Is there anything that says an MCB rating for a ring main has to be 32
amps, could it be 16 amps?
I want to put my kitchen on its own circuit but the MCB's are no longer
made by Crabtree but I have two unused 16 amp MCB's.
If I use the 32 amp that is currently the downstairs ring main for the
kitchen then put the front room/hall on one 16 amp ring and the living
room on another 16 amp ring would I run into problems?
Probably not, obvoiusly the max current available for each circuit would
be lower. You wouldn't have to use rings, 2.5mm on 16A would be
accepyable for a radial circuit.
What you must not do is attach one end of the ring to one MCB and the
other end of the same ring to the other MCB...
Are you are refering to the Crabtree MCBs that came before the plug in type?
ie is it a screw the fastens the MCB to the bus bar?
If so then I was able to buy a 40 amp one from Newey and Eyres last week. I
did have to order it in but it did arrive the next day. I am also sure I
will have an old type 32 amp MCB knocking about in the garage, it will be
guaranteed second hand but I will post it to you if you want.
It is a plugin type. The current starbreakers have exactly the same
type if fitting and are interchangeable but the newer ones stick out
more and are supposed to foul the front cover slightly. I may just try
one and see. The cover has a lip that I could file down a bit if the
Good idea to have a seperate final ring circuit in the kitchen.
Most sparks I know who do this would use 4m T&E as well.
If you have the kettle on, switch the toaster on while the kettle boils
then warn the beans up in a Micro wave, what happens ?
Didn't trip, Umm, now the fridge kicks in and it does trip!!
Is it worth the cost of a 32 Amp MCB ?
Who is going to Insect and Test this for your Part P certificate?
I would probably fail it on the grounds of the MCB being too small to
take a kitchen load by exposing the MCB to too much in overcurrent
operations. What if the MCB failed in the ON position, Presto - No
protection and you would not know - hence the need for Part P.
Then, perhaps the Kettle is on the cooker switch and all is fine.
Serves you right for putting the fridge on the socket circuit, then.
Fridges better on their own non-RCD circuit (except TT installs,
obvoiusly). But that's usually too much work for a "pro" sparks.
Anyway, kettle = 2kW, toaster = 2kW, m/wave = 1kw
total = 7kW, within loading of 32A ring.
If kettle 3kW then a short term overload, which should be well within
the parameters of the ring if the cable and MCB are correctly sized
Exactly how does Part P make any difference to the IEE wiring regs and
the BS for circuit breakers?
If you think having kettles on cooker switches is fine I'm not letting
you near my house.
I will put the existing 32 amp on the kitchen. That would leave two
unused 16 amp MCB's, one for the front room/hall and one for the living
room. If I could get an old type plugin 32 amp I would but tracking one
down is the problem.
16th ed IEE wiring regs is very lean on requirements for ring circuits.
Most of the stuff in earlier editions has been taken out and appears
(if it does) in the IEE On-Site Guide - mostly in Appendix 8. There's
some relevant additional info in Appendix 1 (diversity).
Appendix 8 implies that ring final circuits should be protected at 30A
or 32A & be run using cable rated >= 20A - ie >=2.5mmsq if in FTE.
Possibly there is a further implication in Appendix 8, that if you use
BS1363 socket outlets (the std uk flat pin type) then any ring they are
inserted in must comply table 8A - ie for a ring circuit you need
Frankly a false economy. Use some of the money you are saving by DIY
to do a proper job with fittings that will last. Invest in a new CU if
you possibly can.
Although there's nothing wrong with the OP's idea of using the one
available 32 A MCB for the kitchen ring and splitting the other ring
into two radial circuits, assuming each circuit covers <50 m^2 floor
area. However, these should be 20 A circuits, not 16 A. With 16 A MCBs
they would be non-standard circuits and could be seen as under-rated for
general household loading - likely to be overloaded if the CH breaks
down and all those old electric heaters come out, for example.
...but unless I have very much misunderstood the original post, the
suggestion was to break the circuit into 2 separate 16A rings.
As you suggest breaking it into 2 radials is perfectly OK (or even just
making it into 1 20A radial) & might make the OP's MCB hunt easier as
finding a 20A MCB would also put him back in business.
How does the risk of reusing old MCBs untested rate versus installing a
brand new reputable make CU with only basic continuity testing ? My
money would go on a the new CU as the safer bet.
If the OP is reusing old second hand MCBs then perhaps he should
consider testing them properly.
Pass current through it and see if it trips. Use a variac and a
low-voltage high current transformer, and wind the current up slowly.
Not really necessary though, since (IME) they never fail, unlike RCDs.
I remember testing in that manner, (after it had twice failed to trip,
resulting in the 100A incoming fuse (and its replacement) blowing.) a 6A MCB.
Everything was in spec, It just wasn't up to the (particularly high) PFC
present on that circuit. I've been a little wary of MCBs since.
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