In a stadard modern house, what is the maximum current the immersion
cable can carry? The cable is already there and the house is about 5
years old. Can I replace the 3kW immersion with a higher rated
The immersion circuit is frequently run in 2.5mm cable. This would be enough
to run 2 3kW immersion heaters, or a single one up to 6kW.
1. The cable might be run in 1.5mm, which can only run one 3kW heater. The
cable must be positively identified as 2.5mm.
2. Installation method may affect current capability. In particular, it
might not be possible if it is run in insulation at any point.
3. You may find it very difficult to source the required 25A MCB to replace
the 15/16A one (although they are available, such as RS item 455-1215). You
could possibly claim that a 32A MCB would be fine as cable overload is
handled by the characteristics of the load, or by having 2 13A FCUs at the
far end driving two immersions. Someone else might be able to confirm this,
as I'm not entirely sure.
4. You may need to calculate that the voltage drop and earth loop impedence
are still within limits on the length of cable that you have and for the
protective device you decide on at the consumer unit.
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