I am looking to replace my consumer unit with a split load Wylex 12
way Consumer unit, since i have run out of ways on my exisiting
consumer unit. At the same time I want to run a spur to a shed for a
small ring main and lights. The shed will have my old consumer unit
(Which has a 63A/30mA Incomer providing RCD and Isolator
Is it acceptable/possible to connect the spur to the shed via an RCBO,
or a second dual pole isolator or should I just use a standard MCB?
Also what rating would be suitable for running a spur to a shed, would
a 16A MCB/RCBO be suitable? (Internaly I was going to use 2.5mm t&e
ring main cable, while externaly I was going to use 2.5mm armoured
My worries are that if there are problems on the external wiring for
any reason, then there are safety issues by just connecting it via a
As a slightly seperate point does anyone know if it possible to add a
second RCD to a wylex unit, so as to provide aditional seperate RCD
protection for the lighting circuits from the ring mains?
I just used an mcb so the cable to my detached garage is protected.
Should be fine, you could use a higher rated mcb if you required more power
in the shed.
I put my ext supply on a 16A mcb on the non-RCD side
I guess it could depend on how your ext cable is run. Mine is buried under a
concrete path and so I'm unlikely to hit with a garden tool.
Why? Its unusual to RCD protect lighting circuits these days..
Presumably because its very difficult to get a shock from fixed light
fittings unless you try really hard :-)
You don't have to work on them live because with their mcb(s) off you can
provide light from a power circuit.
It should be possible but you may have to cut the provided busbar. Note RCDs
are 2 mcb widths and the Wylex prebuilt kit only has 2 separate spare mcb
positions, so it wouldn't fit anyway without rebuilding the kit in a
Note if you want to get an extra mcb for the Wylex and you want it to use
their busbar, then be sure the new mcb is also Wylex brand.
Not really, if you going to wire it as a ring circuit you can use a 32A mcb and
separate 6A for lights.
Q Have you thought about earthing, as discussed in this group power to
outbuiildings should be RCD protected at the outbuilding itself and use a TT
earthing system ie, earth rod . And if your outbuilding cable run from your
distribution board is getting near to 30M use 4mm sq to allow for v drop
Generally speaking, you should run the outside line via a standard MCB and
not allow an RCD near the thing until it gets to the outbuilding. This way,
when it trips, you won't have a trek to find which RCD has tripped, possibly
taking out part of the house system if the RCD is shared. However, you have
to do the calculations and measurements properly to ensure adequete earth
loop impedence to trip the MCB in the event of an earth fault. If this can't
be achieved (i.e. TT) system, an RCD is needed on the supply end too.
What is the length of the cable? A quick calculation suggests 30m max for
2.5mm on a 16A MCB. It might be better to run a 32A MCB if you are expecting
to run more than the occasional lawnmower. This would require 6mm cable. If
the distance is a little shorter, then 4mm may be allowed.
Don't. It is dangerous to RCD protect lighting, except for TT systems, where
adequete earth loop impedence can't be obtained. In these cases, a time
delayed 100mA type is used, so that an earth fault on a socket circuit
doesn't cut the lights as the socket circuit's 30mA immediate RCD is
guaranteed to trip first.
If you do need to provide separate RCD protection to a circuit, replace the
MCB with an RCBO.
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