BS 7671 Appendix 4 Tables Current Carrying Capacity

Looking for Amendment 4 (the A4:2026 update to BS 7671) rather than Appendix 4 (the cable tables)? See the BS 7671 Amendment 4:2026 guide for every change and what it means for your work.

Appendix 4 of BS 7671 is the section that every electrician turns to most frequently. It contains the current-carrying capacity tables for all standard cable types used in UK electrical installations. When you are sizing a cable, Appendix 4 is where you look up the maximum current a cable can carry under specific installation conditions.

The tables cover every combination of cable type (PVC, XLPE, MICC, SWA, flexible), conductor material (copper, aluminium), conductor configuration (single-core, multicore), and installation method (Reference Methods A through G). The values in these tables are based on defined reference conditions: an ambient temperature of 30 degrees Celsius, a single circuit (no grouping), no thermal insulation in contact with the cable, and the cable operating at its maximum conductor temperature (70 degrees Celsius for PVC, 90 degrees Celsius for XLPE/LSF).

When the actual installation conditions differ from these references — as they almost always do — correction factors must be applied. Appendix 4 also contains the correction factor tables: Table 4B1 for ambient temperature and Tables 4C1 to 4C5 for grouping.

BS 7671 Appendix 4 Tables: Cable Current Ratings & Volt Drop

BS 7671 Appendix 4 explained: current-carrying capacity, volt drop (mV/A/m) and correction factors (Ca, Cg, Ci) for every cable type and method.

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The Appendix 4 table numbering follows a systematic pattern that, once understood, makes navigation much faster. The pattern is:

The single most important point: the letter alone does not tell you the insulation. Table 4D1A is 70°C thermoplastic (PVC) twin and earth, while Table 4D5A — still in the 4D series — is 90°C thermosetting (XLPE) twin and earth. Both are “4D”. Pick the table by what is printed in its heading.

This is the table most electricians come to Appendix 4 for: the current-carrying capacity (Iz) of twin and earth cable. The values below are for Reference Method C (clipped direct), two loaded conductors, at the standard reference conditions — 30°C ambient, single circuit, no grouping and no thermal insulation. Apply correction factors for any condition that differs.

The XLPE (4D5A) column is consistently around 25% higher than the PVC (4D1A) column for the same conductor size, because the 90°C insulation tolerates more heat than 70°C PVC. For any installation method other than clipped direct, the capacity changes — read the correct column for your reference method.

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A cable's current-carrying capacity depends as much on how it is installed as on its size — the same 2.5mm² T&E carries very different currents clipped to a wall versus buried in loft insulation. BS 7671 calls the installation arrangement the reference method, and it sets which column of the Appendix 4 table you read.

Regulation 521.201 of BS 7671:2018+A4:2026 requires the installation method to be in accordance with Table 4A1 of Appendix 4, and Table 4A2 illustrates each numbered method. The two methods you will meet most often are:

Methods A, B, E, F and G cover conduit in a thermally insulating wall, conduit or trunking on a surface, and cables in free air. The full A–G breakdown, with the Table 4A2 illustrations, is in our cable reference methods guide. Choosing the wrong method is one of the most common cable-sizing errors — a cable rated for clipping direct can be significantly overloaded once it is enclosed in insulation.

The 4D series of tables covers thermoplastic insulated cables — by far the most commonly used cable type in UK domestic and light commercial installations. PVC-insulated cables include twin and earth (flat profile cable with earth), singles for use in conduit and trunking, and PVC-insulated flexible cables.

PVC cables have a maximum conductor operating temperature of 70 degrees Celsius. All current-carrying capacity values in the 4D tables are calculated on the basis that the conductor does not exceed this temperature under sustained full-load conditions at an ambient temperature of 30 degrees Celsius.

When using Table 4D5A for twin and earth cable, the column you select depends on the reference method. Column 6 (two loaded conductors) is the standard column for single-phase circuits in twin and earth cable under the various reference methods.

The 4E series covers thermosetting insulated cables — cables with XLPE (cross-linked polyethylene) or LSF (low smoke and fume) insulation. Thermosetting cables have a higher maximum conductor operating temperature of 90 degrees Celsius, compared to 70 degrees Celsius for PVC. This 20-degree advantage translates directly into higher current-carrying capacity for the same conductor size.

Thermosetting cables are used in several common scenarios: where higher current capacity is needed without increasing cable size (reducing material cost and conduit fill), in locations where the fire performance of PVC is inadequate (escape routes, public buildings), and where ambient temperatures are elevated and PVC derating would be excessive.

The capacity advantage of XLPE over PVC is approximately 25% for the same conductor size and installation method — for example, 2.5mm² T+E clipped direct: 30A (XLPE) vs 24A (PVC). This means that in situations where PVC cable sizing leads to an impractically large cable, switching to XLPE T+E of the same size may provide sufficient capacity without increasing the conductor cross-section.

The voltage drop tables provide mV/A/m (millivolts per ampere per metre) values for each cable type and size. These values are used to calculate the voltage drop across a cable run using the formula:

BS 7671 Regulation 525 limits voltage drop to 3% for lighting circuits and 5% for all other circuits, measured from the origin of the installation. From a 230V single-phase supply, this gives maximum permissible voltage drops of 6.9V for lighting and 11.5V for power. From a 400V three-phase supply, the limits are 12V for lighting and 20V for power.

The voltage drop tables provide separate values for single-phase (two-core) and three-phase circuits. For three-phase calculations, the three-phase mV/A/m values are used, which account for the different phase relationships in a balanced three-phase system. The voltage drop for three-phase is calculated as: VD = mV/A/m (3-phase) x Ib x L / 1000.

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Understanding which table to use for each cable type is essential for accurate cable sizing. Here are the most commonly encountered cable types and their corresponding Appendix 4 tables.

Using the Appendix 4 tables correctly is the core skill of cable sizing. Here is the step-by-step process for looking up a current-carrying capacity value.

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Here are the most frequently needed values from Appendix 4 for everyday domestic and light commercial work. These are for XLPE (90°C) twin and earth cable from Table 4D5A, Reference Method C (clipped direct), two loaded conductors. For PVC (70°C) T+E use Table 4D1A — values are lower (e.g. 2.5mm²=24A, 4mm²=32A, 6mm²=41A).

Remember: these are the tabulated values under reference conditions. Once you apply correction factors, the effective capacity of the cable is reduced. A 2.5mm² XLPE T+E cable with Iz = 30A may only be able to carry around 21A after derating for grouping and insulation. Always calculate the required It before selecting from the table.