Helpful video reference. This installation guide from The Electric Heating Company, "Fitting an EHC Electric Boiler - Installation Considerations & Using The Right Cable", covers how to size the supply cable for different boiler outputs and the considerations that affect the installation method. Useful background before starting your own installation.
1. Read the boiler data plate and confirm the supply specification
Before ordering cable or MCBs, find the manufacturer's data plate fixed to the boiler casing. It states the rated power (in kW or W), rated supply voltage (230 V for single-phase domestic models), and the required supply cable size. Some manufacturers also state the maximum permitted MCB rating.
Do not estimate from the kW rating alone. Cable sizing depends on the installation method, and the data plate is the authoritative starting point. Download the installation manual from the manufacturer's website and read the electrical requirements section in full.
2. Size the cable using BS 7671 Appendix 4 derating factors
The rated current drawn by the boiler is the starting point, not the final answer. BS 7671 Table 4D5 (for flat two-core-and-earth thermoplastic cable) lists current-carrying capacities for different installation methods. Where the cable passes through or is surrounded by loft insulation, its current-carrying capacity is significantly reduced and a larger cross-section may be needed.
As a general guide for a typical domestic installation with cable clipped direct and no significant derating: a 6 kW boiler (approximately 26 A) can use 6 mm2 T&E; a 9 kW boiler (approximately 39 A) may need 10 mm2 T&E depending on the route. If in doubt, calculate the total route and apply the relevant derating factors, or ask a registered electrician to confirm before you buy cable.
3. Fit an RCBO or RCD-protected MCB at the consumer unit
All circuits in a domestic property require 30 mA RCD protection under BS 7671 (18th Edition, Amendment 2, 2022). The neatest solution is an RCBO (a combined MCB and RCD in a single unit) sized to the boiler's rated current, fitted in the consumer unit. Alternatively, use a standard MCB on an RCD-protected way.
Switch off the main isolator at the consumer unit before opening it. Never work inside a consumer unit with it energised. If the consumer unit is full, a second consumer unit or distribution board may be needed -- see the separate guide on adding a second consumer unit.
Label the new way clearly with a durable label: "Electric boiler" with the circuit size is sufficient.
4. Run the cable from the consumer unit to the boiler location
Plan the cable route carefully before cutting any clips or chasing any plaster. Cable must run in the prescribed zones under BS 7671 Regulation 522.6.6: within 150 mm of a ceiling or wall corner, or vertically between an accessory and the ceiling or floor level. Outside these zones, the cable must be protected by earthed metal conduit or an equivalent mechanical protection.
Fix the cable with appropriate cable clips at 250 mm intervals on horizontal runs and 400 mm on vertical runs. Where the cable passes through joists, drill centrally to avoid the stressed zone near the top of the joist. Where it passes through a firebreak wall or floor, fill the hole with intumescent putty or cable transit material.
5. Wire the local double-pole isolator
BS 7671 regulation 537.3 requires local isolation for every item of equipment. For an electric boiler, fit a 45 A or 50 A double-pole switched fused connection unit (FCU) or a dedicated double-pole isolator within sight of the boiler -- typically on the wall immediately adjacent to it.
Connect the incoming supply to the line-in and neutral-in terminals. Connect the outgoing conductors (to the boiler) to line-out and neutral-out. Connect the earth to the earth terminal on the FCU or back box. The FCU fuse element is not needed on a dedicated circuit -- use a fuse rated to match the boiler's current, or replace the FCU with a plain double-pole isolator if the consumer unit already provides the overcurrent protection.
6. Connect to the boiler terminal block
Isolate at the local switch and confirm dead. Remove the access panel on the boiler to expose the terminal block. Follow the wiring diagram in the manufacturer's installation manual exactly -- the diagram is specific to your model and must take precedence over any general guidance.
Most domestic electric flow boilers accept a three-core supply: brown to the line terminal (marked L or with a red dot), blue to neutral (marked N), and green-and-yellow to earth (marked with the earth symbol). Tighten to the torque value specified in the manual. Spring-cage terminals do not require a torque tool but should be fully inserted. Replace the access panel and secure all fixings.
7. Test, certify, and notify Building Control
Before switching on, carry out the required tests under BS 7671 Chapter 61: insulation resistance between line and earth, and between neutral and earth (minimum 1 MΩ); continuity of the circuit protective conductor; and polarity. Then test the RCD or RCBO at the consumer unit with a loop tester to confirm it trips within the required time at 30 mA.
This circuit is notifiable work under Part P. If you are a registered electrician, issue an Electrical Installation Certificate (EIC) and notify via your competent person scheme. If you are a homeowner carrying out your own work, notify your local Building Control authority before work begins, and arrange their inspection and completion certificate on finishing.
When to call us
Richard is NICEIC-registered and can wire, certify, and notify the supply for an electric boiler across Sandwich and east Kent. If the job also involves moving the consumer unit or upgrading the main fuse, he can quote for the full scope at once.
Electric boiler supply in Sandwich or east Kent?
Richard wires dedicated circuits for electric boilers, air source heat pumps, and other high-demand appliances. NICEIC registered, all work certified.
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