ITC15: Single Bypass Electrical Installation Guide
1. Definition of Single Bypass
A Single Bypass is a part of the electrical installation that supplies electricity from the main power line to a user’s installation. It starts at the main connection point and includes safety fuses, measuring devices, and general protection equipment.
Individual derivations within a Single Bypass can consist of:
- Insulated tubes inside electrical cabinets.
- Insulated tubes within buried pipes.
- Insulated tubes in surface-mounted installations.
- Insulated tubes within protective channels that require tools to open.
- Busbar trunking compliant with UNE-EN 60,439-2.
- Insulated tubes within masonry ducts specifically designed for electrical installations.
In all these cases, the pipes, channels, and their installation must adhere to the guidelines outlined in ITC-BT-21, unless otherwise specified in this instruction. These pipes must always include a protective conductor.
Each individual derivation will be entirely independent of derivations for other users.
2. Installation Guidelines
Protective tubes and channels should have a nominal cross-section that allows for a 100% expansion of the initially installed wire size. Under these installation conditions, the minimum nominal outer diameter of pipes for individual derivations should be 32 mm.
When two or more individual derivations share the same route, they can be installed together within a channel protected by a cable protector, ensuring adequate separation between them.
Regardless of the configuration, there should be one reserve pipe for every ten individual derivations, or a portion thereof, starting from the point where they merge into homes or premises. This is to facilitate future extensions.
In areas where partitioning is not defined, at least one tube should be installed per 50 m2.
Joints in rigid tubes should be threaded or embedded to prevent separation.
In buildings primarily used for residential, commercial, office, or industrial purposes, individual derivation points should be strategically placed for common use or in easily accessible locations.
When individual derivations run vertically, they should be housed within a groove or through masonry walls with a fire resistance rating of RF 120. These should be exclusively dedicated to this purpose and either embedded or attached to the stairwell or common areas, except where safe harbors are established as per NBE-CPI-96. They should be free of bends, changes in direction, and properly sealed. To prevent falling objects and the spread of flames, firewalls and sealed manhole covers should be installed at least every three floors, matching the dimensions of the groove to facilitate inspection and installation. These characteristics should be defined by NBE-CPI-96. Manhole covers should have a minimum fire resistance rating of RF 30.
The minimum dimensions of the groove or through masonry should adhere to the following table:
Table 1: Minimum Dimensions of Groove or Through Brickwork
| Number of Derivations | Width L (m) | Depth P = 0.15 m (One Row) | Depth P = 0.30 m (Two Rows) |
|---|---|---|---|
| Up to 12 | 0.65 | 0.50 | – |
| 13-24 | 1.25 | 0.65 | – |
| 25-36 | 1.85 | 0.95 | – |
| 36-48 | 2.45 | 1.35 | – |
For identified individual derivations, the necessary number of channels or grooves should be provided.
The minimum height of the access covers should be 0.30 m, with a width equal to that of the groove. The top of the cover should be installed at least 0.20 m from the ceiling.
To simplify installation, sealable manholes, common to all individual derivation tubes, can be placed every 15 m. These manholes should not contain conductor joints. The boxes should be insulated, non-flame propagating, and have a flammability rating of V-1, according to UNE-EN 60695-11-10.
For insulated cables within buried pipes, individual derivations should comply with the guidelines outlined in ITC-BT-07 for underground networks, unless otherwise specified in this instruction.
3. Cable Specifications
The number of conductors will determine the number of phases required for the corresponding receptors and their power rating, with each line having its corresponding neutral and protective conductor. For individual supplies, the connection of the protective conductor is left to the discretion of the installation designer. Additionally, each individual bypass should include a control wire to allow for the application of different tariffs. The use of a common neutral wire or protective conductor for multiple supplies is prohibited.
When determining the number of phases for an individual derivation, the single-phase power required by the distribution company should be considered if the user requests it.
Cables and their joints should have a uniform cross-section, except for connections made at the meter and protection devices.
Conductors should be made of copper or aluminum, typically unipolar and insulated, with a rated voltage of 450/750 V. The color coding specified in ITC-BT-19 should be followed.
For multi-conductor cables or individual derivations within buried pipes, the conductor insulation should be rated for 0.6/1 kV.
Cables and cable management systems should be installed in a manner that does not compromise the fire safety features of the building structure.
Cables should be non-fire propagating and have reduced smoke emission and opacity. Cables equivalent to those specified in UNE 21,123 Part 4 or 5, or UNE 211002 (depending on the cable’s rated voltage), 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 also meet this requirement.
The minimum cross-sectional area for phase, neutral, and protective conductors is 6 mm2, and 1.5 mm2 for the control wire, which should be red.
When calculating the wire size, the following factors should be considered:
a) Expected Demand: This is the minimum demand set by RBT-010, and its intensity will be controlled by private control and protection devices.
For permissible current ratings for each cross-section, refer to ITC-BT-19. For insulated cables within buried pipes, refer to ITC-BT-07.
b) Maximum Allowable Voltage Drop:
- For installations with meters concentrated in more than one location: 0.5%.
- For installations with fully concentrated meters: 1%.
- For individual derivations to single-user supplies without a general supply line: 1.5%.