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Can a 3 kW immersion heater be connected to a 13 A fused connection unit to BS 1363-4?

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A question that is often asked is whether a 3 kW immersion heater can be connected via a BS 1363-4 fused connection unit (FCU) or switched fused connection unit (SFCU), or perhaps, a plug and socket-outlet combination to BS 1363-1 and BS 1363-2. How the answer to this question is determined is not quite as simple as it would, at first, appear. 

3 kW immersion heater, rated at 240 V

Most 3 kW immersion heaters sold in the UK at the present time are rated 3 kW at a nominal voltage of 240 V. Load currents for calculations, according to BS 7671, should be determined at a nominal voltage of 230 V, so we cannot simply use the power formula to calculate the load current. Instead, we calculate the resistance of the heating element by combining Ohm’s law with the power formula for the rated nominal voltage and power:

If we substitute this value of I in the power formula, we can get a value of resistance in terms of the rated power and rated nominal voltage:

where, in the above:

U is the nominal utilization voltage
I is the load current
R is the resistance of the heating element
P is the rated power

Therefore, for an immersion heater element rated 3 kW at 240 V:

To calculate the current at a nominal voltage of 230 V, we use Ohm’s law again, but this time using U = 230 V:

11.98 A is clearly below the rating of a 13 A fuse and a 13 A connection unit; however, the following should also be considered:

  • Immersion heaters may draw their load current for between 30 and 120 minutes at a time. 
  • Supply voltage variations may increase the current by up to an additional 10 % (bringing it well above 13 A). The designated product standard for immersion heaters, BS EN 60335-2-73, which in turn refers to BS EN 60335-1 for marking and tolerances of rated power or current, also permits a tolerance on acceptable load current. It can be shown that it is highly likely that load currents exceeding 13 A will be observed across a range of installations.
  • Clause 4 of BS 1363-4:2023 (see Figure 1) states the conditions in which the connection unit to the standard should be used; in ambient temperatures of −5 °C to +40 °C, where the average value over 24 hours is not exceeding 25 °C, and in situations not subject to exposure to direct radiation from the sun or other sources of heat likely to raise temperatures above these limits. BS 1363-1:2023 and BS 1363-2:2023 contain the same provision.
  • Regulation 134.1.5 of BS 7671 states that “electrical equipment shall be installed in such a manner that the design temperatures are not exceeded.”

It is therefore clear that FCUs or SFCUs meeting only the basic requirements of BS 1363-4, and plugs and socket-outlets to BS 1363-1 and BS 1363-2, are unlikely to be designed to operate at its rated current if located in an airing cupboard or small room containing a hot water cylinder, because of the increased ambient temperature for long periods.

Figure 1 Clause 4 of BS 1363-4:2023

3 kW immersion heater, rated at 230 V

So, what is the rated current of a 3 kW immersion heater at a nominal voltage of 230 V?
If the element is rated 3 kW at 230 V, we can simply rearrange the power formula:

where:

P is the rated power
U is the nominal utilization voltage
I is the load current

This clearly exceeds 13 A. Regulation 433.1 of BS 7671 states that circuits shall be designed so that small overloads of long duration are unlikely to occur. In addition, Regulation 134.1.5 of BS 7671 states that “electrical equipment shall be installed in such a manner that the design temperatures are not exceeded.” Therefore, a heating element rated 3 kW at 230 V is generally unsuitable for connection via an overcurrent protective device with a nominal rating of 13 A (including FCUs and SFCUs).

What could be the result of connecting 3 kW immersion heaters via BS 1363 fused connection units, or plug and socket-outlet combinations?

Whilst designs can vary, it is often the case that fuses, switches and plug/socket-outlet contacts in BS 1363 products are a source of heat when current is passing through the product. BS 1362 requires that fuses to the standard do not dissipate more than 1 W when conducting their rated current. In type tests for plugs and fused connection units to the BS 1363 series, a ‘calibrated link’ that dissipates 1 W is used for temperature rise tests.

It should therefore not come as a surprise that there is a possibility of components, such as fuses, fuse-holders/covers and switches, overheating over time, where 3 kW immersion heaters are supplied by SFCUs to BS 1363-4, or plug and socket-outlet combinations to BS 1363-1 and BS 1363-2. High ambient temperatures and loose connections can also lead to the same outcome of excessive heating at terminals. 

Conclusion

Immersion heaters rated 3 kW at 240 V are generally unsuitable for connection via a connection unit to BS 1363-4, unless the manufacturer of the connection unit confirms otherwise. They are also generally unsuitable for connection via a plug and socket-outlet combination to BS 1363-1 and BS 1363-2.

Immersion heaters rated 3 kW, at a nominal voltage of 230 V, are unsuitable for connection via a protective device with a nominal current rating less than 15 A. They should therefore not be connected via plugs and socket-outlets, or connection units, to BS 1363 series, as this presents a likely risk of fire or overheating.

It is recommended that, unless the manufacturer of a fused connection unit confirms otherwise:

  • In general, the maximum domestic appliance loads that use full power for periods above 30 minutes, connected to BS 1363 plugs and fused connection units, is limited to 2.7 kW (11.7 A).

NOTE: Some appliances supplied with a BS 1363 plug, such as tumble dryers and kettles, can be rated above 2.7 kW; however, these appliances only use their full rated power for relatively short periods of time.

  • Fused connection units to BS 1363-4, and socket-outlets to BS 1363-2, are not located where they will be subject to ambient temperatures outside the range −5 °C to +40 °C, the average value over 24 hours not exceeding 25 °C, or in locations with sources of thermal radiation that might cause these limits to be exceeded. See Clause 4 of BS 1363-4.

Immersion heaters should be supplied via an appropriately-rated double-pole switch (for example, 20 A for a 3 kW immersion heater) within easy reach of the heater, and a flexible cable outlet (integral with or separate to the switch). The immersion heater switch is generally used for the function of isolation when replacing the heater element or thermostat, therefore, the switch should conform to BS EN 60669-2-4 and be marked with the symbol illustrated in Figure 2.

Immersion heaters fitted to storage vessels in excess of 15 litres capacity should be installed on their own final circuit. Regulation 554.2.1 of BS 7671 also states: “Every heater for liquid or other substance shall incorporate or be provided with an automatic device to prevent dangerous rise in temperature.”

Figure 2 Example of marking on switches to BS EN 60669-2-4

Acknowledgements

  • BEAMA Ltd
  • Mark Coles
  • Jonathan Elliott
  • Gary Gundry
  • Calum Mansell
  • Michael Peace.

References

  1. BS 1363-1:2023 13 A plugs, socket-outlets, adaptors and connection units. Part 1: Rewireable and non-rewireable 13 A fused plugs – Specification
  2. BS 1363-2:2023 13 A plugs, socket-outlets, adaptors and connection units. Part 2: 13 A switched and unswitched socket-outlets – Specification
  3. BS 1363-4:2023 13 A plugs, socket-outlets, adaptors and connection units. Part 4: 13 A fused connection units: switched and unswitched – Specification
  4. BS 7671:2018+A2:2022+Corrigendum:2023+A3:2024 Requirements for Electrical Installations. IET Wiring Regulations 18th Edition
    BS EN 60335-1:2012+A11:2014+A13:2017+A1:2019+A14:2019+A2:2019 Household and similar electrical appliances - Safety Part 1: General requirements
  5. BS EN 60335-2-73:2003+A11:2021 Household and similar electrical appliances - Safety. Part 2-73: Particular requirements for fixed immersion heaters
  6. BS EN 60669-2-4:2005 Switches for household and similar fixed electrical installations – Part 2-4: Particular requirements – Isolating switches