Our old Wylex CU has got plug in MCBs, however I'd like to put in an RCD
onto the shower circuit and I don't want to be replacing the CU just yet.
So - can you get plug in RCBOs (MCB and RCD in one) - or am I hoping for one
step too far and just be grateful that I've got plug-in MCBs?
No such luck, I usually fit the appropriate fuse/MCB in the CU and an
inline RCD and enclosure to the shower cable, such as this one.
http://tinyurl.com/23g8b the 2 module one.
http://tinyurl.com/2ym2d CD263 would be what you want.
No chance: an RCBO needs a feed from both the live and the neutral rail,
while a plug-in would only get at the live side. Either fit a dedicated
RCB downstream, or split the feed to the CU with a Henley block and
take your shower and other RCB-needing circuits through a new CU with
an 80A or 63A RCD as its incomer.
You couldn't actually do that, unless your main service fuse was rated
at 80A or 63A. Equipment has to be rated to the next downstream CPD,
in the case of a CU that would be the service fuse.
Easiest way of wiring in and RCD to the shower is the way I said
originally. If you did it your way and put another CU in you may as
well change the one thats there and have only one CU.
And it's RCCB, (residual current circuit breaker, (RCD with mechanical
switch, usually used as main switch on CU's)), or RCD, (residual
current device, (to just be operated under fault conditions)), in this
instance we only need RCD.
Incorrect, the main fuse is NOT your property and may be changed at the wim
of the supply company. Either all the fuses / CBs must add up to less than
the rateing of the consumer unit or fit YOUR own switch fuse. The rateing of
this fuse to match your consumer unit.
Diversity should be applied by the selection of the loads connected to the
diss board, in fact if you examine a wylex consumer unit it states load must
not exceed N amps. The main fuse up stream of the meter is to provide over
load protection to the meter and the suppliers works, overload protection
for your instalation includeing the main swith is up to YOU. Either fit
100A rated consumer uints or fit a suitable switch fuse to provide overload
protection. All power systems must be designed so that no single fault may
I think we're getting a little too involved for what we needed here.
As a rule I always fit CU's with ALL RCD's etc rated at 100A. Unless
you specifically ask for it you wont get a bigger service fuse than
If the REC did decide to change the size of the service fuse they
wouldn't just turn up, pull it out and change it, you would be
informed so you could add a switch fuse before say a 63A was upgraded
to a 100A.
It also depends on who you get from the REC to do any work on the
meter, some will check on the size of tails etc before changing a
service fuse or before upgrading their own length of tails, as I have
experienced in the past when the REC have been to upgrade a supply
from 63/80A to 100A and I have not had the new CU and tails in place.
It is rather difficult to keep the rating of CPD's below the rating of
the DB as even the most basic of installations would have cooker, ring
main, immersion heater and lights. Take this as 2 x 30A + 10A + 5A and
you're already over 63A. Possibly a CU feeding a garage or shed would
manage to come in under this threshold but not one feeding the entire
house, especially if you are going to put a shower on it as the OP was
so this would put it well over the 80A mark then. So the OP's CU would
really want to be rated at 100A.
Surely not. Almost every house in the country has more MCBs/fuses than
incoming capacity. It isn't likely to all be in use at once.
I've got 4x32A + 2x16A + 3x6A = 178A. My incoming is only 60A. I'm unlikely
to use more than 32A at a time, as the house is gas heated and uses gas
cooking. I have no electric shower.
Even when the gas fails, the most I'd use is:
5kW electric heating (3kW fan + 2kW convector)
2kW washing machine
2kW tumble dryer
1kW everything else
Which is a fraction over 65A. Even then, the chances of all the thermostats
in all those appliances coming on simultaneously for long enough the blow
the fuse is tiny. At 65A, it would probably never blow anyway, even if left
The 60A fuse in my house blew recently (before I replaced the CU!). I wasn't
even drawing much power. No faults could be found. No MCB tripped. It
probably just died of old age. Rather naughtily, I just replaced it myself.
I try to keep spares around, rather than relying on the REC to bother
You'd think so, wouldn't you. However, in practice, not only do you frequently
see shower circuits and similar dedicated feeds Henley-blocked off from
the main feed - where you could make a sensible Nature Of The Load argument
which says that there's no way for the one shower to pull a consistent
overload of, say, 60A through switchgear rated only for 45A, making a 100A
mainfuse an adequate protection for short-circuits somewhere in the switchgear
upstream of the MCB/RCBO which provides overload protection; it's also
standard for split-load CUs to have the RCD/RCCB rated for a continual
current of only 63A or 80A, while the main incomer is rated for 100A, and
for these boxes to be regularly and unremarkably connected to installations
where the service fuse is 100A. It's Diversity which makes such practice
reasonable - even with the RCD side of split-load CUs being populated with
a couple of 32A MCBs for rings and a 40A for an electrick shower, the
likelihood of the notional 104A "total" (or the indefinitely-sustainable
120A you could pull if you put enough fanheaters on the rings and kept the
leccy shower going at full pelt) would be unlikely enough in practice to
make it sound engineering practice to use an RCD across all these circuits
whose continuous rating is a "mere" 63A or 80A. Like all uses of diversity,
though, it depends on actual occupancy/usage patterns: as counter-example,
in the case of the youth hostel warden who asked here about installing three
electric showers, it's quite foreseeable there that a group of hostellers
will all turn up soggy, cold, and anxious to dive into the showerroom
pausing only to have their boots cut off their feet by the warden to save
the communal carpet ;-) and it would not be good practice to apply diversity
to those three showers in such a case.
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