What Size Cable Do I Need? A UK Guide to BS 7671 Cable Sizing

Every correctly sized circuit starts with one question: how much current will it actually draw? That figure is the design current, written Ib in the regulations.

For a single-phase circuit you get it from the load and the voltage. A 7.4 kW load on a 230 V supply at a power factor of 1.0 pulls about 32 A. For resistive loads like heaters and showers the power factor is close to 1, so the sum is simple. For motors and some lighting it's lower, which pushes the current up for the same power.

Only once you know Ib do you pick the protective device. The rule is that the device rating, In, has to be at least the design current. A 32 A load gets a 32 A device, not a 30 A one, because there isn't a 30 A MCB in the standard range.

Takeaway: size the circuit around the current it draws, then choose the fuse or breaker to match. The cable comes after both.

Here's the part people skip. A cable's tabulated current-carrying capacity in BS 7671 assumes ideal conditions. Real installations are rarely ideal, so the regulations apply correction factors that make the required cable bigger.

Three matter most on everyday jobs:

Ambient temperature (Ca) accounts for how warm the surroundings are. The tables assume 30°C. A cable in a boiler room or a hot roof space runs warmer, carries less, and needs derating. Grouping (Cg) accounts for cables bunched together, because they warm each other up. Six circuits in one trunking each carry far less than one circuit on its own. Fuse type (Cf) applies a 0.725 factor if you're protecting the circuit with an old BS 3036 rewireable fuse, which is less precise than a modern MCB.

You divide the device rating by these factors to get the capacity the cable must have on paper. In symbols: It = In ÷ (Ca × Cg × Cf). Because every factor is one or less, the required capacity only ever goes up.

Takeaway: if you size straight off the current tables and ignore temperature, grouping and fuse type, you'll undersize any circuit that isn't in perfect conditions.

The single biggest swing in cable capacity isn't temperature or grouping. It's how the cable is installed.

The same 2.5 mm² twin and earth carries 27 A clipped directly to a wall, but drop it into an insulated wall and that falls sharply. Insulation traps heat, so the conductor overheats at a lower current. This is why a cable that tested fine on the bench can be undersized the moment it disappears into a loft packed with insulation. BS 7671 gives different capacity columns for clipped direct, in conduit on a wall, in conduit in an insulated wall, and cables in or above insulation. Pick the wrong column and the whole calculation is off from the start.

Takeaway: decide how the cable is really going to be installed before you open the tables, not after.

Passing on current-carrying capacity isn't the finish line. A cable also has to deliver usable voltage at the far end. Every metre of cable drops a little voltage, and over a long run that adds up.

BS 7671 recommends the drop stays within 3% of nominal voltage for lighting and 5% for everything else. On a 230 V supply that's roughly 6.9 V for lighting and 11.5 V for power. You work it out from the mV/A/m figure in the tables, the design current and the length: volts dropped = (mV/A/m × Ib × length) ÷ 1000.

Short runs almost never fail this. Long runs often do, and the fix is a bigger cable, which has a lower mV/A/m value. A garage submain that's fine on 6 mm² at 10 m can need 10 mm² at 35 m purely because of voltage drop, even though nothing about the load has changed.

Takeaway: on any run over about 20 m, check voltage drop before you commit to a size. It's the thing that quietly forces the cable up.

Picture a detached workshop at the end of a garden in Sheffield. The owner wants a 40 A supply for tools and a small heater, single-phase at 230 V, run in twin and earth clipped along a fence-line cable tray for 30 m, ambient around 30°C, on its own with no grouping, protected by a 40 A Type B MCB.

Design current is roughly 40 A, so the device is 40 A. With Ca, Cg and Cf all at 1.0, the required capacity is 40 A. Looking at twin and earth clipped direct, 6 mm² carries 47 A, so it clears on current. Now the voltage drop check: 6 mm² has an mV/A/m of about 7.3, so over 30 m at 40 A the drop is (7.3 × 40 × 30) ÷ 1000, about 8.8 V, or 3.8% of 230 V. That's inside the 5% limit, so 6 mm² holds.

Change one thing. Run that cable through 4 m of loft insulation instead of open tray, and 6 mm² no longer carries enough. The sizing jumps to 10 mm², and the voltage drop improves as a side effect.

Takeaway: real jobs are decided by conditions, not by the load alone. Two identical loads can need different cables.

Cable itself is a commodity, but price and availability still shape decisions. A UK merchant like CEF (City Electrical Factors) or a wholesaler such as Screwfix will stock the common sizes off the shelf, and the jump from 6 mm² to 10 mm² twin and earth is a real cost difference over a long run. That's a genuine reason to get the sizing right rather than reaching for the next size up "to be safe" on every circuit. Oversizing everything wastes copper and money; undersizing risks a fail. The calculation exists so you land on the right one.

There's a habit on site of going up a size "just in case". It feels cautious. It isn't always the right call. An oversized cable on a small protective device can mask a problem, costs more, is harder to terminate, and doesn't fix a voltage drop issue that's really about run length. The regulations are built around matching the cable to the circuit, not padding every run. Size it correctly, check voltage drop, and only step up when the maths tells you to. That's more defensible than a blanket "always go one bigger", and it's cheaper.

What size cable for a 9.5 kW shower? A 9.5 kW shower on 230 V draws about 41 A, so it needs a 45 A or 50 A device and typically 10 mm² twin and earth, though the run length and installation method can change that. Check it properly rather than assuming.

Is 2.5 mm² enough for a ring final circuit? For a standard 32 A ring final in twin and earth, 2.5 mm² is the usual size because the load is shared across two legs. A radial on a 32 A device is a different sum and often needs 4 mm².

Does cable length really matter that much? Yes. Length is the main driver of voltage drop, and voltage drop is what forces many long runs up a size even when the current is fine.

What's the UK voltage drop limit? BS 7671 recommends 3% for lighting and 5% for other circuits, measured to the far end of the circuit.

Do I need to be qualified to do this work? Sizing a cable is fine to learn. Installing and certifying notifiable work under Part P of the Building Regulations needs a competent person, so use a registered electrician for the actual job if you're not qualified.

The steps above are exactly what our Cable Sizing Calculator automates: design current, correction factors, capacity and a voltage drop check to BS 7671. If your run is long, it'll also tell you when it has stepped the cable up to keep voltage drop in check. For the partner calculation on the same circuit, the Voltage Drop Calculator shows the drop on a size you've already chosen. You can also browse all BuildByJai calculators for the rest of a job.

Professional advice notice: this article is general guidance, not a design. Electrical work in the UK is subject to Part P of the Building Regulations. Have circuits designed, installed and certified by a competent person where required, for example an NICEIC or NAPIT registered electrician.