How to Choose the Line Configuration: A Brief Answer
The number of cores is determined by the earthing system and the type of load. For a three-phase 380 V network in the TN-C system, four cores are sufficient: three phase cores and one combined PEN. In the TN-S and TN-C-S systems, five cores are used for a three-phase load: three phase cores, a neutral N and a protective PE. For a single-phase 220 V network in the TN-S system, three cores are needed: phase, N and PE. Cables with a number of cores from 1 to 5 are manufactured according to GOST 31996-2012 for voltages of 0.66 and 1 kV[reference:0].
The main rule: the number of cores is determined not by power, but by the network scheme and the requirements for protective earthing. If in the TN-C-S system the neutral and protective functions are separated along the entire length of the line, a five-core cable is used. If in the TN-C system the PEN conductor combines the functions of N and PE, four cores are sufficient. More details about the differences in configurations can be found in the material about single-core and multi-core cable.
What Data Is Needed for Selection
- Earthing system — TN-C, TN-S, TN-C-S or TT. Determines whether a separate protective PE core is needed.
- Number of load phases — single-phase (220 V) or three-phase (380 V).
- Design load current Idesign (A) — for selecting the cross-section of the phase cores.
- Short-circuit current (kA) — for checking the thermal stability of the PE core.
- Laying method — openly, in a pipe, in the ground, in a tray. Affects the permissible current.
- Conductor material — copper or aluminium.
- Insulation type — PVC or cross-linked polyethylene (XLPE).
Earthing Systems and Number of Cores
| Earthing system | Single-phase 220 V network | Three-phase 380 V network | Composition of cores |
| TN-C | 2 cores | 4 cores | L + PEN (single-phase); 3L + PEN (three-phase) |
| TN-S | 3 cores | 5 cores | L + N + PE; 3L + N + PE |
| TN-C-S (on the TN-C section) | 2 cores | 4 cores | L + PEN; 3L + PEN |
| TN-C-S (on the TN-S section) | 3 cores | 5 cores | L + N + PE; 3L + N + PE |
The TN-C system is an outdated one with a combined PEN conductor. It is used in old networks. The TN-S system has separate N and PE along the entire length, five-wire for three-phase consumers[reference:1]. The TN-C-S system is combined: on the section from the substation to the service entrance device, PEN is used, and after that, separate N and PE.
For a three-phase network in the TN-S system, five wires arrive at the consumer: three phase, neutral and protective[reference:2]. A four-core cable with three phase and one neutral core is used in systems where protective earthing is provided by a separate conductor or loop. A five-core cable for three-phase consumers is used in the TN-S and TN-C-S systems[reference:3].
Cross-Section of the Neutral and Protective Cores
The cross-section of the neutral working core N and the protective core PE may differ from the cross-section of the phase cores. The rules are given in the PUE RK and GOST 31996-2012.
| Phase core cross-section, mm² | N or PE cross-section, mm² |
| up to 16 | equal to the phase one |
| 25 | 16 |
| 35 | 16 |
| 50 | 25 |
| 70 | 35 |
| 95 | 50 |
| 120 | 70 |
For three-phase lines in residential buildings, the cross-section of neutral conductors is taken equal to the cross-section of phase conductors if the phase conductors have a cross-section up to 25 mm² for aluminium[reference:4]. In single- or three-phase networks, the cross-section of the neutral working conductor and the PEN conductor is equal to the phase one when its cross-section is 16 mm² and below for a copper core and 25 mm² and below for an aluminium one[reference:5].
The protective PE core must withstand the short-circuit current until the protection operates. The thermal stability check of the PE core is performed using the same formulas as for the phase cores, taking into account the coefficient k for the corresponding material and insulation. VVG or VVGng — differences in design and number of cores.
Step-by-Step Example: Choosing a Configuration for a Three-Phase Line
Initial data. A three-phase 380 V line from a distribution board to a motor. Design load current Idesign = 55 A. Three-phase short-circuit current at the beginning of the line Ip0 = 12 kA. Tripping time ttrip = 0.13 s. Earthing system — TN-S. Laying in the ground, soil temperature +15 °C. Copper conductors, PVC insulation.
Step 1. Selection of the number of cores.
TN-S system, three-phase load — we accept five cores: 3L + N + PE.
Step 2. Selection of the cross-section of phase cores by heating.
According to the table of permissible currents for copper conductors with PVC insulation when laying in the ground (Table 1.3.6 of the PUE), for Idesign = 55 A, a cross-section of 16 mm² is suitable (tabulated current 90 A for standardised conditions: +15 °C, single cable).
Step 3. Thermal stability check.
For copper with PVC insulation, k = 115 A·s1/2/mm². Thermal impulse: Bk = Ip0² · ttrip = 12 000² · 0.13 = 18 720 000 A²·s. Minimum cross-section: Smin = √18 720 000 / 115 ≈ 4327 / 115 ≈ 37.6 mm². A cross-section of 16 mm² does not pass the thermal stability check. We accept a cross-section of 50 mm².
Step 4. Cross-section of the neutral and protective core.
For a phase core of 50 mm², the cross-section of N and PE according to Table 2 is 25 mm². We accept a cable 5×50 + 1×25 (or 5×50 with N and PE of 25 mm² each).
Step 5. Heating check for N and PE.
The N core carries only the unbalance current. Under a symmetrical three-phase load, the current in N is close to zero. A cross-section of 25 mm² passes with a margin. The PE core carries no current in normal mode; the check is only for thermal stability during a short circuit, which was performed in Step 3 for the phase core. For a PE core of 25 mm², thermal stability is checked separately if a short-circuit current can flow through it. The design and characteristics of a power cable are described in a separate material.
Step 6. Result.
We accept a 5×50 cable with copper conductors and PVC insulation (N and PE — 25 mm²). The TN-S system requires five cores. A comparison of copper and aluminium conductors affects the choice of the table of permissible currents and the coefficients k.
What Can Change the Result
| Parameter | How it affects the choice |
| Earthing system | TN-C — 4 cores for a three-phase network; TN-S and TN-C-S — 5 cores. Determines the presence of a separate PE core. |
| Type of load | A symmetrical three-phase load (motor) allows an N core of a smaller cross-section. An asymmetrical one (lighting, household consumers) requires N equal to the phase one. |
| Short-circuit current | Determines the minimum cross-section of the PE core by thermal stability. The higher the short-circuit current and tripping time, the larger the cross-section. |
| Laying method | Affects the permissible current of the phase cores, and hence the preliminary cross-section. For grouped laying, reducing coefficients are introduced. |
| Conductor material | Copper allows a higher current for the same cross-section than aluminium. Comparison of copper and aluminium. |
| Insulation type | XLPE allows conductor heating up to +90 °C, PVC — up to +70 °C. For XLPE, the permissible currents are higher. |
Typical Mistakes
- Using a four-core cable in a TN-S system. In TN-S, the neutral and protective functions are separated along the entire length. Combining N and PE in one core violates electrical safety requirements.
- Using a five-core cable in a TN-C system without separating PEN. If the system is TN-C and the PEN separation is not performed, the fifth core remains unused, which increases the cost of the line without a functional necessity.
- Selecting an N core of an underestimated cross-section for an asymmetrical load. In the presence of higher harmonics (computers, switched-mode power supplies), the current in the N core can reach the current of the phase cores or more. In such cases, N is taken equal to the phase one regardless of the cross-section.
- Checking thermal stability only of the phase cores. The PE core must also withstand the short-circuit current. If a short-circuit current flows through PE, its cross-section is checked using the same formulas as the phase cores.
- Neglecting protection selectivity. For selective protection at the service entrance, the tripping time is longer than on outgoing lines. This increases the thermal impulse and the required cross-section of all cores, including PE.
Brief Selection Algorithm
- Determine the earthing system (TN-C, TN-S, TN-C-S) from the project or the actual network scheme.
- Determine the number of load phases: single-phase or three-phase.
- Select the number of cores according to Table 1.
- Calculate the design load current Idesign.
- Select a preliminary cross-section of the phase cores from the table of permissible currents, taking into account corrections.
- Check the thermal stability of the phase cores and the PE core during a short circuit.
- Determine the cross-section of N and PE according to Table 2 or by calculation.
- When choosing the cable type, take into account the installation conditions. VVG or VVGng — differences in insulation.
FAQ
When is a five-core cable needed, and when are four cores sufficient?
A five-core cable is needed in the TN-S and TN-C-S systems for a three-phase load, where the neutral and protective functions are separated. A four-core cable is used in the TN-C system, where the PEN conductor combines the functions of N and PE, or in systems with a separate earthing loop.
Can a four-core cable be used in a TN-C-S system?
On the section from the substation to the service entrance device, where the TN-C-S system operates as TN-C — yes. After the point of separation of PEN into N and PE — no, a five-core cable is required. The separation point must be marked and earthed.
What cross-section should the neutral core have?
The cross-section of the N core depends on the nature of the load. Under a symmetrical three-phase load, the N core may have a smaller cross-section than the phase cores (according to Table 2 of GOST 31996-2012). Under an asymmetrical load and in the presence of higher harmonics, N is taken equal to the phase one.
Does the laying method affect the choice of the number of cores?
The laying method affects the permissible current and, consequently, the cross-section of the cores, but not their number. The number of cores is determined by the earthing system and the type of load.
How to check the thermal stability of the PE core?
The calculation is similar to the check of the phase cores: Smin = √Bk / k, where Bk is the thermal impulse of the short-circuit current, k is the coefficient for the material and insulation of the PE core. If the PE core has a smaller cross-section than the phase cores, the check is performed separately for its cross-section.
What to do if in a TN-C system there is no possibility of switching to a five-core cable?
In the TN-C system, the separation of PEN into N and PE is performed in the service entrance device. If the cable from the substation to the entrance is four-core, a busbar is installed at the entrance, to which PEN is connected, and separate N and PE conductors are laid from it. Further through the building, five-core cables are used.
Sources Used
- Electrical Installation Code of the Republic of Kazakhstan 2015, as amended by the order of the Minister of Energy of the Republic of Kazakhstan dated 31.10.2022 No. 340 — Committee for Technical Regulation and Metrology of the MIR RK — https://base.spinform.ru/show_doc.fwx?rgn=71151
- GOST 31996-2012 Power cables with plastic insulation for rated voltages of 0.66; 1 and 3 kV. General technical conditions — Interstate Council for Standardization, Metrology and Certification — https://docs.cntd.ru/document/1200104301
- GOST 24334-2020 Power cables for non-stationary installation. General technical requirements — Interstate Council for Standardization, Metrology and Certification — https://docs.cntd.ru/document/1200171038
- Technical Regulation of the Customs Union TR CU 004/2011 “On the safety of low-voltage equipment” — EAEU — https://docs.eaeunion.org/docs/ru-ru/0145005
