Cable size calculator (twin & earth + SWA)
Size a circuit properly: BS 7671 table ratings for twin & earth and SWA, ambient and grouping correction factors, and a voltage drop check, all in one go. It shows its working, so you can sanity-check it.
- Required rating It ≥ In ÷ (Ca × Cg)–
- Ambient factor Ca–
- Grouping factor Cg–
- Chosen cable rating (4D5)–
- Voltage drop at Ib–
- Voltage drop limit–
Guidance only, not a design. This uses the published BS 7671 Table 4D5 ratings for flat twin & earth (with Table 4B1 ambient and Table 4C1 grouping factors) and Table 4E4A for SWA clipped direct. It doesn't cover rewirable (BS 3036) fuses, buried SWA runs, adiabatic CPC checks or sizes beyond 16 mm² T&E / 25 mm² SWA. The installation method selector applies to T&E only, SWA is assumed clipped direct or on tray. Always verify against BS 7671 / the On-Site Guide, or get a qualified designer to sign it off.
How cable sizing actually works
Every circuit has to satisfy one golden rule: Ib ≤ In ≤ Iz. The design current can't exceed the device rating, and the device rating can't exceed what the cable can carry as installed. That last bit is where people slip up, because the same 2.5 mm² T&E carries 27 A clipped to a joist but only 13.5 A buried in loft insulation.
The calculator runs the standard method: it takes your device rating In, divides by the correction factors (Ca for ambient temperature, Cg for grouping), and finds the smallest cable whose tabulated rating beats that number. Then it checks voltage drop over your run, because on longer circuits it's usually voltage drop, not current, that forces the size up.
Table 4D5 ratings this tool uses (flat twin & earth, amps)
| Size | C, clipped | 100 | 101 | 102 | 103 | A, conduit |
|---|---|---|---|---|---|---|
| 1.0 mm² | 16 | 13 | 10.5 | 13 | 8 | 11.5 |
| 1.5 mm² | 20 | 16 | 13 | 16 | 10 | 14.5 |
| 2.5 mm² | 27 | 21 | 17 | 21 | 13.5 | 20 |
| 4.0 mm² | 37 | 27 | 22 | 27 | 17.5 | 26 |
| 6.0 mm² | 47 | 34 | 27 | 35 | 23.5 | 32 |
| 10 mm² | 64 | 45 | 36 | 47 | 32 | 44 |
| 16 mm² | 85 | 57 | 46 | 63 | 42.5 | 57 |
SWA ratings this tool uses (Table 4E4A, 90°C, clipped direct, amps)
| Size | 2-core (single-phase) | 3/4-core (three-phase) |
|---|---|---|
| 1.5 mm² | 27 | 23 |
| 2.5 mm² | 36 | 31 |
| 4.0 mm² | 49 | 42 |
| 6.0 mm² | 62 | 53 |
| 10 mm² | 85 | 73 |
| 16 mm² | 110 | 94 |
| 25 mm² | 146 | 124 |
SWA to BS 5467 with 90°C thermosetting insulation runs hotter than T&E, so it carries noticeably more current for the same copper. These figures are for clipped direct or on tray; buried runs use different tables (and a whole different conversation about ducts and derating), so treat underground sizing separately.
A worked example: 9.5 kW shower
9.5 kW at 230 V is 41.3 A, so you'd protect it at 45 A. Running through a loft with insulation up to 100 mm (method 100), the required rating is 45 ÷ (1.0 × 1.0) = 45 A. From the table, 6 mm² gives you 34 A (fail) and 10 mm² gives 45 A (pass, just). Add a second grouped circuit and Cg drops to 0.8, the required rating jumps to 56.3 A, and you're at 16 mm². That's why "what size cable for a shower" has no one-line answer, the route decides.
- Diversity first: cookers, showers and EV chargers aren't all sized at nameplate rating. Work out Ib properly before you size anything.
- The worst point of the run governs. Ten metres clipped neatly and one metre buried in loft insulation? Size for the insulation.
- Long run? Check the voltage drop even when the current check passes easily.
Cable sizing FAQs
What size cable do I need for a 9.5 kW shower?
About 41 A at 230 V, so typically 10 mm² T&E on a 45 A or 50 A device. Short, favourable runs can sometimes justify 6 mm² by calculation, but 10 mm² is the safe norm. The installation method and run length decide, run your actual numbers through the calculator above.
What size cable for a cooker circuit?
Most domestic cookers end up on 6 mm² T&E with a 32 A device once diversity is applied. Big range cookers can need 10 mm². Work out the diversified current first, then size for your route.
What size cable for a 7 kW EV charger?
A 7.4 kW charger draws 32 A continuously, no diversity. That usually means 6 mm² T&E minimum on favourable routes, stepping up to 10 mm² for longer runs or anything near insulation, and voltage drop bites hard on garage and driveway runs. EV circuits also have their own regs (Section 722), so treat this as a starting point only. The EV charger cable size calculator runs the numbers for 7 kW and 22 kW chargers.
What are reference methods 100 to 103?
They describe how flat T&E sits relative to thermal insulation. 100: above a plasterboard ceiling with up to 100 mm of insulation over it. 101: same but more than 100 mm. 102: in a stud wall with the cable touching the inner wall surface. 103: in a stud wall completely surrounded by insulation, the harshest case.
What do Ib, In and Iz stand for?
Ib is the design current of the circuit, In is the protective device rating, Iz is the cable's current-carrying capacity as installed. Design rule: Ib ≤ In ≤ Iz.
What size SWA cable do I need?
Switch the cable type to SWA in the calculator above and it sizes from Table 4E4A (90°C thermosetting, clipped direct). As a feel for it: 2-core SWA carries 36 A at 2.5 mm² and 62 A at 6 mm², well above T&E, because the insulation runs hotter. Buried runs are a different table and aren't covered here.
Why does the tool stop at 16 mm² T&E and 25 mm² SWA?
Because beyond that you're realistically into parallel runs, singles in containment or supply upgrades, different tables, different questions, and firmly design-engineer territory. If the biggest size here doesn't pass, the answer isn't just bigger cable.
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