How many large MCBs in a CU

[diy_dude]

by large a mean the Amp value. I can easilly see what can physically fit in.

For example, could I fit my split load board with 10 x 40A MCBs? even though the RCD is 63A, the main incommer is 100A and my supply fuse is 100A.

The above is hypothetical, but my current CU is already oversubscribed, ie it has 3x 32A MCBs 1x 40A MCB several 6A MCBs.

 

[Chri5]

On design for a new build, a re-wire or additional circuits you have to considered the maximum demand, the service head size, the CU's capability and diversity.

400A of demand on a peak (all on) would blow a 100A service fuse, it would likely heat and melt something in the CU if the service fuse took a while to go.

Some old boards limit the board to 1 x 40A MCB, most will cope with a couple of high rated MCB's- none that I know of are designed for the load levels you question mentions.

 

[diy_dude]

The hypothetical load of 10 x 40A was just really to illustrate my point.


the example below that is what I currently have:
RCD side
40A shower
32A upstairs sockets
32A downstairs sockets
32A kitchen sockets

non-RCD side
32A cooker
16A shed CU
6A upstairs lights
6A downstairs lights
6A central heating
6A intruder alarm

Is this too much for my CU, its a SquareD Dome CU. I think its a 14 way, but with the isolator and RCD installed there are 10 available for use.

 

[Click-Sure]

You really need to contact Schneider Electric on 0870 608 8 608, they make Square D boards and will be able to advise.

Normal practice is to install highest rated MCB's closest to the main switch or RCCB. If the MCB is to be continuously (more than an hour at a time) run at a level close to its rated current then air gaps should be left around it to let it breathe.

 

[JohnD]

You really need to work out what the actual peak load will be. Whether the house has a ring per floor (like mine) or just one (like some older installations), the people will be using about the same number of appliances. Only a shower or immersion run at full load for sustained periods, electric cookers run at full load only briefly on Christmas day as they warm up. in most houses other circuits run way under capacity. Hopefully the residents will be using CFLs so lighting loads are trivial.

For example my house has three 32A rings (one per floor), two 32A kitchen radials, three 6A lighting circuits, a 16A Immersion circuit, a 6A alarm circuit and a 20A outdoor sockets and garage circuit. However average usage over the year is 20kWh per day... less than 1 kWh per hour. CH is by gas but the tumble drier uses a lot. The immersion is only used if the boiler is out of action.