UK Cable Sizes & Current Ratings — BS 7671 Reference

Twin and Earth (6242Y) Current Ratings

Flat twin and earth cable (6242Y) is the standard cable for domestic wiring in the UK. Current ratings below are for Reference Method C (clipped direct to a surface) from BS 7671:2018+A2:2022 Appendix 4 Table 4D5, column 6:

Current ratings shown are for Reference Method C (clipped direct). Ratings reduce for Method A (enclosed in insulation) or Method B (enclosed in conduit/trunking). Always apply correction factors for grouping, ambient temperature, and thermal insulation.

SWA (Armoured) Cable Current Ratings

Steel wire armoured (SWA) cable is used outdoors and underground, for submains to outbuildings, garages, and EV chargers, wherever the run needs mechanical protection that flat twin and earth cannot provide. It follows a different BS 7671 table from 6242Y because the round, armoured construction has different heat-dissipation characteristics. Figures below are two-core, single-phase, 70°C PVC-insulated copper SWA to BS 7671:2018+A2:2022 Appendix 4 Table 4D4A:

SWA is not made below 1.5 mm² (unlike 6242Y, which starts at 1.0 mm²). Three- and four-core SWA for three-phase supplies is rated lower than the two-core, single-phase figures above for the same size, so size three-phase submains separately, see the three-phase FAQ below. XLPE-insulated (90°C) SWA carries a noticeably higher rating than the 70°C PVC figures shown here for the same cross-sectional area: confirm which insulation type is specified before comparing ratings. The buried figures assume the standard soil thermal resistivity BS 7671 uses for ground cables around buildings; a deeper or more thermally resistive burial reduces the rating further.

Common Domestic Circuits

Earth (CPC) Conductor Sizes

The circuit protective conductor (CPC, commonly called the earth) has its own sizing rule in BS 7671 Table 54.7, based on the cross-sectional area of the circuit's line conductor:

Factory-made 6242Y twin and earth cable does not follow the simple Table 54.7 rule directly: the CPC column in the ratings table above shows a smaller factory-fitted earth core than the line conductor for most sizes (a 2.5 mm² T&E has a 1.5 mm² CPC, for example, not 2.5 mm²). This is permitted because the cable is manufactured to BS 6004, which justifies the reduced CPC size against the adiabatic equation (Regulation 543.1.3) rather than the simpler Table 54.7 sizing rule. Where you are running a separate, non-factory CPC alongside a cable, or sizing the armour of an SWA cable as the protective conductor (Regulation 543.2), apply Table 54.7 directly.

Main protective bonding conductors (bonding gas, water, and other extraneous-conductive-parts back to the main earthing terminal) are a separate calculation under BS 7671 Table 54.8, not Table 54.7: use our Bonding Conductor Calculator for that figure. For earth electrode sizing on TT or TN-S systems, see the Earthing System Calculator. CPC and bonding conductor sizing both interact with earth fault loop impedance and disconnection times: get any earthing design checked by a qualified electrician, and any new circuit or altered earthing arrangement notified under Part P where required.

Key Points for Cable Selection

Correction Factors for Cable Ratings

Every rating on this page assumes standard reference conditions: 30°C ambient air temperature, a single circuit, and no contact with thermal insulation. Real installations often need one or more correction factors applied, reproduced here from BS 7671 Tables 4B1 and 4C1 for 70°C PVC-insulated cable:

Multiply the tabulated rating by every factor that applies (ambient temperature, grouping, and thermal insulation contact) to get the cable's actual current-carrying capacity for your installation. The grouping figures above are for cables touching, bunched, or enclosed together; cables clipped in a single spaced layer are derated less. For the full table including XLPE-insulated (90°C) figures, Reference Methods A and B, and thermal insulation contact, see the Cable Ampacity Table.

Frequently Asked Questions

What is Reference Method B in BS 7671?

Reference Method B in BS 7671 (18th Edition, Appendix 4) covers cables enclosed in conduit or trunking — for example, run on a wall — whereas the ratings in the table above are for Reference Method C, cable clipped direct to a surface. Enclosed cables dissipate heat less effectively, so the tabulated ratings are lower for Method B than for Method C at the samecable size, and generally lower still where the cable is surrounded by thermal insulation. That is why the installation method must be fixed before a rating is read from the tables. Our Cable Sizing Calculator applies the rating for your chosen installation method rather than assuming clipped-direct figures.

What size cable do I need for a 32 A circuit?

It depends on whether the circuit is a ring or a radial. A 32 A ring final circuit normally uses 2.5 mm² twin and earth (27 A tabulated rating). This is not undersized: a ring has two legs running from the consumer unit back to it, so the load current splits between them, and each leg only needs to carry a fraction of the total design current, provided the ring is wired correctly with no interconnections or spurs beyond the permitted limits. A 32 A radial circuit is different: the full current flows through a single cable run, so the cable itself needs a tabulated rating of at least 32 A once correction factors are applied. 2.5 mm² T&E (27 A clipped direct) is not sufficient for a 32 A radial; 4.0 mm² (37 A) is the minimum practical size. Always confirm the specific circuit design, including any correction factors, with the Cable Sizing Calculator or a qualified electrician.

Is there a difference between 17th Edition and 18th Edition cable sizes?

No, not for the common domestic cable sizes covered on this page. The Table 4D5 current ratings this page uses are from the current BS 7671:2018+A2:2022 (18th Edition), and the core ampacity figures for standard PVC twin and earth have not changed materially between the 17th Edition (BS 7671:2008) and the current tables across both editions. What did change between the 17th and 18th Editions was elsewhere in the regulations, not the cable ampacity tables: expanded RCD protection, arc fault detection device (AFDD) recommendations, surge protection requirements, and a dedicated EV charging section. See our BS 7671 18th Edition Changes guide for the full list. Always design and install to the edition of BS 7671 in force at the time of the work.

Do three-phase circuits use the same cable sizes as single-phase?

No. BS 7671's cable tables give a separate, lower current rating for three- or four-core three-phase cable than for two-core single-phase cable of the same cross-sectional area, because a three- or four-core cable generates more heat per core in the same space. A cable sized correctly for a single-phase 32 A radial is not automatically correct for a three-phase circuit carrying a similar current per phase. Three-phase supplies are also more common for larger installations (workshops, larger EV charging setups, some heat pumps), where load balancing across the three phases matters as much as the cable rating itself. Use our Cable Sizing Calculator or Three-Phase Balancing Calculator as a starting point, and have three-phase designs checked by a qualified electrician or electrical engineer.

Why isn't there an 8 mm² twin and earth cable?

UK cable manufacturers follow the IEC 60228 preferred conductor size series: 1.0, 1.5, 2.5, 4, 6, 10, 16, 25, 35 mm² and upward. There is a deliberate jump from 6 mm² straight to 10 mm², with no 8 mm² size in between, for both flat twin and earth and SWA cable. If a 6 mm² cable is not rated highly enough for a circuit once correction factors are applied, the next available standard size is 10 mm², not an intermediate size.

What cable size do I need for a 100 A supply (meter tails)?

Meter tails, the short cables between the electricity meter and the consumer unit's main switch, are conventionally sized to match the supply fuse rating rather than read directly off the domestic tables above: 25 mm² tails are the long-standing industry norm for 80 A and 100 A single-phase domestic supplies, which is why the sub-main row in the twin and earth table stops at 16 mm² (85 A) and larger meter tails sit outside that table. 100 A supplies are increasingly requested to accommodate EV chargers, heat pumps, and induction cooking on top of existing loads. The cut-out fuse itself is DNO property and can only be changed by the DNO or an authorised meter operator; the tails and consumer unit are the installer's responsibility. Always get a supply upgrade assessed and notified by a qualified electrician.

Calculate Your Cable Requirements

• Cable Sizing Calculator — determine the correct cable size to BS 7671
• Voltage Drop Calculator — check voltage drop compliance
• Circuit Breaker Sizing Calculator — determine correct MCB rating
• EV Charger Cable Sizing Calculator — cable sizing for EV charger installations
• Bonding Conductor Calculator — size main protective bonding conductors to Table 54.8
• Earthing System Calculator — earth electrode resistance for TT and TN-S systems