Consumer unit and shower upgrade in a 1960s house: earthing, bonding and Part P
A 1960s detached house in England, declared TN-S. Ze is 0.35 Ω and PFC 1.6 kA. The main earthing conductor is 6 mm²; the main protective bonding to gas and water is 4 mm² with clamps in the right places. The client wants a new metal-clad consumer unit with 30 mA RCBOs and a 10.5 kW shower replacing an 8.5 kW unit. The existing shower cable is 6 mm², clipped in a warm loft, protected by a 32 A MCB, with a ceiling pull cord at 2.4 m just outside the bathroom door. The builder wants the isolator moved onto the bathroom wall at 1.8 m beside the shower. The client asks whether Building Control need to know, 'as it's only a fuse board'.
1. What must be done about the earthing conductor and the main protective bonding, and how do you establish the requirements?
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Both current sizes are almost certainly wrong, whichever system this is.
- TN-S: Table 54.7 sizes the main earthing conductor from the supplier's neutral. For a neutral up to 16 mm² the earthing conductor must be the same size; 6 mm² will not do.
- TN-C-S: Table 54.8 requires 10 mm² for a supply neutral up to 35 mm². 6 mm² still fails.
- Main protective bonding: Table 54.8 requires 10 mm² for both TN-S (neutral ≤ 35 mm²) and PME. The existing 4 mm² must be upgraded, with BS 951 clamps, within 600 mm of the point of entry, on the consumer's side of the meter, and labelled.
How you establish it
Do not rely on the declared TN-S. A declared arrangement can differ from what is actually there, and the earthing conductor is sized from the supplier's neutral, which you may not be able to see. Confirm the arrangement and the neutral size with the DNO in writing, or measure and record what you can: Ze, PFC, and the physical arrangement at the cut-out.
Sequencing
The earthing and bonding upgrades are part of the board change, not a separate favour. A new metal-clad board full of RCBOs sitting on a 6 mm² earthing conductor and 4 mm² bonding is a worse outcome than the old board, because it looks compliant. Upgrade before energising, record the sizes and Ze/PFC on the schedule, and note the DNO confirmation.
Judgement
The tempting answer is 'Ze is fine, so the earthing is fine'. Ze tells you the loop works; it says nothing about whether the conductor is the right size for the fault current and duration it may have to carry.
2. Assess the 10.5 kW shower and the proposal to move the isolator to 1.8 m in the bathroom.
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The shower
10.5 kW at 230 V is 45.7 A, so the design current is about 46 A with no diversity on a single shower.
- A 32 A MCB cannot protect it. A 50 A device is needed.
- 6 mm² clipped direct is 47 A tabulated - already below the 50 A device, and it is in a warm loft where Ca and any insulation contact reduce it further. It fails.
- 10 mm² clipped direct is 64 A; derated for the loft and any quilt contact it remains comfortably above 50 A. Volt drop at 4.4 mV/A/m over a typical 15 m run is about 3 V, roughly 1.3%, so cable size is set by current-carrying capacity, not drop.
- Note that the existing 8.5 kW unit (37 A) on a 32 A device was already inadequate - do not repeat the mistake because it 'worked for years'.
- The bathroom circuit needs 30 mA additional protection; the new RCBO provides it, and with main bonding in place and all bathroom circuits RCD protected, supplementary bonding may not be needed.
The isolator
At 1.8 m beside the shower it is inside zone 1 or zone 2. Accessories there must be at least IPX4 and suitable for the location, and a wall switch within reach of someone using the shower is poor practice regardless of the IP rating. Manufacturer's instructions for showers normally require a double-pole isolator, readily accessible, outside the zones. The existing ceiling pull cord at 2.4 m is above the 2.25 m zone boundary, which is exactly why it is where it is.
Recommendation
Keep the pull-cord arrangement, or relocate the isolator to a wall outside the zones. Refuse the 1.8 m position and explain why in one sentence the builder can repeat: showers and wet hands do not mix with wall switches.
Judgement
The builder's proposal is not unlawful in every reading - it is simply the wrong engineering call, and the case wants you to say so and offer the alternative.
3. What is notifiable, what do you issue, and how do you answer the client's question?
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Notifiable in England
- Replacing a consumer unit: notifiable.
- A new final circuit for the shower: notifiable.
- Moving the shower isolator as part of that work: covered by the same notification.
- Swapping a like-for-like accessory would not be notifiable - that is not what this is.
Notification happens before work starts, through a competent person scheme, or to the local authority building control with their fee. The Building Regulations compliance certificate comes via the scheme, or from building control.
What you issue
One EIC covering the board change, the new shower circuit, the earthing and bonding upgrade, with a schedule of inspections and a schedule of test results. One certificate for one job - not a shower EIC plus a separate board EIC. Give copies to the client and retain one; the building control body or scheme gets what the scheme requires.
An EICR is not a substitute. It is a report on an existing installation, not a certificate for new work.
What goes on the certificate
The Ze and PFC measured, the upgraded earthing and bonding conductor sizes, the shower circuit's design current, cable size, reference method and protective device, the RCD test results, and any departures from BS 7671 with the reason. If the existing installation has defects that the board change exposes - undersized CPCs, a borrowed neutral - they are recorded and, where they are dangerous, rectified before energising.
Answer to the client
'Only a fuse board' is still a notifiable alteration, and the shower is a new circuit. What protects him is not the notification itself but the certificate and the test results behind it - the paperwork he will be asked for when he sells, and the reason the insurance company will pay out rather than argue.