Reference Methods: How Cable Installation Affects Capacity

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Installation reference methods are standardised descriptions of how a cable is physically installed in a building or site. Table 4A2 of Appendix 4 of BS 7671:2018+A4:2026 lists around 70 numbered installation methods and gives, against each one, the reference method to be used to determine current-carrying capacity. Regulation 521.2 requires the installation method of a wiring system to be in accordance with Table 4A2, and Regulation 523.2 requires the current in the cable not to exceed the value read from Appendix 4 with reference to Table 4A2, subject to any necessary rating factors. The reference method you select determines which column of the Appendix 4 current-carrying capacity tables you read, and therefore the maximum current the cable can safely carry.

The reason reference methods matter is heat dissipation. A cable generates heat when it carries current (I²R losses). How effectively that heat can escape into the surrounding environment depends entirely on the physical installation arrangement. A cable in free air on an open tray can shed heat freely in all directions. The same cable enclosed in conduit inside a masonry wall, surrounded by plaster and possibly thermal insulation, cannot dissipate heat nearly as well — so its safe current-carrying capacity is lower.

BS 7671 uses eight reference methods: A, B, C, D1, D2, E, F and G. (Cable calculation software sometimes shows the IEC subdivisions A1/A2 and B1/B2 for single-core versus multicore cables; the Appendix 4 tables in BS 7671 do not use them — the single-core and multicore cases are separate tables instead.) Each method represents a different installation arrangement, from the most restrictive (enclosed in a thermally insulated wall) to the least restrictive (free air). Selecting the correct reference method is not optional — it is a required step in every cable sizing calculation. Table 4A2 is not a closed list: Regulation 521.2 permits other installation methods provided they meet the requirements of Chapter 52, and Appendix 4 notes that the evaluation of current-carrying capacity may then need to be based on experimental work.

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BS 7671 Reference Method C: Current Capacity

BS 7671 Reference Method C for cables clipped direct to surface. Check Appendix 4:2026 current ratings in seconds. Avoid undersizing on site.

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Method A covers cables in conduit inside a thermally insulated wall. Appendix 4 describes the wall precisely: an outer weatherproof skin, thermal insulation, and an inner skin of wood or wood-like material with a thermal conductance of at least 10 W/m²K. The conduit sits close to — but not necessarily touching — the inner skin, and heat from the cables is assumed to escape through the inner skin only. The conduit may be metal or plastic.

When it applies: Conduit run inside a timber-framed or otherwise thermally insulated wall, and — per Appendix 4 — a cable installed in a ceiling, which it treats as similar to Reference Method A. Appendix 4 also warns that higher ambient temperatures can arise in ceiling-mounted junction boxes serving luminaires, in the region of 40 °C to 50 °C, in which case a rating factor from Table 4B1 must be applied.

Capacity example: A 2.5 mm² T+E cable under Method A has a current-carrying capacity of just 20 A. Compare this with 27 A under Method C. That 7 A difference can determine whether you need to use 4 mm² cable instead of 2.5 mm².

Method B covers cables enclosed in conduit or trunking on a wooden or masonry wall, and conduit or ducting buried in masonry. Appendix 4 describes the reference case as conduit mounted such that the gap between the conduit and the surface is less than 0.3 times the conduit diameter; the conduit may be metal or plastic. It notes that where the conduit is fixed to a masonry wall, rather than a wooden one, the actual capacity may be higher than tabulated.

Method B gives higher current-carrying capacities than Method A because the enclosure is not surrounded by thermal insulation. It is still lower than Method C (clipped direct) because the conduit or trunking restricts airflow around the cable. Note that the tabulated Method B values are for a single circuit: where more than one circuit shares the trunking, the group rating factor from Table 4C1 applies, whether or not there is an internal barrier or partition.

Common use: Commercial and industrial installations where cables are run in surface-mounted conduit or trunking for protection and appearance. Also used in domestic garages, workshops, and utility rooms where surface wiring is acceptable.

Method C is the most common reference method for domestic installations using flat twin-and-earth (T+E) cable. Appendix 4 describes it as single-core or multicore cable fixed on, or spaced less than 0.3 times the cable diameter from, a wooden or masonry wall (Installation Method 20) — no conduit or trunking enclosure. Where the cable is fixed to or embedded in a masonry wall rather than a wooden one, Appendix 4 notes the actual capacity may be higher than tabulated.

Method C provides good heat dissipation because the cable is in contact with the mounting surface on one side and exposed to air on the other. The mounting surface (wood, masonry, plaster) absorbs and conducts some heat away from the cable, while the exposed side radiates and convects heat into the air.

Watch the ceiling case. A cable clipped to joists is Method C, but a cable fixed directly under a wooden or masonry ceiling is Reference Method B in Table 4A2, because convection around the cable is reduced. And a T+E cable above a plasterboard ceiling or inside a stud wall containing thermal insulation is not Method C at all — it falls under Reference Methods 100 to 103, covered below.

Capacity example: A 2.5 mm² T+E cable under Method C has a current-carrying capacity of 27 A — a 35% increase over Method A (20 A) for the same cable. This is why the reference method selection has such a significant impact on cable sizing decisions.

Important note: If a cable that is mostly clipped to joists (Method C) passes through a section of conduit in a wall (Method B) or through a thermally insulated wall (Method A) at any point, the more restrictive method applies to the entire run. Design for the worst case.

BS 7671 splits the buried case into two reference methods, and mixing them up is a common source of undersized cable. This is the standard territory for armoured cable (SWA) runs supplying outbuildings, garages, garden offices, external lighting, and any other installation that requires an underground route.

Method D is unique because the thermal environment (soil) is very different from air. The D1 and D2 capacities are given in separate columns of the armoured cable tables in Appendix 4 — Table 4D4A for 70 °C thermoplastic armoured cable and Table 4E4A for 90 °C thermosetting (XLPE) armoured cable. Grouping of buried cables (multiple circuits in the same trench) also has a significant impact: use Table 4C2 for cables buried directly in the ground and Table 4C3 for single cables in ducts buried in the ground — not the Table 4C1 factors used for cables in air.

Methods E, F and G all cover cables installed in free air — on perforated trays, ladders, brackets, cleats, ties or hangers — where air can circulate freely around the cable. These methods give the highest current-carrying capacities because heat dissipation is maximised. What separates E from F from G is not the support system, as is often assumed: Table 4A2 gives “E or F” for perforated tray, wire mesh tray, brackets, ladder and support wire alike. The split is by cable type and spacing.

Appendix 4 sets thresholds for whether a support system counts as free air at all. A perforated tray must have a regular pattern of holes occupying at least 30 % of the base area — a tray with no holes, or with less than 30 % open area, is Reference Method C, not E or F. A ladder system qualifies where the metalwork under the cables occupies less than 10 % of the plan area. And free-air capacities may only be used where the clearance between the cable and any adjacent surface is at least 0.3 times the cable diameter for multicore cables, or 1.0 times the cable diameter for single-core cables.

These methods are rarely used in domestic work but are essential for commercial and industrial installations. The higher current-carrying capacities they provide can mean smaller (and cheaper) cables for the same circuit, which makes a significant cost difference on large installations with many long cable runs.

Alongside the lettered reference methods, Table 4A2 of BS 7671 carries a separate section headed “Installation methods for flat twin and earth cables in thermal insulation”, giving four numbered methods — 100, 101, 102 and 103. These are the methods that apply to almost every domestic rewire. They are not an On-Site Guide invention: they are in BS 7671 itself, and the ratings for all four come from Table 4D5.

Table 4A2 adds the overriding instruction: wherever practicable, a cable is to be fixed in a position such that it will not be covered with thermal insulation, and points to Regulation 523.9 and BRE guidance on avoiding overheating. The IET On-Site Guide reproduces the same material in Tables 7.1(iii) and 7.1(iv), noting that Methods 100, 101 and 102 all require the cable to be in contact with the plasterboard or the joists. Where a cable is instead surrounded by thermal insulation for more than 0.5 m and no more precise information is available, the On-Site Guide requires the capacity to be taken as 0.5 times the Method C value.

Practical note: These scenarios are very common in domestic rewires and first-fix work. Any T+E cable that runs through a stud wall or above a plasterboard ceiling in a void containing thermal insulation must be assessed against Methods 100 to 103 rather than simply defaulting to Method C.

Selecting the correct reference method is a three-step process:

In practice, for most domestic work the choice is between Method C (clipped direct), Method B (conduit or trunking, including conduit chased into masonry, and cable fixed directly under a ceiling) and Methods 100 to 103 (T+E above a plasterboard ceiling or in an insulated stud wall). Method A only comes in where the wall is a thermally insulated construction. For commercial work with tray and trunking, you may be choosing between Method B (trunking) and Method E or F (perforated tray, ladder or cleats, depending on whether the cable is multicore or single-core). Always document your reference method choice — it forms part of the design records required by BS 7671.

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The difference in current-carrying capacity between reference methods is substantial. Here is a comparison for common cable sizes using Table 4D5 (multicore PVC copper T+E):

As you can see, the difference between Method A and Method C is approximately 25 to 40% for the same cable. This can easily mean the difference between a 2.5 mm² and a 4 mm² cable, or a 6 mm² and a 10 mm² cable. The cost and physical size difference is significant — especially for long runs or when conduit sizing is a constraint.

These tabulated values are before correction factors for grouping, ambient temperature, and thermal insulation are applied. After applying those factors, the effective capacity is lower still. The Elec-Mate cable sizing calculator handles all of this in one step.

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The table above gives the tabulated current for each reference method. That figure is the starting point, not the answer. Take the value from the column for your reference method — 27 A for 2.5 mm² T+E on Method C, 20 A on Method A — enter it as the base current rating, then set the conditions the cable actually sees: ambient temperature, how many circuits are grouped together, whether it is surrounded by thermal insulation, and for a buried run the soil thermal resistivity and cable spacing.

The calculator applies the rating factors from Tables 4B1 (Ca), 4C1 and 4C2 (Cg), Appendix 4 Section 2.6 (Ci) and Table 4B3 (Cs), plus Cf for a BS 3036 semi-enclosed fuse and Cc for a buried installation, and returns Iz. Give it a design current and device rating as well and it checks Ib ≤ In ≤ Iz for you.

Here are the reference methods for the most common cable installation scenarios you will encounter: