I am in the process of renovating a house and intend to re-wire the whole
building & install a new consumer unit. Fixed appliances likely to be
installed include an electric cooker, boiler, water softener, waste
disposal, central heating pump, shower pump, fire alarms, security sensors.
There will also be an outside socket in the garden shed.
The house has three floors and an extension. I want to choose a CU that
gives me enough MCB/RCBO slots for all the circuits, plus some spare for the
I suspect that my two areas of ignorance (below) have been discussed at
length on this newsgroup, although I can't find any information at
http://www.diyfaq.org.uk/electrical.html . So I wonder if someone could
kindly point me to threads or website links that cover the following:
A. The typical electrical circuits to be found in a house (e.g. 2 x ring
mains, 2 x lighting, 1 x cooker etc...). I want to find a list of appliances
that would have their own circuit protection.
B. The relative merits of RCDs and RCBOs.
Thanks in advance for your assistance.
Technically, you have a maximum of 100m2 for a ring circuit. However, it is
better to anticipate the likely load and use this as maximum. In particular,
a well stocked kitchen often has more equipment than should be installed on
a single ring main, especially if, like me, you cycle the machines all
simultaneously whilst cooking.
As an example, my house has the following circuits installed into a 16
outgoing way Contactum CU. TN-S earthing, 100A DP switch incomer. (60A
cutout). RCBOs are double width, as they were the cheapest Type B RCBOs I
1. B32A/0.03 RCBO. Ring main for entire house except kitchen. (May split
into two RCBO circuits in future when loft conversion built).
2. B32A/0.03 RCBO. Ring main for kitchen. (including 2.2kW dual fuel cooker)
3. B32A/0.03 RCBO. External electrics (outside socket, outside lighting,
future shed supply)
4. B32A MCB. Radial circuit for kitchen fixed integrated appliances.
(dishwasher, washing machine, dryer) remotely switched sockets concealed
5. B16A MCB. Radial circuit for fridge/freezer only. Reduces risk of
defrosting, as only a fault on that circuit, or the main service fuse will
cut the power.
6. B16A MCB. Heating system. (Immersion, central heating, heat bank).
7. B6A MCB. Most of house lighting circuit.
8. B6A MCB. New lighting circuit (kitchen only currently)
9. B6A MCB. Alarms (Interlinked smoke/heat detectors + intruder alarm)
If I had an electric hob, there would be a separate B32A MCB circuit for it.
When I moved in, the house had 3 operational circuits, a single ring main, a
single lighting circuit and a cooker circuit used solely to power the skanky
fridge freezer they left behind, as only 2 double sockets were installed in
the entire kitchen.
If designing from scratch, rather than upgrading the previously stingy, but
safe and well executed system, I would use at least two lighting circuits.
It is traditional to do upstairs/downstairs split. However, I would prefer
to randomise the circuits, so in the event of the MCB popping, a nearby
light will work. It does have the disadvantage that someone might assume the
upstairs/downstairs thing and work on a live circuit, though. Quite frankly,
if you make assumptions like those, a quick jolt could help you change your
I would also split the rest of the house ring main into two. However, there
is no real reason to change my preexisting circuit. It comes well within the
100m2 requirement. Loading is likely to be very light, with no fixed
appliances or heating loads expected. The major requirement is during a gas
failure, where I might use up to around 5kW of electric heating. My entire
house heating requirement at -3C outside is only 8kW, so it would be
difficult to blow the ring using thermostatically controlled heaters anyway.
30mA RCD (whole house) = BAD!!!
30mA RCD (split load) = Much cheaper than RCBOs
30mA RCBO = Much less prone to nuisance tripping. Tripping has fewer
consequences. Warm feeling of having done it right.
Obviously, if a TT earthing system, the 100A DP incomer switch should be
exchanged for a 100A/100mA time delay RCD. The use of split load RCD/RCBO
would remain exactly the same, though. If I had such a system, I would also
consider using an RCBO on the kitchen radial, to prevent any earth leakage
in the kitchen fixed appliance heating elements taking out my lighting and
As for your list of appliances:
electric cooker = Own dedicated circuit on B32A MCB. Do not install cooker
unit with built in socket. They look bad and trail leads over the cooker.
boiler = put on a dedicated central heating circuit MCB (not RCBO). Can
share with immersion, especially if sharing controls, such as programmers.
All central heating valves, programmers and other controls should be from
the same central heating circuit. The CH side (as opposed to the immersion)
should be fused down to 3A, either internally in the boiler or in a separate
FCU. If there is no immersion on this circuit, use a B3A MCB instead, and a
DP switch to isolate the entire circuit next to the boiler.
water softener = 3A FCU off nearby ring main/kitchen fixed appliance radial.
Some softeners don't require electrical supply.
waste disposal = 3A FCU (or as advised in installation instructions) off
nearby ring main/kitchen fixed appliance radial
central heating pump = off same circuit as boiler. Will be internal to
boiler, or connected directly to it in a modern system.
fire alarms = either wire from a regularly used lighting circuit, or give
own small radial circuit (i.e. B6A or B3A MCB).
outside socket in the garden shed = separate RCBO circuit only shared with
other outside electrics. DO NOT spur from internal socket circuits. DO NOT
share the RCD/RCBO with any other circuit or appliances. The outside socket
should be off a 13A DP switched FCU rather than directly on the radial. This
enables you to turn it off inside, preventing nuisance RCD trips and
Mine's pretty identical to yours. House was originally rewired
around 1970, although the original Wylex CU was probably older.
Cabling still perfect, but sockets were crap quality and worn out.
Completely rewired kitchen and bathroom during refit and brought
service and bathroom bonding up to spec and fitted new CU. Most
of the ground floor lighting has been rewired as a side effect
of bringing it under home automation control.
These two I have on one circuit, but not RCD protected.
I would suggest using C6A for lighting circuits. It may reduce
the chance of a filament lamp tripping the MCB. BS1361 cartridge
fuse is another option (probably a better option).
I put the circuit in although I don't ever intend having an
electric hob myself. For now, it has an unswitched 13A socket
on the end with the gas hob spark generator plugged into it,
and a 45A cooker switch ;-)
Strangely, the 1970 rewire did install two ring circuits, but they
were joined into one big ring in the CU, which also makes me think
the CU was older and not changed at that time.
and doesn't conform to the regs
One disadvantage of most RCBO's is they give no clue what tripped
them, earth leakage or current draw. There were some types where
the dolly only moved halfway to the off position when the RCD part
caused the trip, but I haven't seen them for a long time now.
...and the cooker doesn't want RCD protection, but all the worktop
sockets do, which is incompatible with a combined socket.
Moreover, all the central heating controls should all be switched off
by the one boiler isolation switch.
There are some rules (which I don't remember, and didn't seem very
sensible to me) about sharing smoke detectors with lighting circuits,
and relate to the type of battery backup the smoke detectors have.
In my case, the smoke detectors run off the alarm anyway, and use
the alarm battery in the event of a mains failure.
I'm not worried. I do not possess a filament light bulb in the house, except
in my Maglite. I prefer MCBs to fuses whenever possible, as it is easier to
reset at 2am, especially when you've got no spares.
Mine is similar, but not quite. The ring went down from the old CU, round
the ground floor and then up, past the CU to the upstairs and back down
again. I can split it simply by cutting the cable and crimping an extension.
I'm just too tight to buy another RCBO until I have to!
"Andrew Gabriel" wrote
| > fire alarms = either wire from a regularly used lighting circuit,
| > or give own small radial circuit (i.e. B6A or B3A MCB).
| There are some rules (which I don't remember, and didn't seem very
| sensible to me) about sharing smoke detectors with lighting circuits,
| and relate to the type of battery backup the smoke detectors have.
| In my case, the smoke detectors run off the alarm anyway, and use
| the alarm battery in the event of a mains failure.
IIRC if you use a lighting circuit (where you will probably notice a failure
fairly quickly) you have to have battery-back-up smoke detectors, and if
you use a dedicated circuit (which would go faulty for weeks without being
noticed) you don't have to have battery-back-up.
 Although some have a whopping big capacitor these days, I think
Thanks for such a comprehensive reply.
Based on your excellent response and a bit of background reading, I have
pencilled in the following circuits (and protection) for my house:
Ring Main A ...32A RCBO
G/F Kitchen (incl. fridge/freezer)
Ring Main B ...32A RCBO
G/F Dining Room, Lounge, Hall
Ring Main C ...32A RCBO
1/F Bedrooms, Bathroom, Landing
Ring Main D ...32A RCBO
Extension: Playroom, Utilities (washer/dryer), Study
Radial A ...6A MCB
Lights on east side of house
Radial B ...6A MCB
Lights on west side of house
Radial C ...6A MCB
Lights in extension
Radial D ...32A MCB
Radial E ...16A RCBO
Immersion Heater/boiler controls/pumps
Radial F ...6A MCB
Radial G ...20A RCBO
Garden sockets & lights
Assuming that the RCBOs are double width, I guess that I need at least a
17-way consumer unit. Did you put your RCBOs side-by-side? I read somewhere
on this NG that a RCBO can "trip" an adjacent RCBO.
The garden lights must be switched on/off from the house. The shed
sockets/lights must be switched on/off in the shed. Is it still OK to run
the whole lot off one circuit ("Radial G", above)?
My earthing system is TN-S. I guess you'd advise that I would be better
served by a switch than a 100A/100mA time delay RCD at the incomer.
My thanks again for your help.
Absolutely do not put your fridge/freezer on an RCBO circuit, unless you
like coming home from holiday to a rotting stinking mess. It is better to
put it on its own circuit, or at least shared on another MCB only circuit.
You can always cut out one of your lighting circuits to make room.
OK. Although containing high current devices, having just a washing and a
dryer on the circuit shouldn't cause any overload.
Might be overkill, could probably combined with another lighting circuit.
Use an MCB, rather than an RCBO. Although an RCBO is less prone to nuisance
trips than a shared RCD, there's still a chance of a spurious pop when you
are on your winter skiing break, leading to your house freezing, possibly
You can put them side by side, no problem. When populating the consumer
unit, it is best to order as reducing ratings, starting with the heaviest
circuits next to the incomer. Remember that with an RCBO installation, you
want a "standard" isolator incomer (except for TT), rather than a fancy
split load unit.
Absolutely. Consider having a DP switch (or even FCU) on the run to the
shed. Then you can ensure that everything is turned off without having to
brave the weather.
So, with these modifications, you get:
1. B32A RCBO - Kitchen ring
2. B32A RCBO - Reception ring
3. B32A RCBO - Upstairs ring
4. B32A RCBO - Extension/utility ring
5. B32A MCB - Cooker
6. B20A RCBO - Outside
7. B16A MCB - Heating
8. B16A MCB - Fridge/Freezer
9. B6A MCB - Lighting A
10. B6A MCB - Lighting B
11. B6A MCB - Alarm
I make this 16 outgoing ways using double width RCBOs, or 11 outgoing ways
using single width RCBOs. You've probably got some overkill on the ring
mains and could combine some, too. For example, you might swap the Utility
RCBO for a 32A MCB for just the appliances and use the Reception ring RCBO
to cover the sockets in the extension. This would save you an RCBO (and give
you an extra way if using double width units). It also removes likely earth
leakage culprits (i.e. the washing machine and tumbler) from the general
purpose ring main.
Hager do single module RCBO's in a double pole flavour. Cant remember
what the numbers were off the top off my head, I'm thinking AD010B but
don't rely on that as correct!
Bear in mind you want some spare capacity, around 20-30% I usaually
If you were to use Hager then a 14 way board would be ample.
Could you use a single pole RCBO though to protect an outdoor circuit (or
any internal socket close to the back door to the garden)?
Looking though the catalogues, there is a much wider range of single pole
(solid neutral) RCBOs than there is for double pole RCBOs.
A DP is nice, as it can act as an isolator for the circuit. Particularly
useful for maintainance, or if a neutral-earth short has caused the trip on
that circuit (will prevent other RCBOs tripping too). However, there is no
requirement, AIUI. After all, an MCB is single pole.
No, it could affect an RCBO at the same level.
Consider two SP RCBO circuits in a CU, called A and B.
There is a neutral-earth short on circuit A.
A high current device on A draws some current. A small proportion of the
return current from this goes through the earth fault rather than the
neutral, tripping the RCBO. All well and good, exactly what you want.
However, now a device on circuit B draws some current. As RCBO A has only
cut the live, the neutral current from B could go into the A circuit neutral
and through the same neutral-earth short, causing B to trip also, even
though the earth fault is on a different circuit.
Had A been fitted with a DP RCBO, this neutral would have been cut, so B
wouldn't trip. It is possible that B would trip first whilst A is still
operating. However, once A had tripped or been manually pulled, B would not
repeat the experience.
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