Electrical Supply Line Requirements: Design & Installation Standards

General Supply Line Requirements: Definition

A general supply line links the General Protection Fund with the centralization of meters. From the same general supply line, different meter centralizations may be fed.

General power lines shall comprise:

• Insulated conductors within cabinets.
• Insulated conductors within buried conduits.
• Insulated conductors within surface-mounted conduits.
• Insulated conductors within protective channels with lids requiring a tool to open.
• Busbar trunking systems compliant with UNE-EN 60439-2.
• Insulated conductors within masonry ducts, specifically designed and built for this purpose.

In the aforementioned cases, conduits and ducts, along with their installation, shall comply with ITC-BT-21, unless otherwise specified in this instruction. The wiring shall, in all cases, include the protective conductor.

Electrical Supply Line Installation Standards

The general supply route should be as short and straight as possible, running through common areas.

When installed within conduits, their thickness, depending on the cable section to be installed, shall conform to Table 1. The dimensions of other conduit types must allow for a 100% increase in conductor section.

For installations of insulated conductors and protective conductors within buried conduits, compliance with ITC-BT-07 is required, except as specified in this instruction.

Conduit connections shall be threaded rigid conduit or encased, ensuring ends cannot be separated.

Furthermore, when the general supply line passes vertically through a groove, recessed brickwork, or is attached to a stairwell in common areas, curves, changes in direction, and temperature influence from other building pipes should be avoided. This conduit shall be registered and sealed at each floor, with at least three floors providing a firewall, and its walls shall have a fire resistance of RF 120 according to NBE-CPI-96. Access covers shall have a minimum fire resistance of RF 30. The minimum duct size shall be 30 x 30 cm and must be used exclusively for the general supply line and protective conductor.

Cable Specifications for Electrical Systems

Conductors to be used, three-phase and one neutral, shall be copper or aluminum, unipolar and insulated, with an insulation level of 0.6/1 kV.

Cables and cable management systems must be installed without compromising the fire safety features of the building structure. Cables shall be non-fire propagating with reduced smoke emission and opacity. Cables equivalent to those specified in UNE 21123, parts 4 or 5, meet this requirement.

Cable management elements with characteristics equivalent to those classified as "non-flame propagating" according to UNE-EN 50085-1 and BS EN 50086-1 meet this requirement.

Where aluminum conductors are used, connections must be made using appropriate techniques to prevent conductor deterioration due to potential hazards arising from galvanic effects.

The cable section must be uniform throughout its length and without splices, except for derivations made inside power meter centralization boxes. The minimum cross-sectional area is 10 mm² for copper or 16 mm² for aluminum.

For calculating cable sections, the maximum allowable voltage drop and maximum permissible current must be considered.

The maximum allowable voltage drop is:

• For general supply to fully centralized meters: 0.5%.
• For general supply to partially centralized meters: 1%.

The maximum permissible current to consider is specified in UNE 20460-5-523, with correction factors for each assembly type, in accordance with the anticipated power levels established in ITC-BT-10.

For the neutral conductor section, the maximum expected imbalance, harmonic currents, and behavior based on established overload and short-circuit protections must be considered. The neutral conductor section shall not be less than 50% of the phase conductor's section, nor less than the values specified in Table 1.

Table 1