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 air temperature, Table 4B2 for ambient ground temperature, Table 4B3 for soil thermal resistivity (Cs), Table 4B4 for depth of laying (Cd), Table 4B5 for cables with more than four loaded cores, and Tables 4C1 to 4C6 for grouping (Cg).

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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: you do not have to guess. Appendix 4 opens with Table 4A3, which lists every cable construction against the table that gives its current rating. Read the series letter as the insulation and conductor material — 4D is 70°C thermoplastic (PVC) with copper conductors, 4E is 90°C thermosetting (XLPE/LSF) with copper conductors — and the number as the construction within that series.

This is what most electricians come to Appendix 4 for: the current-carrying capacity (Iz) of a cable. Before you read a number, fix two things — the table (from Table 4A3, by construction and insulation) and the column (your reference method). For flat twin and earth the table is Table 4D5; for 90°C thermosetting multicore it is Table 4E2A. The indicative figures below are for two loaded conductors clipped direct at the standard reference conditions — 30°C ambient, single circuit, no grouping and no thermal insulation. Always read the actual value off the printed table for your exact construction and reference method, then apply correction factors for any condition that differs.

The 90°C thermosetting column is consistently higher than the 70°C PVC column for the same conductor size, because the 90°C insulation tolerates more heat. 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 521.1 of BS 7671:2018+A4:2026 requires the installation method of a wiring system, in relation to the type of conductor or cable used, to be in accordance with Table 4A1 of Appendix 4, provided external influences are taken into account under Section 522. Regulation 521.2 then requires the installation method in relation to the situation to be in accordance with Table 4A2, which is where each numbered installation method is illustrated and mapped to its reference 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.

Whichever table you are in, the column you select depends on the reference method and on the number of loaded conductors — the two-loaded-conductor columns are the ones for single-phase circuits, the three- or four-loaded-conductor columns for three-phase. Read the column headings; do not count columns across from another table.

The 4E series covers thermosetting insulated cables with copper conductors — cables with XLPE (cross-linked polyethylene) or LSF (low smoke and fume) insulation. It mirrors the 4D series construction for construction: 4E1A single-core non-armoured, 4E2A multicore non-armoured, 4E3A single-core armoured, 4E4A multicore armoured (XLPE/SWA). 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 thermosetting over thermoplastic is meaningful for the same conductor size and installation method, so where PVC cable sizing leads to an impractically large cable, switching to a 90°C thermosetting cable of the same size may provide sufficient capacity without increasing the conductor cross-section. Two cautions. First, the gain is only usable if every terminal and accessory in the circuit is rated for the higher operating temperature: the introduction to Appendix 4 lists the limiting temperatures for the terminals of equipment (Section 526) as one of the considerations that affects conductor size. Second, Regulation 523.1 NOTE 3 expressly allows the 70°C tables (4D1 to 4D5) to be used for 90°C thermosetting cables where the rating is to be based on 70°C, which is often what you must do for that reason.

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 sizes most often needed for everyday domestic and light commercial work, with indicative capacities for a 90°C thermosetting cable clipped direct with two loaded conductors. Read the tabulated value from Appendix 4 for the cable you are actually using — Table 4D5 for 70°C flat twin and earth, the 4E series for 90°C thermosetting — and remember the protective device rating (In) must sit between the design current and the cable capacity: Ib ≤ In ≤ Iz, per Regulation 433.1.1.

Remember: tabulated values apply under reference conditions only. Once you apply correction factors, the effective capacity of the cable is reduced — a cable grouped with others and in contact with insulation can lose a third of its tabulated capacity or more. Always calculate the required It before selecting from the table.