Cable reference methods categorize the various ways in which a cable is installed. These methods are critical because the surrounding environment directly affects the cable’s ability to dissipate heat; this thermal performance, in turn, dictates its current-carrying capacity under BS 7671 standards.

When describing these methods further, it is helpful to keep these three variables in mind:

• Enclosure: Is the cable in the open air, a conduit, or a tray?

• Surroundings: Is the cable clipped to a masonry wall or buried in a glass-fiber insulated ceiling?

• Heat Dissipation: The “Method” (A, B, C, etc.) essentially acts as a shorthand for how easily the cable can stay cool.

The examples below should provide a clearer understanding of the various installation scenarios covered by the reference methods in BS 7671:2018+A2:2022. For more detailed information, refer to Appendix 4 of the BS 7671 standards. Here is a summary of the main reference methods and associated codes defined in BS 7671:

Main Reference Methods

1. Reference Method A:

   • Enclosed in Conduit in a Thermally Insulated Wall: Multi-core cables directly installed in a thermally insulated wall.
   • Example 1: Electrical cables run through plastic conduits embedded within the thermal insulation of an external wall in a residential building.
   • Example 2: Power cables enclosed in a metal conduit inside a thermally insulated partition wall in an office space.

2. Reference Method B:

   • Enclosed in Conduit or Trunking on a Wall: Cables enclosed in conduits or trunking systems fixed on walls.
   • Example 1: Electrical wiring inside a PVC conduit mounted on the surface of a brick wall in a commercial warehouse.
   • Example 2: Data cables enclosed within trunking systems running along the walls of an IT server room.

3. Reference Method C:

   • Clipped Direct: Cables fixed directly to a surface like a wall or ceiling.
   • Example 1: Power cables clipped directly to the surface of a masonry wall in a garage.
   • Example 2: Lighting cables fixed directly to the wooden beams of a ceiling in a barn.

4. Reference Method D:

   • Buried Directly in the Ground or Within a Duct in the Ground: Cables installed underground either directly buried or placed within protective ducting.
   • Example 1: Armoured cables buried directly in the ground to supply power to a garden shed.
   • Example 2: Electrical cables run within a plastic duct laid underground to connect outdoor lighting in a park.
   • Note:
     • provided that the cable is spaced more than 0.3 x D (overall
       diameter of the cable) from the wall.
     • that cables installed on unperforated cable trays are classified under Method C.
   •  

5. Reference Method E: – Multicore cable in free-air

   • In Free Air – Multi-Core Cables on Perforated Cable Tray or Ladder Rack:
   • Example 1: Multi-core power cables laid on a perforated cable tray along the ceiling of a data center.
   • Example 2: Multi-core cables installed on a ladder rack in an industrial plant’s control room.
   • Note:
     • provided that the cable is spaced more than 0.3 x D (overall
       diameter of the cable) from the wall.
     • that cables installed on unperforated cable trays are classified under Method C.

6. Reference Method F: Single core cables touching in free-air

   • In Free Air – Single-Core Cables on Perforated Cable Tray or Ladder Rack: Cables supported in free air, ensuring good heat dissipation.
   • Example 1: High-voltage single-core cables laid on a perforated cable tray in a factory’s production area.
   • Example 2: Single-core cables run on a ladder rack system in a large electrical substation
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7. Reference Method G:

   • Bare or Non-Sheathed Cables on Insulators: Cables supported on insulators, often used in high-voltage applications.
   • Example 1: Bare overhead transmission lines supported by ceramic insulators in a high-voltage transmission network.
   • Example 2: Non-sheathed copper cables on insulators within an indoor electrical testing facility.

Specific Methods for Twin and Earth Cables

These methods apply specifically to flat twin and earth cables in thermal insulation environments:

1. Reference Method 100:

   • Above a Plasterboard Ceiling with Insulation Not Exceeding 100mm: Cables installed above ceilings with a limited amount of insulation.
   • Example 1: Electrical cables installed above the ceiling in a modern apartment, with insulation limited to 100mm.
   • Example 2: Twin and earth cables running above the ceiling in a renovated home with minimal insulation.
   •  

2. Reference Method 101:

   • Above a Plasterboard Ceiling with Insulation Exceeding 100mm: For thicker insulation scenarios.
   • Example 1: Electrical wiring above the ceiling in a passive house with thick insulation exceeding 100mm.
   • Example 2: Power cables installed in an attic space with heavy insulation for energy efficiency.

3. Reference Method 102:

   • In a Stud Wall with Thermal Insulation Touching the Inner Wall Surface: Cables touching the wall surface within an insulated stud wall.
   • Examples: Wiring within partition walls in residential buildings.

4. Reference Method 103:

   • In a Stud Wall with Thermal Insulation Not Touching the Inner Wall Surface: Cables not touching the wall surface, providing better cooling.
   • Example 1: Cables run within the stud walls of a new residential construction, with insulation in direct contact with the wall surface.
   • Example 2: Wiring for electrical outlets in a well-insulated interior partition wall in a commercial building.