Selecting low-voltage cables for building projects: steps and TCVN standards

1. Select the cable type by voltage, installation location and environment

Low-voltage power cables with extruded insulation such as CVV and CXV are rated 0.6/1 kV and fall within the scope of TCVN 5935-1:2013. Wires and cables with PVC insulation to TCVN 6610 have rated voltages up to 450/750 V. The insulation material determines the maximum operating temperature of the conductor: 70 °C for PVC, 90 °C for XLPE (TCVN 7447-5-52:2010, Table 52.1). Therefore, for the same cross-section and installation method, an XLPE cable carries a higher current.

The insulation must suit the supply voltage, the installation method and the environmental conditions (TCVN 9207:2012, clause 5.4). The wiring system type is also chosen by environment (Table 5 of this standard). For example, in damp and very damp locations and outdoors, steel conduit and closed steel trunking with a conduit or trunking wall thickness of 2 mm or less are prohibited. For the designation of each cable type, see how CV, CVV, CXV and DSTA differ. Cable ranges are listed in the low-voltage copper cables and low-voltage aluminium cables categories.

2. Determine the design current IB

The design current IB is taken from the load calculation. Under TCVN 9206:2012, the supply voltage to equipment in residential and public buildings (except motors) shall not exceed 380/220 V (clause 4.2). The design power factor of residential networks is taken as 0.80 to 0.85 (clause 5.8). How to convert power to current, with a table of current by power, is covered in how to size cables by power.

3. Select the cross-section by current-carrying capacity, then compare with the minimum cross-section

Select the smallest size whose corrected current-carrying capacity is not less than IB. The current-carrying capacity is looked up by conductor material, insulation, number of loaded conductors and installation method, then multiplied by the temperature and grouping factors. Look it up in the cable current-carrying capacity tables or enter the values in the cable size calculator. TCVN 9207:2012 (clause 10.2) gives priority to values declared by the manufacturer. The tables of TCVN 7447-5-52 are used only when the manufacturer does not specify them.

The selected cross-section shall not be smaller than the minimum cross-section for each type of circuit given in the table below.

• The cable must be protected against overload according to IB ≤ In ≤ Iz (QCVN 12:2014/BXD, clause 2.6.3.1). For how to select the circuit-breaker, see selecting an MCB by load current.
• Current-carrying capacity shall be determined for the section of the route with the poorest heat dissipation, except for wall penetrations shorter than 0.35 m (QCVN 12:2014/BXD, clause 2.1.4.6).

4. Check the voltage drop to the most distant equipment

In residential and public buildings, the voltage drop from the origin of the supply to any equipment shall not exceed 5 % of the nominal voltage (QCVN 12:2014/BXD, clause 2.1.7). TCVN 9207:2012 (Table 9) also takes 5 % for lighting, motors and other loads, and 10 % for 12–42 V equipment. On long runs, the cross-section often has to be increased because of voltage drop rather than load current. Calculation method and examples are in permissible voltage drop and how to calculate it. For a quick result, use the voltage drop calculator.

5. Neutral conductor and PE protective conductor

The neutral conductor must be equal to the phase conductor in single-phase two-wire circuits and in three-phase circuits with phase conductors up to 16 mm² (QCVN 12:2014/BXD, clause 2.1.5.2). The neutral may be smaller than the phase conductor only when all conditions are met: the phase conductor is larger than 16 mm², the load is balanced, 3rd harmonic content does not exceed 15 % and the neutral conductor is protected against overcurrent. Even then, the neutral must not be less than 50 % of the phase conductor and not less than 16 mm². How to read a 3-phase + 1 neutral cable designation is explained in reading cable designations and specifications under TCVN.

The PE conductor is of the same metal as the phase conductor, with its cross-section according to TCVN 9207:2012 (Table 12):

• Phase conductor S ≤ 16 mm²: PE equal to S.
• 16 < S ≤ 35 mm²: PE 16 mm².
• 35 < S ≤ 400 mm²: PE equal to S/2.
• PE conductor not forming part of the supply wiring system: minimum 1.5 mm² if protected against mechanical damage, 4 mm² if not (TCVN 9207:2012, clause 10.11).

6. Fire requirements

Wires and cables supplying fire-fighting and evacuation loads must have a sheath of fire-resistant material (TCVN 9207:2012, clause 5.3). These loads include exit lighting, central processing units for fire alarm and automatic fire suppression, fire pumps, stairwell pressurisation fans and lifts. Where the building requires reduced fire spread, QCVN 12:2014/BXD (clause 2.1.9.1) requires materials that minimise the risk of fire spread. How fire-resistant cables differ from flame-retardant cables is explained in fire-resistant and flame-retardant cables. Cable ranges are listed in the fire-resistant and flame-retardant cables category.

When ordering

For each run, state the cable designation, number of cores × cross-section, voltage rating, brand and length in metres. Specify clearly which runs require armour, fire-resistant or flame-retardant cable. Certification documents differ by brand and product, so state your requirements when you send the list. A sample cable line entry is in preparing a BOQ to receive a quotation.

Hoang Nhat is an authorised dealer of Cadi-Sun, an official distributor of GoldCup and a distributor of TAYA; other cable groups are listed in the wires and cables category.