Everything you need to know about installing an electric boiler: types, correct sizing, BS 7671 circuit requirements, Part P notification, and realistic costs for UK properties.
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Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.
It depends on the boiler kW rating and supply. As a guide, a 9kW single-phase electric boiler draws around 39A and is commonly wired in 10mm² twin and earth on a 40A breaker; larger 12kW+ units (50A+) often need a higher-rated dedicated circuit or a three-phase supply. Always size from the design current and apply correction factors (BS 7671 Reg 433.1.1), on a dedicated circuit with 30mA RCD protection.
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Electric boilers are available in several configurations, each suited to different property types and heating requirements. Understanding the differences helps electricians specify the right product and size the electrical supply correctly.
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Correct sizing is critical for both comfort and efficiency. An undersized electric boiler will fail to maintain design temperatures during cold weather. An oversized boiler wastes capital and may draw more current than the electrical installation can support.
These figures are indicative only. The correct approach is a full heat loss calculation per BS EN 12831 for the specific property, accounting for wall U-values, window area, floor and roof insulation, and local design temperature (typically -3\u00b0C for most of England, colder for Scotland and upland areas).
For properties with existing radiators sized for a gas boiler running at 70\u00b0C flow temperature, the radiator system should cope well with an electric boiler. If upgrading to a heat pump at a later date, radiators sized for 55\u00b0C or lower may be needed.
Electric boiler installation is one of the more demanding domestic electrical jobs in terms of current-carrying requirements. The dedicated radial circuit must be sized correctly and installed in accordance with BS 7671:2018+A4:2026.
Above figures assume installation method C (clipped direct, single circuit, no grouping derating) at an ambient temperature of 30\u00b0C. Adjust for actual installation conditions per BS 7671 Appendix 4.
The installation of an electric boiler in a dwelling involves adding a new circuit and is therefore notifiable work under Part P of the Building Regulations in England. The same principle applies in Wales. Scotland uses a different system (Building Standards) and Northern Ireland has its own regulations.
Failure to notify can result in difficulty selling the property, as solicitors routinely request evidence of Part P compliance for electrical work done since 2005. A retrospective inspection can be arranged but costs more than the original notification.
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Installation costs depend on the boiler output, type, distance from the consumer unit, whether an existing cylinder is being retained or replaced, and whether the consumer unit needs upgrading.
The choice between electric and gas is not always straightforward. Electric boilers have advantages that are often overlooked when comparing headline fuel costs.
Every electric boiler installation in a dwelling requires an Electrical Installation Certificate. Where the consumer unit is also upgraded, this must be included in the scope of the EIC. Elec-Mate provides all the certification tools needed for boiler installation jobs.
Under BS 7671:2018+A4:2026 (Regulation 644.3) and GN3, an Electrical Installation Certificate must be accompanied by Schedules of Circuit Details and Schedules of Test Results. For a new boiler circuit the following tests are mandatory:
The Schedule of Test Results shall be given to the person ordering the work together with the signed EIC (OSG Reg 9.1, GN3 Reg 1.3).
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