Radial to a garden workshop: derating, grouping and an exported PME earth
An 18 m² timber-framed workshop stands 22 m from a house on a TN-C-S supply. You are to install a 20 A RCBO supplying a 2.5 mm² T&E radial to three double socket outlets. The design current is taken as 16 A (2 kW heater, 1.5 kW saw, IT equipment). The route leaves the consumer unit, runs 12 m clipped direct along the garage wall, then 8 m through a stud partition filled with mineral wool insulation, where for 4 m it is bunched with two other T&E cables. Ambient in the partition is 30 °C; Ca = 1.0. The client has asked for no earth rod because 'the garden has just been turfed'. A metal water pipe runs from the house to the workshop and is used for a sink.
1. Show whether 2.5 mm² T&E is acceptable on a 20 A device for this route, including the voltage drop check.
Read the worked answer
Take the worst condition, not the average
The design rule is In ≤ It × Ca × Cg × other factors, where It is the tabulated value for the most onerous reference method on the route, not for the easy part of it.
- In the insulated partition the reference method is A (enclosed in a thermally insulating wall, method 101): 2.5 mm² T&E, It = 20 A.
- Ca at 30 °C for 70 °C thermoplastic = 1.0.
- Cg for 3 circuits bunched = 0.70 (Table 4C1).
It × Ca × Cg = 20 × 1.0 × 0.70 = 14 A against In = 20 A. It fails, and even with the grouping ignored the margin is zero.
Voltage drop (separate check, not a substitute)
18 mV/A/m × 16 A × 22 m = 6.34 V = 2.75% of 230 V. Within the 5% permitted for a power circuit, so volt drop is not the limiting factor - thermal capacity is.
Note on insulation
If any part of the cable were completely surrounded by thermal insulation for 500 mm or more in a ceiling, Table 52.2 factors (down to 0.50) apply on top of the tabulated value. The tabulated Method 101 value already assumes the cable is in an insulated wall, but it does not cover a cable buried in a quilt.
Conclusion
2.5 mm² on a 20 A device in this route is non-compliant. The calculation, not an opinion, is what the assessor wants.
2. Give the realistic remedial options and state which you would adopt, with reasons.
Read the worked answer
Option 1 - re-route (preferred)
Take the 8 m out of the insulation: clip to the face of the stud or run in the void clear of the mineral wool, and separate it from the other two cables so grouping disappears. 2.5 mm² clipped direct is 27 A, comfortably above 20 A, volt drop already passes, and the CPC stays adequate. Cheapest and it removes the cause rather than compensating for it. It depends on the client accepting surface clipping or a slightly deeper chase.
Option 2 - upsize
4 mm² in the insulated wall is 27 A; × 0.70 = 18.9 A, still short of 20 A. So the honest answer is that you would need 6 mm² (32 A × 0.70 = 22.4 A) to keep the route and the grouping, with a larger CPC and a harder pull through the partition.
Option 3 - reduce the device
A 16 A device does not rescue it: 14 A derated capacity is still below 16 A, and it also limits the sockets more than the client needs.
Recommendation
Option 1, with Option 2 as the fallback if the client refuses surface wiring. Whichever is chosen, the design current, the reference method, Ca and Cg must be recorded on the EIC so the next person can see why the cable is the size it is. Also confirm the 30 mA RCBO: socket outlets ≤ 32 A need additional protection regardless of the earthing arrangement.
3. The client refuses an earth rod. How do you deal with the earthing of the workshop and the metal water pipe?
Read the worked answer
The issue
The house is TN-C-S. The combined neutral and earth (PEN) is only earthed at the supplier's transformer and at PME points along the network. Exporting that earth to a detached building means that if the PEN is lost, the workshop's exposed metalwork - and its metal water pipe - can rise to a dangerous potential with no low-impedance path to clear it. That is why exporting PME is normally avoided and why an earth rod is the standard solution.
What I would do
- Explain the risk in writing, in terms the client can act on: losing the PEN with exported PME can put the shower of a fault current through anyone touching the workshop metalwork and the true earth at the same time.
- Confirm the earthing arrangement with the DNO rather than relying on the declaration.
- Recommend a TT arrangement for the workshop: local earth electrode, main protective bonding of the incoming metal water pipe to the workshop's earth terminal, and 30 mA RCD protection (the RCBO already gives it) with the electrode resistance low enough for the RCD to operate, typically well under 200 Ω and recorded on the schedule.
- If the client still refuses, I do not simply export the PME earth and say nothing. The refusal, the advice given, and the limitation go on the certificate and in a letter.
Judgement
'No rod because of the turf' is a cost objection, not a technical one. The professional answer is not to comply quietly; it is to price the rod, document the risk, and let the client make an informed decision in writing - with the paperwork protecting whoever follows you.