BS 7671 Reference Methods
Reference Methods are codes used to describe the various installation methods for the installation of electrical wiring and cables as set out in BS 7671 Appendix 4, Section 7. Methods of Installation, and are used to determine the current carrying capacity of a cable depending on the reference method employed for each cable type which is required for accurate cable sizing.
This explanation will guide you through the all the installation reference methods, codes and their applications, uses and descriptions.
Updated October 2026 for BS 7671:2018+A4:2026. Amendment 4 has not changed the installation reference methods or the current-carrying capacity tables in Appendix 4.
Reference Method Codes
BS 7671 Reference Methods A–G & 100–103 Explained (Table 4A2).
BS 7671 electrical reference method wiring codes include installation reference methods A, B, C, D, E, F, G, 100, 101, 102 and 103.
Methods 100, 101, 102 and 103 refer only to flat twin and earth cables installed in thermal insulation.
See BS 7671 Table 4A2 – Schedule of installation methods of cables of BS 7671 for determining the current-carrying capacity of cables.
Reference Method Table
| Reference method | Installation description | Typical cable / example |
|---|---|---|
| A | Enclosed in conduit in a thermally insulating wall, or multicore cable direct in a thermally insulating wall | Singles in conduit in an insulated cavity wall |
| B | Enclosed in conduit or trunking on a wall, or in conduit in masonry | Singles in PVC trunking or conduit on a wall |
| C | Clipped direct to a surface, on an unperforated tray, or embedded direct in masonry or plaster | Twin & earth clipped to a wall or joist; SWA clipped to a wall |
| D1 | In a duct in the ground | SWA in a duct to an outbuilding |
| D2 | Buried direct in the ground | SWA buried direct (T&E is not suitable) |
| E | Multicore cable in free air: on perforated tray, ladder rack, brackets or cleats, spaced from the wall | SWA or multicore on perforated tray |
| F | Single-core cables touching, in free air: on perforated tray, ladder rack, brackets or cleats | Single-core cables on ladder rack |
| G | Single-core cables spaced apart in free air, including bare or insulated conductors on insulators | Spaced singles on cleats or insulators |
| 100 | Flat twin & earth above a plasterboard ceiling, covered by thermal insulation not exceeding 100 mm | T&E in a loft under 100 mm insulation |
| 101 | Flat twin & earth above a plasterboard ceiling, covered by thermal insulation exceeding 100 mm | T&E in a loft under 270 mm insulation |
| 102 | Flat twin & earth in a stud wall with thermal insulation, cable touching the inner wall surface | T&E in an insulated stud wall, touching the plasterboard |
| 103 | Flat twin & earth in a stud wall with thermal insulation, cable not touching the inner wall surface | T&E surrounded by insulation in a stud wall |
Methods 100–103 apply to flat twin & earth cable only. Current ratings for T&E are in Table 4D5. Always check the full descriptions in Table 4A2.
Reference Methods
The main reference methods and descriptions are,
Reference Method A = Enclosed in conduit in a thermally insulated wall.
Reference Method A = Multi-core cable direct in a thermally insulated wall.
Reference Method B = Enclosed in conduit or trunking on a wall.
Reference Method C = Clipped Direct.
Reference Method C = On unperforated tray, (with item 2 of table 4C1).
Reference Method D = Buried directly in the ground (D2), or within a duct in the ground (D1).
Reference Method E = Multi-core cables on perforated cable tray or ladder rack.
Reference Method F = Single-core on perforated cable tray or ladder rack.
Reference Method G = Bare or non-sheathed cables on insulators.
In BS 7671 2018 Amendment 4 2026 Reference Method D has been split to D1 and D2.
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BS7671 table 4A2 details the specific applications of the reference methods as various installation methods may be covered by the same reference method code.
It is important to note that not all cable types are suitable for every type of Reference Method, for example, it is not permitted to install flat twin and earth cables directly in the ground (Reference Method D2).
Twin and Earth Reference Methods
The following reference methods are only applicable to flat twin & earth cables in thermal insulation.
Reference Method 100 = Above a plasterboard ceiling covered by thermal insulation NOT exceeding 100mm.
Reference Method 101 = Above a plasterboard ceiling covered by thermal insulation EXCEEDING 100mm.
Reference Method 102 = In a stud wall with thermal insulation with the cable TOUCHING the inner wall surface.
Reference Method 103 = In a stud wall with thermal insulation with the cable NOT TOUCHING the inner wall surface.
Reference methods 100, 101, 102 and 103 do not apply to any other cable type other than flat twin & earth cables, see tables 4A2 and 4D5 of BS 7671.
For a full and exact description of all the different electrical installation reference methods listed in BS 7671, Appendix 4, section 7.
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Reference Methods FAQs
Clipped direct to a surface, or laid on an unperforated tray. It is the most common method for twin and earth and gives the highest current rating for T&E.
Method 100 if the ceiling insulation over the cable is 100 mm or less, or method 101 if it is more than 100 mm. If the cable is clipped to a joist above the insulation, it is method C.
Method 102 if the T&E is touching the inner surface of the wall, or method 103 if it is surrounded by insulation and not touching the wall.
Method D, whether it is buried direct or run in a duct.
No. The installation methods and current-carrying capacity tables are unchanged in BS 7671:2018+A4:2026.
Appendix 4, Table 4A2 (schedule of installation methods). Current ratings for flat T&E are in Table 4D5.
