Electrical

Cable Sizing to BS 7671

Cable sizing comes down to one chain — Ib ≤ In ≤ Iz — with correction factors pulling the cable's usable rating down and a volt drop check making sure the far end still gets a decent voltage; get the factors and the checks right and the cable follows.

Updated 20 Aug 202610 min read

Sizing a cable isn't guesswork or picking the same size you used last time. BS 7671 gives a defined method that ties the load, the protective device and the cable together, applies correction factors for how and where the cable is installed, and then checks the result against volt drop, disconnection time and thermal withstand.

This guide walks through the core sequence: the Ib ≤ In ≤ Iz relationship, the four correction factors, how to read the tabulated current ratings, and the volt drop check that often decides the answer on a long run.

Key takeaways

The core rule is Ib ≤ In ≤ Iz: design current ≤ device rating ≤ cable's installed current-carrying capacity.
Correction factors Ca (ambient), Cg (grouping), Ci (thermal insulation) and Cf (BS 3036 fuse) reduce the usable rating.
Required tabulated rating It ≥ In ÷ (product of correction factors) — then pick a cable from the Appendix 4 tables at the right installation method.
Volt drop must stay within limits — typically 3% for lighting, 5% for other uses of a 230V supply.
Finish with the earth-fault (Zs) and adiabatic checks — a cable that passes on current and volt drop can still fail on fault protection.

The Ib ≤ In ≤ Iz chain

Everything hangs off one inequality: Ib ≤ In ≤ Iz. Ib is the design current — the load the circuit actually has to carry. In is the rated current of the protective device — it must be at least Ib so it doesn't trip on normal load. Iz is the current-carrying capacity of the cable as installed — it must be at least In so the device protects the cable rather than the cable failing first.

Read left to right it's a chain of protection: the load sets the minimum device, the device sets the minimum cable. Get Ib first from the connected load (applying diversity where appropriate), choose In as the next standard device rating at or above it, and then the whole job of cable sizing is finding a cable whose installed Iz is at least In.

Correction factors: Ca, Cg, Ci, Cf

A cable's tabulated rating assumes reference conditions. Real installations are hotter, more crowded or more enclosed, so BS 7671 applies rating factors that reduce the usable capacity. Ca corrects for ambient temperature (ratings assume 30°C in air, 20°C in ground). Cg corrects for grouping — cables bunched together can't shed heat, so a group of circuits derates each one. Ci corrects for thermal insulation the cable passes through or is surrounded by. Cf accounts for the use of a semi-enclosed BS 3036 rewireable fuse, which needs extra headroom.

The factors combine by multiplication, and each is 1 or less, so together they pull the rating down. The cable's installed capacity is its tabulated rating multiplied by all the applicable factors: Iz = It × Ca × Cg × Ci × Cf. Turned around to size the cable, you need a tabulated rating It ≥ In ÷ (Ca × Cg × Ci × Cf) — divide the device rating by the product of the factors to find the minimum tabulated rating, then pick a cable that meets it.

Factors multiply, and they hurt

Two correction factors of 0.9 and 0.8 don't leave you much — 0.72 of the tabulated rating. Grouping and thermal insulation together are what force cables up a size or two on real jobs.

Reading the tabulated ratings

The current ratings live in the Appendix 4 tables of BS 7671, and the number you read depends on the installation method — clipped direct, in conduit, in a wall with thermal insulation on one side, buried in the ground and so on. The same 2.5mm² cable has very different ratings clipped to a wall versus buried in loft insulation, so you must pick the row and column that match how the cable is actually run (reference methods A to E and beyond).

So the flow is: work out It ≥ In ÷ (product of factors), go to the correct table for your cable type and installation method, and read down until you find the smallest conductor whose tabulated rating meets or exceeds It. That's your candidate cable on current-carrying capacity — but it isn't confirmed until it passes volt drop and the fault checks.

The volt drop check

A cable big enough to carry the current can still deliver too little voltage at the far end of a long run. BS 7671 limits volt drop to preserve the voltage at the point of use — commonly taken as 3% of nominal for lighting circuits and 5% for other circuits from a 230V supply, which is about 6.9V and 11.5V respectively.

Calculate it from the tabulated millivolts-per-amp-per-metre (mV/A/m) figure for the cable: volt drop = mV/A/m × Ib × length ÷ 1000. On long runs this frequently forces a cable up a size beyond what the current rating alone demanded — a 2.5mm² circuit fine on current can fail volt drop over 30-plus metres, and the fix is 4mm². Always run the volt drop check; on longer circuits it's the deciding factor, not the current rating.

Length decides long runs

Short circuit? Current rating usually wins. Long circuit? Volt drop usually wins and pushes you up a size. Check both every time and take the larger cable.

Finishing the job: Zs and adiabatic

A cable that passes on current-carrying capacity and volt drop still isn't signed off. Two fault checks remain. First, earth-fault loop impedance: the circuit's Zs (Ze + R1+R2) must be low enough that the protective device disconnects within the required time — a longer or thinner CPC raises R2 and can bust the max Zs even when the line conductor is comfortable. Second, the adiabatic check: the CPC must be large enough to survive the fault current for the disconnection time without exceeding its temperature limit.

In practice these tie back to the cable choice: sometimes you increase the CPC size (or the whole cable) not for current or volt drop but to bring Zs within limits or satisfy the adiabatic equation. Cable sizing is the loop of all four checks — current capacity, volt drop, disconnection time and thermal withstand — and the cable you fit is the one that passes every one.

Frequently asked questions

What does Ib ≤ In ≤ Iz mean?

It's the core cable-sizing rule: the design current (Ib) must be no more than the device rating (In), which must be no more than the cable's installed current-carrying capacity (Iz). The load sets the device, the device sets the minimum cable.

What are the correction factors in cable sizing?

Ca for ambient temperature, Cg for grouping of cables, Ci for thermal insulation, and Cf for the use of a BS 3036 rewireable fuse. They multiply together and reduce the cable's usable rating: Iz = It × Ca × Cg × Ci × Cf.

What's the maximum volt drop allowed?

For a 230V supply it's commonly taken as 3% for lighting (about 6.9V) and 5% for other circuits (about 11.5V), measured from the origin to the point of use. On long runs this often forces a larger cable than the current rating alone would require.

Does passing current and volt drop mean the cable is fine?

Not on its own. You still have to check earth-fault loop impedance (Zs) against the disconnection-time limits and run the adiabatic check on the CPC. A cable can pass on current and volt drop yet need increasing to satisfy fault protection.

From guidance to action

Cable sizing calculatorApply the correction factors and Ib ≤ In ≤ Iz to find the cable size.Voltage drop calculatorCheck mV/A/m volt drop against the 3% and 5% limits on any run.