For electricians
Circuit design tool
Answer the design questions — supply and earthing, load, protective device, cable type, installation method, route and location — and this tool works through the BS 7671:2018+A4:2026 sequence with you: design current, correction factors, cable size, cpc, voltage drop, earth fault loop impedance, protective device selection, fixing and support requirements, and the mistakes that most often catch people out. Export the whole thing as a printable A4 PDF.
Pair with the risk assessment and method statement generators. Everything runs in your browser — nothing you type is uploaded.
Design aid only — no liability accepted
This document is produced by an online design aid. Every value, factor, cable size and requirement in it is indicative only and MUST be independently verified by a competent person against BS 7671:2018+A4:2026, the relevant manufacturer's instructions and the actual conditions on site before any work is carried out. Tabulated values are reproduced as a guide and may not reflect the current published edition. Site-measured values (Ze, Zs, PFC, ambient temperature, grouping and insulation conditions) take precedence over any figure shown here. All Cabling© Limited accepts no liability whatsoever for any loss, damage, injury or cost arising from the use of, or reliance on, this document or the tool that generated it. Use of this output is entirely at the user's own risk.
1. Project details
2. Supply and earthing
The earthing arrangement drives fault protection. Ze should be measured, not assumed — the figures below are typical maxima only (TN-C-S 0.35 Ω, TN-S 0.8 Ω, TT 21 Ω).
3. The load
4. Protective device
5. Cable and installation method
6. Where does it run?
Select every location and condition the circuit passes through. Each one adds its own installation requirements to the output.
Installation requirements
- Method 100 — above a plasterboard ceiling, insulation ≤ 100 mm. Cable laid on a joist or in contact with a plasterboard ceiling, covered by not more than 100 mm of loft insulation.
- TN-C-S (PME): main protective bonding to Table 54.8 — normally 10 mm² for a 25 mm² supply neutral, 16 mm² above that. Confirm the DNO's requirement.
- 30 mA RCD/RCBO required. Select the correct type: Type AC is no longer appropriate for most modern loads — use Type A as the minimum, Type F for variable-speed drives and washing machines, Type B for EV charging and PV where d.c. fault current can exceed 6 mA.
- Surge protection (SPD): required unless the consequence of an overvoltage is limited to the equipment itself and a documented risk assessment justifies omission (Section 443). In practice most domestic consumer units now include a Type 2 SPD.
- Cable supports must resist premature collapse in fire (Regulation 521.10.202) — metal clips, saddles or metal cable ties at the intervals in the support table.
- Flat twin & earth: the cpc is bare and must be sleeved green/yellow at every termination. Terminate the cpc first and last, and leave it long enough to reach the earth terminal without strain.
- Cables concealed at less than 50 mm from the surface must run in a permitted safe zone: within 150 mm of the top of the wall, within 150 mm of a corner, or vertically/horizontally in line with an accessory (Regulation 522.6.202).
- In a safe zone the cable must additionally have 30 mA RCD protection, OR mechanical protection against penetration by a nail or screw, OR earthed metallic covering, OR be in earthed conduit/trunking (Regulation 522.6.204).
- Where a wall is of metal or metal-framed construction, the cable requires 30 mA RCD protection regardless of depth (Regulation 522.6.203).
- Use oval conduit or steel capping over chases — capping alone is not mechanical protection, it is there to protect the cable from the plasterer's trowel.
- Chases: vertical chases no deeper than one third of the wall thickness, horizontal no deeper than one sixth (BS EN 1996 / Approved Document A).
- Cables through joists must pass through holes drilled not less than 50 mm from the top or bottom edge (Regulation 522.6.201).
- Holes must be in the permitted zone: between 0.25 and 0.4 of the span from the support, diameter not more than 0.25 of the joist depth.
- Notches, where unavoidable, must be in the top edge between 0.07 and 0.25 of the span, not deeper than 0.125 of the joist depth — and a notched cable needs protection.
- Do not lay cables loose over the top of ceiling joists where they will be walked on or covered by boarding without support.
- Where loft insulation will be laid over the cable, apply the derating for the applicable method (100/101) — this catches out more designs than anything else.
- A 32 A ring final in 2.5/1.5 mm² T&E requires a tabulated capacity of not less than 20 A (Regulation 433.1.204).
- Unfused spurs: one single or one twin socket per spur, taken from a socket on the ring or a junction box in the ring.
- Both legs must be continuous — test r1, rn and r2 end-to-end before energising. An interconnected or broken ring is a C2.
- On completion: inspect and test to Part 6 — continuity of protective conductors, ring continuity where applicable, insulation resistance, polarity, Zs, PFC/PSCC and RCD operation — and issue an Electrical Installation Certificate or Minor Works Certificate.
Mistakes to avoid
- Sizing the cable on the breaker rating alone and ignoring the correction factors — Ca, Cg, Ci and Cf all apply together, and 0.81 × 0.8 is 0.65, not 0.9.
- Forgetting that In must be greater than or equal to Ib, and It must be greater than or equal to In. Getting that order wrong is the most common design error there is.
- Designing for current-carrying capacity and never checking voltage drop — on runs over about 20 m, voltage drop usually decides the cable size.
- Assuming Ze rather than measuring it. A design Zs built on an assumed Ze is not a verified design.
- Using the maximum Zs from Table 41.3 without applying the 0.8 rule-of-thumb correction for conductor operating temperature when comparing against a cold measured value.
- Fitting a 30 mA RCD and treating it as a substitute for correct cable sizing or safe-zone routing. It is additional protection, not the primary means.
- Plastic clips or plastic trunking as the sole support — Regulation 521.10.202 requires supports that resist premature collapse in fire.
- Not checking the manufacturer's minimum cable size for the appliance — showers and EV chargers frequently specify more than the calculation gives.
- Ignoring the cpc: the standard cpc in flat twin & earth is smaller than the line conductor and must be verified against the adiabatic equation for the actual fault level.
- Leaving the circuit chart, labels and certification until later — the installation is not complete until the paperwork is.
Cable support and fixing intervals
On-Site Guide Table 4A. Supports must also resist premature collapse in fire — Regulation 521.10.202.
| Cable | Horizontal | Vertical |
|---|---|---|
| Non-armoured, up to 9 mm overall (1.0–2.5 mm² T&E) | 250 mm | 400 mm |
| Non-armoured, 9–15 mm overall (4–16 mm² T&E) | 300 mm | 400 mm |
| Non-armoured, 15–20 mm overall | 350 mm | 450 mm |
| Armoured (SWA), up to 9 mm overall | 300 mm | 400 mm |
| Armoured (SWA), 9–15 mm overall | 350 mm | 450 mm |
| Armoured (SWA), 15–20 mm overall | 400 mm | 550 mm |