The same cable can carry 20 A or 27 A depending on how it is installed. Reference methods A to G define the installation arrangement and directly determine the current-carrying capacity from the BS 7671 Appendix 4 tables. This guide explains each method, when it applies, and how to choose correctly.
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Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.
Reference Method C is "clipped direct" — a cable fixed directly to a non-metallic surface or run on a cable tray, with free air around it. It is one of the installation methods in Appendix 4 (Tables 4A1 and 4A2) that determines which column of the current-carrying capacity tables you read. Method C gives higher ratings than enclosed methods because heat dissipates more easily.
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Installation reference methods are standardised descriptions of how a cable is physically installed in a building or site. They are listed in Table 4A2 of Appendix 4 of BS 7671:2018+A4:2026 and are fundamental to the cable sizing process. 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.
There are seven main reference methods: A1, A2, B, C, D, E, F, and G. Each represents a different installation arrangement, from the most restrictive (enclosed in insulation) to the least restrictive (open air on a perforated tray). Selecting the correct reference method is not optional — it is a required step in every cable sizing calculation.
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 installed in conduit or trunking that is itself enclosed in a thermally insulating wall — typically PVC conduit chased into masonry and then plastered over, or cables directly in a thermally insulating wall.
When it applies: Any time you chase conduit into a masonry or stud wall and plaster over it. This is common for switch drops, socket feeds through walls, and any cable run that needs to be concealed in a solid wall with conduit protection. The surrounding masonry and plaster act as thermal insulation, restricting heat dissipation.
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 that is mounted on a wall or ceiling surface — not buried in the wall. The conduit or trunking is exposed and can dissipate some heat into the surrounding air, but the cable inside is still partially restricted.
Method B gives slightly higher current-carrying capacities than Method A because the conduit or trunking is not embedded in insulating material — it is on the surface and can radiate heat into the air. However, the capacity is still lower than Method C (clipped direct) because the conduit or trunking restricts airflow around the cable.
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. It covers cables fixed directly to a wall, ceiling, joist, or other surface using clips, without any conduit or trunking enclosure.
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.
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 A) at any point, the more restrictive Method A applies to the entire run. Design for the worst case.
Method D covers cables installed directly in the ground. This is the standard method for armoured cable (SWA) runs supplying outbuildings, garages, garden offices, external lighting, and any other installation that requires an underground cable route.
Method D is unique because the thermal environment (soil) is very different from air. The current-carrying capacities for Method D are given in separate columns in the Appendix 4 tables — usually in the 4E series (XLPE/SWA). Grouping of buried cables (multiple circuits in the same trench) also has a significant impact and uses specific grouping factors from Table 4A1.
Methods E, F, and G cover cables installed in free air — on open trays, ladders, or cleats — where air can circulate freely around the cable. These methods give the highest current-carrying capacities because heat dissipation is maximised.
Cables on a horizontal or vertical perforated tray. The perforations allow air to circulate beneath and around the cables. Common in commercial and industrial cable management for distribution cables and submains.
Cables on horizontal or vertical cable ladders, or secured to wall-mounted cleats. Maximum air circulation around the cables. Used for large power cables in industrial installations, switchrooms, and plant rooms.
Cables spaced from a wall or ceiling surface in free air, not touching the surface. Provides slightly better heat dissipation than touching the surface (Method C) because air can circulate on all sides.
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.
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Start free trialIn addition to the lettered reference methods (A to G) in BS 7671 Table 4A2, the IET On-Site Guide defines three numbered reference methods — 100, 101, and 102 — specifically for flat twin-and-earth cable installed inside stud walls and ceiling voids where the cable is in contact with plasterboard or joists.
The detailed current-carrying capacity values and correction factors for Methods 100, 101, and 102 are given in Tables 7.1(iii) and 7.1(iv) of the IET On-Site Guide (OSG 9th Edition, 2022, incorporating A4:2026). These tables reference Table 4D5 for the base cable ratings. Failing to apply these numbered methods when cables are in contact with plasterboard or joists — and instead using the higher Method C ratings — results in undersized cables that may overheat in service.
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 across joists in a void containing thermal insulation must be assessed against Methods 100–102 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 A (conduit in wall) and Method C (clipped direct). If any section of the cable route is in conduit in a wall, Method A applies. For commercial work with tray and trunking, you may be choosing between Method B (trunking) and Method E (perforated tray). Always document your reference method choice — it forms part of the design records required by BS 7671.
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Try it free for 7 daysThe 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):
| Cable Size | Method A (A) | Method B (A) | Method C (A) |
|---|---|---|---|
| 1.5 mm² | 14.5 | 17.5 | 20 |
| 2.5 mm² | 20 | 24 | 27 |
| 4 mm² | 27 | 32 | 37 |
| 6 mm² | 34 | 40 | 47 |
| 10 mm² | 46 | 54 | 64 |
| 16 mm² | 61 | 73 | 85 |
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.
Here are the reference methods for the most common cable installation scenarios you will encounter:
T+E cable clipped to joists under the floor and dropping down through conduit in the wall to socket outlets. The section in conduit in the wall is Method A; the section clipped to joists is Method C. Use Method A for the whole run.
Dedicated circuit from the consumer unit to the shower. Cable typically clipped to joists (Method C) then drops through the bathroom wall in conduit (Method A). Use Method A. Given the high current (40 A+), the reference method has a significant impact on the cable size required.
Armoured cable from the main distribution board, through the wall, buried in the ground across the garden, and into the outbuilding. The buried section is Method D; the section inside each building clipped to the wall is Method C. Use Method D for the buried section (usually the most restrictive for SWA cable).
Submain cables on perforated cable tray from the main switchboard to a sub-distribution board. Method E for the tray section. If the cable enters trunking at any point, that section becomes Method B. Use the most restrictive method — or size each section individually if the design allows.
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