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Cable calculation: the regulations to check firstCan the existing installation take a new circuit?
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Part 1 is "the regs". Part 1 sets out each requirement. The remaining parts are guidance on how to meet those requirements. That's a useful way to think of it.
Before adding a circuit, make sure the existing installation can take it. Check the consumer's equipment and the DNO equipment (size of the main fuse, general condition), and that the earthing and bonding are adequate. That means main bonding to every extraneous-conductive-part, sized correctly. Most dwellings have 16 mm² or 25 mm² tails, so the main bonding would usually be 10 mm² from the MET. In this scenario we're told earthing and bonding are adequate.
BS 7671 · Reg 132.16 · additions and alterations · p.29
Socket-outlets must have additional protection by a 30 mA RCD. Our circuit has a socket on the cooker switch, so it needs one.
The exception is often described as sockets used only by BA4 (instructed) or BA5 (skilled) persons, backed by a risk assessment. That isn't how BS 7671:2018+A4:2026 words it. Under A4 the RCD can only be left off for socket-outlets in locations where they aren't liable to be used by ordinary persons (BA1) or children (BA2), where a suitably documented risk assessment, done with a skilled person (electrically) involved, decides it isn't necessary, and the risk assessment goes with the certificate. A home never qualifies in practice. The point stands: almost every socket needs it (one example of an exception is a socket under a board in a locked switchroom).
BS 7671 · Reg 411.3.3 · additional protection for socket-outlets · p.72
Our cable is chased into the wall at 30 mm. So:
- It must run in a safe zone: horizontally or vertically in line with an accessory, or within 150 mm of the top of the wall (the ceiling) or of a corner. Ours runs straight down to the switch and straight down to the outlet, so it's in a safe zone.
- A cable less than 50 mm deep also needs 30 mA RCD protection.
So there are two reasons this circuit needs an RCD: the socket, and the buried cable. With no socket and the cable in trunking all the way, you could leave the RCD off.
Reg 522.6.204 lists ways to conceal a cable without RCD protection: a cable with an earthed metallic covering (e.g. SWA), earthed conduit, earthed trunking, mechanical protection (such as a metal plate over the chase against nails and screws), or the circuit being part of a SELV or PELV system.
BS 7671 · Reg 522.6.202 · cables concealed in walls (safe zones, < 50 mm) · p.152 BS 7671 · Reg 522.6.204 · the alternatives to an RCD · p.152
Device decision so far
Socket-outlet → Reg 411.3.3 → 30 mA RCD Buried < 50 mm in wall → Reg 522.6.202 → 30 mA RCD Socket circuit in an HMO → Reg 421.1.7 → AFDD → a 30 mA RCBO with AFDD (about £140)
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