"Tim Watts" wrote in message
...
TheScullster
wibbled on Friday 23 April 2010 15:02

Hi all

I am looking at the options for reconfiguring a Clipsal CU with Hager
MCBs
and RCD.
Currently the box is split with 7 non-RCD circuits and 3 power circuits
protected by one 63A RCD.

To comply with the new regs (17th) I was considering moving the RCD 2
units to the right, to make room for the cooker and kitchen lights on the
RCD side of the box.

So here come the questions:

Is moving the cooker MCB well away from the incomer OK - traditionally
the
cooker tends to be the first "user".

I've heard of people who like to put it near the incomer, but if you look
at
the size (CSA) of the busbar, it think it is irrelevant.

It certainly mattered on the old Wylex fuse boxes with their small cross
sectional area bus bars. On a new CU the bus bar is quite substantial. Any
voltage drop (shown by the diming of lights etc) when a large load it used
is down the the impedance of the supply cable not due to a voltage drop
across the bus bar.

I know someone who had a 9kW water heater that caused a voltage drop when it
was powered up. He was able to prove that it was not voltage drop on the CU
bus bar:-)

There is a case for spacing out RCBOs (I have big current, small, small,
gap
where possible) as RCBOs do heat up a bit more than MCBs in theory - not
seen any proof, but if possible, it does no harm.


With 3 power circuits a cooker feed (all 32A rated) and a lighting feed,
do I need to uprate the RCD?

I'll refrain on that one as it will involve digging out teh OnSite Guide
and
doing a diversity calc and I'm only in for an orange juice between
clearing
the garden :_)

Page 96 of OSG. It is important to ensure that distribution boards or
consumer units are of sufficient rating to take the total load connected to
them without the application of any diversity.

What a load of cock. If I pugged everything I owned in at once, switched on
the electric shower, the tumble dryer, washing mashine and dish washer it
would in theory blow the main fuse.

Common sense has to be applied.


The power circuits are upstairs ring, downstairs ring and kitchen in 4
bed
house - nothing too far out of the ordinary.

The worst case would be a boiler failure and you used electric heaters as a
temporary backup. As you have 3 ring main circuits you could plug in more
heaters around the house than most people without tripping a MCB and
possibly overloading the 63A RCCB.

I have no idea what happens if you shove 70A through a 63A RCCB. My best
guess is nothing would happen.

Adam