How Is Cable Size Calculated?

What Determines the Size: The Design Rule I_b ≤ I_n ≤ I_z

UK cable sizing is governed by BS 7671:2018+A2:2022, the 18th Edition of the IET Wiring Regulations (amended by A3:2024 and, from April 2026, A4:2026). At its heart sit three currents that must line up in a strict order (Regulation 433.1.1):

• I_b — the design current: the current the circuit will actually carry in normal service.
• I_n — the protective device rating: the rating of the circuit breaker, RCBO or fuse protecting the cable. It must not be less than I_b.
• I_z — the cable’s current-carrying capacity as installed: what the conductor can carry continuously in its real installed conditions, after every derating factor is applied. It must not be less than I_n.

So the chain is I_b ≤ I_n ≤ I_z. BS 7671 adds a further overload condition: the current that causes the protective device to operate (I₂) must not exceed 1.45 × I_z, so the device disconnects the circuit before a sustained overload can damage the cable’s insulation. Once a candidate conductor satisfies the current chain, the design is then checked against the voltage-drop limit — and a cable that passes on current can still fail on voltage drop. Here is what feeds each step.

Factor 1 — The Design Current (I_b)

Everything starts with an honest assessment of the load: what the circuit supplies, its power demand, and how much of that demand can realistically occur at the same time. For circuits feeding several points, the electrician applies diversity— a recognised allowance for the fact that not every load runs at once. Overstating the load wastes money; understating it produces a cable that runs hot for its whole life. This assessment is a judgement call based on the standard’s guidance, which is one reason it belongs with a professional.

Factor 2 — The Protective Device (I_n)

The device at the consumer unit exists to protect the cable, not the appliance. The electrician selects the next standard device rating at or above I_b— and the cable must then be sized around that device, not around the load alone. The type of device also changes the arithmetic: semi-enclosed (rewireable) fuses, for example, attract an additional correction factor that pushes the design towards a larger conductor than a modern circuit breaker would.

Factor 3 — How the Cable Is Installed (the Reference Method)

A cable’s capacity is set by how fast it can shed heat, so BS 7671 rates every cable against an installation Reference Method(Table 4A2). A cable clipped direct to a surface such as a wooden or masonry wall (Method C) sheds heat more easily than the same cable enclosed in conduit or trunking (Method B), which runs warmer and is rated lower; genuinely free-air installations — cables on ladder, cleats or perforated tray with air flowing around them (Methods E, F and G) — are rated higher again; and a cable buried in the ground (Method D) has its own rules. For flat twin-and-earth in modern homes there is a further set of installed conditions (Methods 100–103) covering cables above insulated plasterboard ceilings and inside insulated stud walls — the more the cable is surrounded by thermal insulation, the harder its heat is to shed and the lower its rating becomes. The identical conductor size can therefore be fine on one route through a house and inadequate on another.

Factor 4 — Derating Factors (C_a, C_g, C_i, C_c)

The current ratings tabulated in BS 7671 Appendix 4 assume reference conditions: an ambient temperature of 30°C, a single circuit, and no contact with thermal insulation. Real installations rarely match, so the electrician applies rating factors:

• C_a — ambient temperature (a hot loft or plant room lowers capacity)
• C_g — grouping (cables bunched together heat each other)
• C_i — contact with thermal insulation
• C_c — circuits buried in the ground

The required rating is divided by the product of the applicable factors to find the minimum tabulated capacity the chosen cable must meet. The factors for adverse conditions — heat, grouping, thermal insulation — are 1.0 or below, so they de-rate the cable and every adverse condition compounds: two or three together can easily force the design up a conductor size, which is exactly what a rule of thumb cannot see. (Where conditions are more favourable than the tabulated reference — an ambient cooler than 30°C, say — the factor can exceed 1.0, another judgement the standard leaves to the designer.)

Factor 5 — Voltage Drop: 3% for Lighting, 5% for Everything Else

Passing on current is not enough. BS 7671 (Appendix 4, Table 4Ab) limits the voltage drop between the origin of an installation supplied from the public low-voltage network and any load point to 3% for lighting and 5% for other uses— on a nominal 230 V single-phase supply that is 6.9 V and 11.5 V respectively. Voltage drop grows with the length of the run and the current carried, using the millivolt-per-amp-per-metre values tabulated for each conductor. On a long run to an outbuilding or a far corner of a large house, voltage drop — not current-carrying capacity — is very often the check that forces a larger conductor.

The Checks That Follow

Sizing the live conductors is still not the end of the design. The electrician also verifies that the earth-fault loop impedance of the finished circuit is low enough for the protective device to disconnect quickly under fault conditions, and that the protective conductor can withstand the energy of a fault without damage. The circuit is then inspected, tested with calibrated instruments and formally certificated. None of those steps can be done from a table on a website — they depend on measurements taken on the actual installation.

Why You Cannot Just Use a Rule of Thumb

“Amps per millimetre squared” tables and forum folklore describe one set of reference conditions only. The same nominal conductor size that is comfortable clipped to a joist can be significantly over-stressed once it is buried under loft insulation, bunched with other circuits, run through a hot space, or asked to feed a distant load. A cable that is too small does not fail on day one — it runs chronically hot, ages its insulation, and creates a fire risk that is invisible once the walls and ceilings are closed up. That is why the regulation is written as a coordinated chain of checks rather than a lookup, and why the person doing it needs the full picture of your installation — not just the load.

Who Must Do This — the Legal Position

BS 7671 is the national standard for electrical installation work in the UK, and in England and Wales domestic electrical work sits under Part P of the Building Regulations, which requires electrical installations in dwellings to be designed and installed so that people are protected from fire and injury. Notifiable work — in England that includes installing any new circuit, replacing a consumer unit, and additions or alterations in special locations such as the zones around a bath or shower — must follow one of three routes set out in Approved Document P: it can be carried out and self-certified by an electrician registered with a government-approved competent person scheme; it can be inspected and certified by a registered third-party certifier appointed before the work begins; or it can be notified to the local authority building control body before work starts. England narrowed the notifiable list in 2013 (the current Approved Document P is the 2013 edition, for use in England), while Wales kept the wider pre-2013 scope — so more work is notifiable in Wales (for example kitchens and outdoor installations) than in England. A registered electrician will know which applies. On completion you should receive the appropriate electrical certification for the work; keep it, as it is routinely requested when a property is sold.