions should be taken to make any risk of short-circuiting highly improbable.
When constructing unprotected circuits, it is crucial to connect the conductors securely. Additionally, the mechanical strength of the conductors should be considered.
Conductors with single insulation, such as H07 V-U/R or H07 V-K, require protection with an additional sheath or need to be placed in ducting to mitigate the risk of contact with parts that may cause injury.
Conductors that possess a high level of mechanical strength and are equipped with PTFE insulation can be utilized directly.
Single-core and multicore cables may be utilized without an extra sheath unless they are exposed to potential hazards, such as sharp edges.
In practical terms, the cross-sections of conductors in unprotected circuits are typically selected based on the circuit's power requirements and must not be too small to ensure adequate mechanical strength. Generally, a minimum cross-section of 4 mm² is adopted.
Protective devices for permanent circuits should be selected based on the circuit's current and the prospective short-circuit current at the supply end of the switchboard. The use of cartridge fuse-type circuit breakers is common when dealing with very high values.
Figure 11 – An example of a connection involves attaching a separate copper plate. The screws are equipped with washers to prevent them from loosening.
Do not rules
Connect to the heads of screws: drilling the thread can weaken even the largest diameter screw!
Connect the wires between the lugs and the connection plate on the device: the wire may be cut and the area of the surfaces is compromised!
Connect the large cross-section power supply cable directly in the terminals on the device: the hold is uncertain!
Figure 12 – Directly connecting a large cross-section power supply cable to the terminals on the device is prohibited.
Warnings!
Permanently energized unprotected circuits are not specifically marked according to IEC 60364. However, it is recommended to clearly label them with warnings such as: "Caution, permanent circuits not disconnected by the main device," and include additional identification for the circuits involved, for instance: "voltage present," "enclosure lighting," "group detection," and so on.
The IEC 60204-1 standard (pertaining to machinery safety) advises that these circuits should be physically isolated from other circuits or marked by orange insulation on the conductors.
Reference: Legrand
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