In uk.d-i-y, mmzz wrote:
i need to install an in line electric water heater
i have a choice of 7kw or 8 - 9.6kw
whichever i choose i may overload my trip switch
is there any way to reduce the current.
obviously this will reduce the heating effect but that is not a problem.
the problem is I may need to keep the current below the level of my trip

With that load you need a dedicated circuit from your consumer unit/
distribution board. The 7kW needs just under 30A, and 4mm cable unless
it's a "long" run (say over 15m) or runs buried in thermal insulation -
then you'll need 6mmsq minimum; the 9.6kw will pull 40A and would be
better wired in 10mmsq (bit of a bummer to work with). No sensible easy way
to reduce this load: water needs a lot of energy to heat it! If you
don't need much temperature rise, try to find a 3kW inline heater instead.

HTH - Stefek