IEC & BS 7671 calculators

Cable size calculator (BS 7671 and IEC 60364)

Find the smallest cable that carries the load as installed and keeps the voltage drop in limit, from the BS 7671 Appendix 4 or IEC 60364-5-52 tables, with every factor shown.

Calculated with electrician.wiki · https://electrician.wiki/calculators/cable-size/

Code / standard

Supply

Load given as

Design current Ib

Voltage, line to neutral

Protective device

Device rating In

Cable (BS 7671 Appendix 4 table)

Installation method

Ambient air temperature

°C

Circuits grouped together

Count this circuit; 1 if it runs on its own.

Grouping arrangement

Overload protection

Route length

Voltage-drop limit

3 % lighting, 5 % other uses (public supply).

Result · BS 7671

2.5 mm² · Iz 27 A · 3.76 % drop

A 16 A load on a B20 needs 2.5 mm² flat twin and earth clipped direct: it carries 27 A as installed and drops 3.76 % over 30 m, within the 5 % limit. 1.5 mm² would carry the current (on a single-source rating), but its drop of 6.05 % is over the limit.

Cable size: 2.5 mm²
Installed rating Iz: 27 A
Tabulated It: 27 A
Minimum It: 20 A
Voltage drop: 8.64 V · 3.76 %
Longest run at 5 %: 39.9 m
Ca × Cg: 1
Size on current alone: 1.5 mm²

Show the work

1. Design current against the device: Ib ≤ In (B20)
$16 A \leq 20 A$
A circuit-breaker trips by 1.45 × In, so In ≤ Iz also keeps I2 ≤ 1.45 Iz (BS 7671 Regulation 433.1).
2. Rating factors
$C_{a} \cdot C_{g} = 1 \cdot 1 = 1$
Ca = 1 (air at 30 °C; BS 7671 Table 4B1); Cg = 1 (one circuit, not grouped; BS 7671 Table 4C1 (same factors as IEC B.52.17))
3. Minimum tabulated rating
$I_{t} \geq \frac{I_{n}}{C_{a} \cdot C_{g}} = \frac{20 A}{1 \cdot 1} = 20 A$
4. Smallest size that passes: 2.5 mm² (BS 7671 Table 4D5, reference method C)
$I_{t} = 27 A \geq 20 A$
Ruled out: 1 mm² drops 9.18 %; 1.5 mm² drops 6.05 %.
5. Voltage drop at 2.5 mm² (BS 7671 Table 4D5 (voltage drop))
$V_{d} = \frac{mV/A/m \cdot I_{b} \cdot L}{1,000} = \frac{18 \cdot 16 A \cdot 30 m}{1,000} = 8.64 V$
3.76 % of 230 V, limit 5 %.
6. Installed rating Iz
$I_{z} = I_{t} \cdot C_{a} \cdot C_{g} = 27 A \cdot 1 \cdot 1 = 27 A$
Ib = 16 A ≤ In = 20 A ≤ Iz = 27 A.

References: BS 7671 Regulation 433.1 (2018): Ib ≤ In ≤ Iz and I2 ≤ 1.45 Iz · BS 7671 Table 4D5 (2018): current-carrying capacity · BS 7671 Appendix 4 (2018): rating factors (Tables 4B1, 4C1) and voltage drop (section 6.4)

Copper conductors; Appendix 4 ratings at 30 °C ambient air (20 °C ground), conductors at their maximum operating temperature.
Voltage drop at the design current, with tabulated or Annex G values at the operating temperature (no correction for a cooler conductor).
Balanced loads without significant third-harmonic current; no parallel cables. Check Zs and the cpc separately.

Report a problem with this result

How to use the cable size calculator

Pick the standard first. BS 7671 sizes the cables UK electricians actually buy, from the Appendix 4 tables: flat twin and earth (Table 4D5) with its insulation methods 100 to 103, single-core PVC in conduit or trunking (4D1A), and PVC or XLPE steel-wire-armoured cable (4D4A and 4E4A). IEC 60364 uses the generic Annex B tables of IEC 60364-5-52 for copper or aluminium, PVC or XLPE, and the ten reference installation methods A1 to G. Visitors outside the US and UK get IEC preselected.

Then describe the circuit: the design current Ib (or the load in kW with its power factor), the protective device and its rating In, how and where the cable runs, the air or ground temperature, how many circuits share the route, the route length and the voltage drop you’ll accept. The calculator works up the size list from the smallest conductor and stops at the first one that carries the current under those conditions and keeps the voltage drop within your limit. Every rating factor, table value and intermediate current appears in the working, with the table it came from.

The sizing rule: Ib ≤ In ≤ Iz

Overload protection rests on three currents. Ib is what the circuit draws in normal service. In is the rating of the fuse or circuit-breaker. Iz is what the cable can carry continuously as installed without its insulation running hotter than its limit: 70 °C for PVC, 90 °C for XLPE. The device must not trip in normal use, and it must open before the cable cooks, so the three must line up (BS 7671 Regulation 433.1, IEC 60364-4-43 clause 433.1):

I_{b} \leq I_{n} \leq I_{z}

A second condition hides behind the first. A device doesn’t trip at exactly In: it is only certain to operate at its conventional current I2, which must not exceed 1.45 × Iz. For circuit-breakers to BS EN 60898 or IEC 60898-1, I2 is 1.45 × In, so In ≤ Iz settles it. BS 7671 treats BS 88-2 and BS 88-3 fuses the same way. A BS 3036 rewireable fuse can need twice its rating to blow, which is why BS 7671 limits it to In ≤ 0.725 × Iz. Under the IEC rule a gG fuse of 16 A or more has I2 = 1.6 × In, so In ≤ 0.9 × Iz, as the ABB handbook points out. The calculator applies the right factor for the device you choose.

The default circuit on one scale. The cable must carry at least the device rating (In ≤ Iz), and the current that is sure to trip the device (I2, 1.45 × In for a circuit-breaker) must stay below 1.45 × Iz, which it does automatically when In ≤ Iz.

The tables don’t give Iz directly. They give It, the rating under reference conditions (one circuit, 30 °C air or 20 °C ground, soil at 2.5 K·m/W), and you correct it with rating factors: Ca for ambient temperature, Cg for grouping, Ci for thermal insulation, and for buried cables a soil factor. Turned round, the minimum tabulated value you need is:

I_{t} \geq \frac{I_{n}}{C_{a} \cdot C_{g} \cdot C_{i}}

BS 7671 Appendix 4 adds two variants. Where grouped circuits can’t all be overloaded at once, It must meet the larger of Ib/Cg and a second condition that credits the unloaded neighbours:

I_{t} \geq \sqrt{{I_{n}}^{2} + 0.48 \cdot {I_{b}}^{2} \cdot \frac{(1 − Cg²)}{{C_{g}}^{2}}}

The 0.48 is 1/1.45², the overload allowance carried into the arithmetic. With three bunched circuits (Cg = 0.7), a 16 A load on a B20 needs It ≥ 28.6 A sized on In, but only 23 A under this variant: 4 mm² becomes 2.5 mm². The third variant covers circuits where overload protection may be omitted (Regulation 433.3, a fixed load that can’t overload): the cable then only has to carry Ib, though its fault withstand still needs checking.

Rating factors that change the answer

Ambient temperature (Ca). Hotter air leaves less headroom. The IEC factors follow √((θmax − θa)/(θmax − 30)), so at 40 °C a PVC cable keeps 0.87 of its rating and an XLPE cable 0.91. Below 30 °C the factor rises above 1, but BS 7671 is more cautious than IEC: at 25 °C it allows 1.03 for PVC where IEC allows 1.06. Between tabulated temperatures the calculator uses the hotter row.
Grouping (Cg). Cables bunched together warm each other. The factor depends on the arrangement (bunched, a single layer on a wall, under a ceiling, on perforated tray or ladder) and on the number of circuits; a single layer gets no further reduction beyond nine. Buried circuits use their own factors, set by the spacing between cables or ducts.
Thermal insulation (Ci). A cable totally surrounded by insulation for 0.5 m or more keeps only half its clipped-direct rating (Regulation 523.9). Shorter runs of small cables lose less, but those Table 52.2 values aren’t in our verified data, so the calculator applies 0.5, which is never less safe.
Soil. Buried ratings assume 20 °C ground, 0.7 m depth and soil at 2.5 K·m/W. Damp clay conducts heat better (a factor above 1); dry sand worse.

Grouping alone can push a cable up two sizes:

A 16 A load on a 20 A circuit-breaker, copper PVC clipped direct (IEC method C), bunched with other circuits.
Circuits	Cg	It needed	Size
1	1	20 A	2.5 mm²
2	0.8	25 A	2.5 mm²
3	0.7	28.6 A	4 mm²
4	0.65	30.8 A	4 mm²
6	0.57	35.1 A	4 mm²
9	0.5	40 A	6 mm²

Voltage drop

A cable that is thermally fine can still deliver too little voltage at the far end. BS 7671 Appendix 4 and IEC 60364-5-52 Annex G both suggest a limit, measured from the origin of the installation: 3 % for lighting and 5 % for other uses on a public supply (6 % and 8 % from a private transformer). In BS 7671 mode the drop comes from the tabulated millivolts per amp per metre, at the design current:

V_{d} = \frac{mV/A/m \cdot I_{b} \cdot L}{1,000}

For cables over 16 mm² the tables split mV/A/m into resistive and reactive parts; the calculator uses the full impedance figure, which is never lower than the power-factor-weighted value, so it errs on the safe side. In IEC mode the drop follows Annex G, with ρ1 = 0.0225 Ω·mm²/m for copper (0.036 for aluminium) and a reactance λ of 0.08 mΩ/m; b is 2 for single-phase circuits and 1 for three-phase:

u = b \cdot (ρ_{1} \cdot \frac{L}{S} \cdot cos φ + λ \cdot L \cdot sin φ) \cdot I_{b}

The two methods agree closely: for 2.5 mm² copper, 2 × 0.0225 / 2.5 = 0.018 Ω/m, the 18 mV/A/m of Table 4D5. The table below shows how far common twin-and-earth circuits can run before the drop becomes the limit; in the default example, 1.5 mm² carries the current but drops 6.05 % over 30 m, so the answer moves up to 2.5 mm².

Longest run of flat twin and earth at the stated current before the voltage drop reaches 3 % and 5 % of 230 V (BS 7671 Table 4D5 mV/A/m, conductors at 70 °C).
Size	mV/A/m	Current	3 % (lighting)	5 % (other uses)
1 mm²	44	6 A	26.1 m	43.6 m
1.5 mm²	29	10 A	23.8 m	39.7 m
2.5 mm²	18	20 A	19.2 m	31.9 m
4 mm²	11	32 A	19.6 m	32.7 m
6 mm²	7.3	40 A	23.6 m	39.4 m
10 mm²	4.4	50 A	31.4 m	52.3 m
16 mm²	2.8	63 A	39.1 m	65.2 m

Twin and earth, insulation and Table 4D5

Flat twin and earth is the UK’s most-searched case and the one where installation matters most. Table 4D5 rates it clipped direct (method C), in conduit in an insulated wall (method A) and in four insulation-specific methods: above a plasterboard ceiling under up to 100 mm of insulation (100) or more (101), and in an insulated stud wall touching the inner surface (102) or clear of it (103). The same 32 A radial, installed seven ways:

A 28 A load on a B32, 20 m of flat twin and earth, 30 °C, not grouped: the size this calculator returns for each installation method.
Installation	Size	Note
Clipped direct (reference method C)	4 mm²	It 37 A, single-source value
In conduit in a thermally insulated wall (method A)	6 mm²	It 32 A, single-source value
Method 100: above a plasterboard ceiling, insulation up to 100 mm thick	check Table 4D5 at 4 mm²	needs It ≥ 32 A; the published value has no source we could verify
Method 101: above a plasterboard ceiling, insulation over 100 mm thick	check Table 4D5 at 4 mm²	needs It ≥ 32 A; the published value has no source we could verify
Method 102: in an insulated stud wall, touching the inner surface	check Table 4D5 at 4 mm²	needs It ≥ 32 A; the published value has no source we could verify
Method 103: in an insulated stud wall, not touching the inner surface	check Table 4D5 at 4 mm²	needs It ≥ 32 A; the published value has no source we could verify
Totally surrounded by thermal insulation (0.5 × method C)	10 mm²	It 64 A, single-source value

Moving the cable from the wall surface into a conduit in an insulated wall takes it from 4 mm² to 6 mm²; burying it fully in insulation needs 10 mm². The rows that say “check Table 4D5” are deliberate. Our research found published values from two independent sources for only a handful of 4D5 cells; 22 of the method 100 to 103 values have no source we could check at all, so the calculator won’t use them. It stops, tells you the minimum It that size must have, and asks you to read the figure from your copy of BS 7671. Where a size can be ruled out on safe ground anyway, it is: a cable in insulation always rates below the same cable clipped direct, so if even the verified clipped-direct value is too low, the size fails without the missing number. Values confirmed by only one publisher are used, with a warning that names the source.

Two UK-specific notes. A 2.5 mm² ring final on a 32 A device follows the ring arrangement of Appendix 15, where the load shares two paths; this calculator sizes radial circuits and will flag the case. And XLPE armoured cable rated at 90 °C (Table 4E4A) may only be loaded that hard if the equipment at both ends is rated for it; otherwise Regulation 512.1.5 holds the design to 70 °C, so choose the PVC SWA option to size on Table 4D4A.

IEC reference methods A1 to G

IEC 60364-5-52 rates every cable against one of ten reference methods. Method A1 and A2 are cables in conduit in a thermally insulated wall (single-core and multicore); B1 and B2 the same on the wall surface or in trunking; C is clipped direct; D1 is in a duct in the ground and D2 direct in the ground; E is a multicore cable in free air or on perforated tray; F single-core cables touching in free air; G single-core cables spaced one diameter apart. Pick the method that matches the worst stretch of the route: a run that spends two metres in an insulated wall is rated as if it were all there.

A 30 A three-phase load on a 32 A circuit-breaker, copper PVC (70 °C), 30 °C air or 20 °C ground, one circuit: the size for each IEC 60364-5-52 reference method.
Method	Size	Tabulated It
A1 · single-core in conduit in an insulated wall	10 mm²	42 A
A2 · multicore in conduit in an insulated wall	10 mm²	39 A
B1 · single-core in conduit or trunking on a wall	6 mm²	36 A
B2 · multicore in conduit or trunking on a wall	6 mm²	34 A
C · clipped direct or on an unperforated tray	4 mm²	32 A
D1 · in a duct in the ground	6 mm²	38 A
D2 · direct in the ground	4 mm²	33 A
E · multicore in free air or on perforated tray	4 mm²	34 A
F · single-core touching, in free air or on tray	25 mm²	110 A
G · single-core spaced one diameter apart	25 mm²	146 A

Every value here is confirmed by at least two publishers. Methods F and G start at 25 mm².

Worked examples

A 20 A radial that voltage drop pushes up a size

A 16 A load on a B20 circuit-breaker, flat twin and earth clipped direct, 30 m from the consumer unit.

1. Design current against the device: Ib ≤ In (B20)
$16 A \leq 20 A$
2. Rating factors
$C_{a} \cdot C_{g} = 1 \cdot 1 = 1$
3. Minimum tabulated rating
$I_{t} \geq \frac{I_{n}}{C_{a} \cdot C_{g}} = \frac{20 A}{1 \cdot 1} = 20 A$
4. Smallest size that passes: 2.5 mm² (BS 7671 Table 4D5, reference method C)
$I_{t} = 27 A \geq 20 A$
5. Voltage drop at 2.5 mm² (BS 7671 Table 4D5 (voltage drop))
$V_{d} = \frac{mV/A/m \cdot I_{b} \cdot L}{1,000} = \frac{18 \cdot 16 A \cdot 30 m}{1,000} = 8.64 V$
6. Installed rating Iz
$I_{z} = I_{t} \cdot C_{a} \cdot C_{g} = 27 A \cdot 1 \cdot 1 = 27 A$

ABB’s worked example: seven circuits on a hot tray

A 100 A three-phase load in multicore copper PVC cable on perforated tray, bunched with six other circuits at 40 °C. The ABB Electrical installation handbook (2010) works this example by hand and lands on 95 mm² with Iz = 111.81 A.

1. Design current against the device: Ib ≤ In (B100)
$100 A \leq 100 A$
2. Rating factors
$C_{a} \cdot C_{g} = 0.87 \cdot 0.54 = 0.47$
3. Minimum tabulated rating
$I_{t} \geq \frac{I_{n}}{C_{a} \cdot C_{g}} = \frac{100 A}{0.87 \cdot 0.54} = 212.9 A$
4. Smallest size that passes: 95 mm² (IEC 60364-5-52 Table B.52.10, method E)
$I_{t} = 238 A \geq 212.9 A$
5. Voltage drop at 95 mm² (IEC 60364-5-52 Annex G)
$u = 1 \cdot (0.0225 Ω·mm²/m \cdot \frac{10 m}{95 mm²} \cdot 1 + 0.00008 Ω/m \cdot 10 m \cdot 0) \cdot 100 A = 0.237 V$
6. Installed rating Iz
$I_{z} = I_{t} \cdot C_{a} \cdot C_{g} = 238 A \cdot 0.87 \cdot 0.54 = 111.8 A$

A 100 A load on a B100 needs 95 mm² copper PVC cable, method E (multicore in free air or on perforated tray): it carries 111.8 A as installed and drops 0.10 % over 10 m, within the 5 % limit.

A rewireable fuse costs a size

An older installation: a 25 A load on a 30 A BS 3036 fuse, twin and earth clipped direct, 10 m.

1. Design current against the device: Ib ≤ In (BS 3036 30 A fuse)
$25 A \leq 30 A$
2. Rating factors
$C_{a} \cdot C_{g} = 1 \cdot 1 = 1$
3. Minimum tabulated rating
$I_{t} \geq \frac{I_{n}}{C_{a} \cdot C_{g} \cdot C_{f}} = \frac{30 A}{1 \cdot 1 \cdot 0.725} = 41.38 A$
4. Smallest size that passes: 6 mm² (BS 7671 Table 4D5, reference method C)
$I_{t} = 47 A \geq 41.38 A$
5. Voltage drop at 6 mm² (BS 7671 Table 4D5 (voltage drop))
$V_{d} = \frac{mV/A/m \cdot I_{b} \cdot L}{1,000} = \frac{7.3 \cdot 25 A \cdot 10 m}{1,000} = 1.83 V$
6. Installed rating Iz
$I_{z} = I_{t} \cdot C_{a} \cdot C_{g} = 47 A \cdot 1 \cdot 1 = 47 A$

A 25 A load on a BS 3036 30 A fuse needs 6 mm² flat twin and earth clipped direct: it carries 47 A as installed and drops 0.79 % over 10 m, within the 5 % limit.

BS 7671 and IEC 60364: what differs

Tables. BS 7671 rates specific UK cable types (Tables 4D5, 4D1A, 4D4A, 4E4A); IEC rates generic constructions by method. Some columns match exactly: the BS armoured-cable ratings in the ground equal the IEC D1 values, and the free-air columns of Table 4D1A equal IEC methods F and G. Others differ, so each mode sizes from its own tables.
Ambient factor below 30 °C. 1.03 in BS 7671 against 1.06 in IEC at 25 °C for PVC.
Fuses. BS 7671 accepts In ≤ Iz for BS 88 fuses; the IEC rule with a gG fuse’s 1.6 × In conventional current asks for In ≤ 0.9 × Iz.
Buried cables. BS 7671 Appendix 4 applies an extra factor of 0.9 to circuits in the ground; the calculator includes it and says so.
Voltage drop. Tabulated mV/A/m in BS 7671; the Annex G formula in IEC mode. The limits are the same.

Common mistakes

Sizing on Ib instead of In. The cable must suit the device, not just the load; only circuits without overload protection size on Ib.
Ignoring the route. One short pass through loft insulation or a bundle in a riser governs the whole cable. Size for the worst metre.
Multiplying factors the wrong way. Factors divide the requirement (It ≥ In / factors) and multiply the rating (Iz = It × factors).
Checking the drop at the device rating. Voltage drop uses the design current; the device rating is for the thermal check.
Stopping at the cable. A cable that passes here still needs its earth fault loop impedance and protective conductor checked.

After sizing, check the earth fault loop impedance for disconnection time, size the protective conductor with the adiabatic equation, and confirm the breaking capacity of the devices with the prospective fault current calculator. For a load given in kW, the three-phase power calculator shows the current.

Method and verification

Every rating and factor comes from our research data set, where each cell is checked against at least two independent publications (ABB, Eupen, Schneider Electric, Siemens, Eland, Prysmian, the IET, NICEIC and NAPIT among them). Cells with a single source carry a warning when they decide an answer; cells with none are refused. The tests reproduce the ABB handbook’s worked examples exactly, recompute the BS 7671 variants by hand, and check properties such as “a longer run, a larger device, more grouping or a hotter room never gives a smaller cable”.

Covers copper (and IEC aluminium) cables up to 300 mm², one cable per phase, no harmonic-loaded neutrals. Updated 2026-10-14. Found a problem? Report it and we log every fix publicly.

Questions people ask

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

It depends on the installation. A 32 A radial in twin and earth clipped direct needs 4 mm² by BS 7671 Table 4D5, but run in conduit in an insulated wall it needs 6 mm². Long runs can need more for voltage drop. A 32 A ring final is a separate case that uses 2.5 mm².

How do you calculate cable size?

Divide the device rating In by the rating factors for temperature, grouping and insulation to get the minimum tabulated current It, pick the first size in the table for your installation method that reaches it, then check the voltage drop at the design current and go up a size if it is too high.

What does Ib ≤ In ≤ Iz mean?

The design current Ib must not exceed the protective device rating In, which must not exceed the cable’s rating as installed, Iz. The device then never trips in normal use, but opens before an overload overheats the cable.

Why does loft insulation increase cable size?

Insulation traps the heat the cable makes. Twin and earth totally surrounded by insulation for half a metre or more keeps only half of its clipped-direct rating (BS 7671 Regulation 523.9), and the 4D5 methods 100 to 103 rate it below method C even when it only touches insulation.

What is the maximum voltage drop allowed?

BS 7671 Appendix 4 and IEC 60364-5-52 Annex G suggest 3 % for lighting and 5 % for other uses, measured from the origin on a public supply, and 6 % and 8 % from a private supply.

Why won’t the calculator give a size for method 100 or 103?

Many Table 4D5 values for the insulation methods have no published source we could verify, so the calculator refuses to rely on them. It tells you the minimum rating the next size must have, so you can read it from your copy of BS 7671.