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Find the minimum BS 7671 compliant cable size for any UK circuit — checks current-carrying capacity (Table 4D2A) and volt drop (Table 4Ab) simultaneously, with derating for installation method and grouping.
Design current of the circuit (Ib) — typically the protective device rating.
One-way cable run from supply point to load.
Recommended Size
2.5 mm²
minimum compliant size
Volt Drop
1.88%
4.32V at 230V
Next size up — 4 mm² (for reference)
Volt drop
1.15%
Current capacity (Iz)
28 A
Volt drop V
2.64 V
| Size | Iz (A) | Req. Iz | Vd% | Status |
|---|---|---|---|---|
| 1.5 mm² | 15.5 | 16 | 3.03% | FAIL |
| 2.5 mm²recommended | 21 | 16 | 1.88% | PASS |
| 4 mm² | 28 | 16 | 1.15% | PASS |
| 6 mm² | 36 | 16 | 0.76% | PASS |
| 10 mm² | 50 | 16 | 0.46% | PASS |
| 16 mm² | 68 | 16 | 0.29% | PASS |
| 25 mm² | 89 | 16 | 0.18% | PASS |
Next step
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BS 7671 (18th Edition) cable sizing is a two-constraint problem: the cable must carry the current without overheating, and it must not drop more than 5% of the supply voltage to the load. This calculator evaluates both simultaneously.
BS 7671 Table 4D2A gives the tabulated current rating (Iz) for each cable size and installation method. The grouping derating factor (Cg) from Table 4C1 reduces this when multiple cables are bunched together, because grouped cables cannot dissipate heat as freely. The required current after derating is: I_required = design current ÷ Cg. The cable must have Iz ≥ I_required.
Using the mV/A·m method from BS 7671 Table 4Ab, volt drop in volts = (mV/A·m × I × L) ÷ 1,000. Volt drop percentage = (Vd ÷ nominal voltage) × 100. This must not exceed 5% (BS 7671 Appendix 12). For 90°C XLPE cables, the mV/A·m value is multiplied by 1.08. For three-phase circuits, by 0.866 (√3/2).
The calculator evaluates every cable size from 1.5mm² to 25mm² and selects the smallest that passes both the current check and the volt drop check. If a small cable passes the current check but fails volt drop (long run), the calculator steps up to the next size. The “binding constraint” field tells you which check was the deciding factor — useful for understanding whether a shorter route or a larger cable is the more economic solution.
This calculator provides a first-pass sizing guide based on standard BS 7671 tabulated values. For final designs, also consider: prospective fault current and disconnection times (loop impedance test), harmonic loading, ambient temperature correction (Ca), and any partial burial that changes installation method along the route. A qualified electrician must verify compliance with the as-installed conditions before energising.
WORKED EXAMPLE
An electrician is sizing a 32A radial circuit to a 7kW EV charger 25m from the consumer unit. Single-phase, 70°C thermoplastic, clipped direct to wall (Method C), single cable (no grouping). What is the minimum cable size?
Required Iz (no derating)
32 A
4mm² Iz Method C
28 A — FAIL
6mm² Iz Method C
36 A — PASS
6mm² Vd (7.3 × 32 × 25) ÷ 1,000
5.84V = 2.54% ✓
Result
6mm² — current limited
4mm² fails current capacity (28A < 32A) · 6mm² passes both checks (36A ≥ 32A, 2.54% ≤ 5%)
This is typical for EV charger installations: current capacity (not volt drop) is the binding constraint on short-to-medium runs. For runs beyond ~55m, volt drop would become the binding factor and require 10mm².
Check BS 7671 volt drop compliance and find the maximum run for any cable size.
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Common questions about BS 7671 cable sizing for UK electricians.
BS 7671 cable sizing involves two parallel checks that must both be satisfied. First, current capacity: the cable's rated current (Iz from Table 4D2A), derated for installation method and grouping, must be equal to or greater than the design current (Ib). Second, volt drop compliance: the voltage loss along the cable must not exceed 5% of the nominal voltage (230V single-phase or 400V three-phase). The cable selected must be the smallest size that passes both tests simultaneously.
Current rating determines whether the cable will overheat: a cable carrying more current than its rated capacity will heat up, degrade its insulation, and eventually become a fire risk. Volt drop determines whether the connected equipment receives adequate voltage: too much resistance in a long cable means the appliance sees a reduced supply voltage, potentially causing malfunction or motor burnout. A short circuit close to the consumer unit may need only a small cable for current capacity, but a long run to a remote workshop may need a much larger cable purely to limit volt drop. Both constraints must be checked.
Derating accounts for conditions that reduce a cable's ability to dissipate heat. When multiple cables are grouped together in a bundle, they cannot shed heat as easily as an isolated cable, so each cable's rated current must be multiplied by a grouping derating factor (Cg) from BS 7671 Table 4C1. For example, three cables grouped together have Cg = 0.70 — so a cable rated at 21A clipped direct would only be considered as 14.7A effective when grouped with two others. The calculator automatically applies the appropriate BS 7671 derating factor.
Installation method determines how easily a cable can dissipate heat into its surroundings. Method A1 (enclosed in insulation) and B1 (enclosed in conduit in a wall or ceiling) provide the least heat dissipation and therefore the lowest current ratings. Method C (clipped directly to a surface) allows better heat transfer and gives higher ratings. Method E (in free air, suspended or on a cable tray) provides the best heat dissipation and therefore the highest current-carrying capacity. The same 2.5mm² cable is rated 18A in conduit but 24A in free air.
6mm² copper cable clipped direct is rated 36A and has a volt drop of 7.3 mV/A·m (70°C). 10mm² clipped direct is rated 50A with 4.4 mV/A·m. For a 32A circuit up to about 70m, 6mm² will pass both current and volt drop checks. For runs beyond 70m or currents above 36A, 10mm² is needed. The key difference is that 10mm² has significantly lower resistance and therefore much lower volt drop on longer runs — making it the standard choice for sub-main feeds in commercial properties and EV charger supplies on longer runs.
The most common reason 2.5mm² is not adequate is volt drop on long circuit runs. A 20A circuit in 2.5mm² cable fails the 5% volt drop limit beyond approximately 32m (one-way). Current capacity is the other limiting factor: 2.5mm² clipped direct is rated 21A. If the protective device is 32A (as on a dedicated socket circuit), you must use at least 4mm². 2.5mm² is also not suitable for electric shower circuits (which are typically 32–40A) or EV charger feeds.
Grouping multiple cables together reduces each cable's effective current-carrying capacity because the cables compete to dissipate heat. BS 7671 Table 4C1 grouping factors reduce from 1.0 (single cable) to 0.57 (six or more cables). This means you may need to select a cable one or two sizes larger than the single-cable calculation would suggest. In practice, grouping is most significant in commercial installations where dozens of cables may share a common cable tray or trunking route.
Yes — always check both. It is a common mistake to size a cable only for current and then discover the volt drop fails, or vice versa. For short, high-current circuits (motor feeds, shower circuits close to the consumer unit), current capacity is typically the limiting factor. For long, lower-current circuits (outbuilding feeds, lighting circuits with long runs), volt drop is often the binding constraint and will require a larger cable than the current check alone would specify. The cable sizing calculator above evaluates both criteria and selects the minimum size that satisfies them simultaneously.