I'll be re-wiring the house over the next few months and replacing all of
the electrical accessories.
Firstly, what type of switch should I use for the immersion heater and the
central heating timer. A 13A FCU (Screwfix p/n D15777) or 20A DP Switch
(Screwfix p/n D13639)?
Secondly, the existing consumer unit is one of the old bakelite ones which
has probably been here since the house was built in the 50s. I've bought
myself a new split load consumer unit with MCBs. I want to disconnect the
existing wiring from the old CU and connect it to the new CU. This should
make it easier for me to changeover from the old wiring to the new wiring as
I have more ways on the new CU than I need.
My idea was to disconnect the existing wiring from the old CU, connect them
to some terminal strips (e.g. Screwfix p/n 17672) mounted in a sealed box
and then extend the wiring from the terminal strips to the new CU using twin
& earth. Is this viable? If not, could someone recommend another method.
The immersion heater will need a 13A FCU. Are you using one or two switches?
It is normal for two switches to be used (ie a FCU downstairs where the
immersion is turned on/off and then a double pole switch where the 2.5 T&E
changes to heat resistant flex in the airing cupboard). If you are using
just one switch (eg it is in the airing cupboard) then use a 13 amp FCU
I assume your CH timer is conneced to a gas boiler or similar. If so you
need a FCU and a 3 amp fuse. This must isolate all the power to the CH
system not just the timer.
This would work as it is only on a temporary basis
I would not use an FCU -- I would use a 16A MCB protected circuit
with no further fuses. The extra 13A fuse adds nothing safety wise,
but is a potential failure point for poor contacts and overheating.
Use a 20A DP switch adjacent to the immersion heater. Any additional
switches can be single pole, but you'll probably find double pole
ones easier to find.
If you can temporarily run the whole house off the highest current
fuseway in the old fusebox, then use this to temporarily supply the
new CU, and connect all the circuits into the new CU properly. If
you have any high current appliances like an electric shower or
electric hob, this isn't viable unless you leave it disconnected
until the new CU is correctly connected in. You will need to
temporarily interconnect tto the new CU with suitably sized cable;
because of the requirement to derate conductors protected by rewirable
fuses, this will need to be suitable for higher current than the
fuse protecting it.
The earthing in a 1950's installation will be well below current
requirements. Suggest you start by bringing that up to current requirements
directly into the new CU, and don't simply take an earth connection
from the old fuse box.
I agree this a most acceptable method (in fact my local council now use this
on their rewires). I have somewhere in the depths of my confused/mad/pissed
brain that BS 1363-4 is a FCU reg and thought it was a requirement. I
believe either method is OK.
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