Distibution board

[Oscar_Whild]

I am trying to fit a 10.5Kw shower. My current board is old and I can't get new MCB for it. I have just bought a new Crabtree 5 + 5 split load which I want to set up as follows:
Incoming 100A (unprotected) with cooker (32A), Heater (16A), Lighting (6A), Lighting (6A) = 60A.
63A RCCB (protected) with Shower (50A), Sockets (32A), Sockets (32A), Pond (16A) = 130A.
I am worried that the rating of the MCBs is greater than the main switches. Can anyone tell me if this is OK ?

Thanks,
Oscar.

[ZenStalinist]

Yes, It should be okay. But you say incoming is unprotected? You mean just the switch is unprotected? I mean, you have a service fuse rated at 100A in the meter cabinet right?

[Oscar_Whild]

Thanks for the quick response. The electricity board wires come from the metre into the main incoming device on the Crabtree (100A). As a split board the first 5 MCB are not covered by the RCCB only the last 5 are.

It would be a great help to me if you could briefly, and simply explain why it doesn't matter about the apparently quite large difference between the MCB total (60A + 130A = 190A total) and the ratings of the Incoming device (100A) and the RCCB (63A) = 163A.

Thanks.

[ZenStalinist]

Well, you see, not all of the circuits are going to be in use at once. And since the incomer must be protected by a service fuse of the same value, we allow for some diversity, based upon this assumption. It's perfectly safe, in fact most houses have three socket circuits or so, and many only have a 60A supply. For example, in the kitchen, one could have the washing machine, the tumble drier and the dishwasher going, if one only had up to the service fuse amount for the protective devices to add up to, one would most likely only have one circuit, and therefore if someone were doing a spot of vacuuming, or plugged in an electric heater elsewhere, you'd overload it. Now if we have more circuits, rated at 30A, this problem is far less likely to occur, yet it is very unlikely that one would draw the maximum amount of power, since it is highly unlikely for there to be that much power in use at once. That's basically one of the reasons, sometimes you more power in some places, and are usually less likely to need the power in others. I probably did a lousy job of explaining that, but I hope I more or less made at least part of the reason clear. Also, that way we can have smaller supplies, since 190A would be a heck of a capacity to install, if we did things this way we'd end up with pylons and huge wires all over.

It might be a good idea to upgrade your RCD, given that your shower is 50A.

[Oscar_Whild]

Thanks very much for that, I had hoped as much. I don't like the last line about the RCD though. This bloody shower is rapidly becoming the "job from hell"!!

[andy]

it should be higher because the shower is 50A, giving you 13A spare. say someone then starts the hoovering. gettin close to the limit. the TV computer is also on. even closer. then someone decides to have a cup of tea..

[Oscar_Whild]

Do you think that the best way forward would be to take the 50A MCB (shower) from the non RCCB side of the box and use a separate stand alone RCD solely for the shower then ?

[plugwash]

yes

i would have suggested a rcbo but a 2 module would be a bit big in your CU and all the crabtree 1 modules ones are type C (which you really don't want on a 50A cuircuit domestically)

[ban-all-sheds]

Oscar_Whild wrote:

Thanks very much for that, I had hoped as much. I don't like the last line about the RCD though. This b****y shower is rapidly becoming the "job from hell"!!

Then perhaps you shouldn't have gone out and bought a CU without knowing what you were doing.

If you'd bothered to find out some more, you might not have bought something unsuitable.

What are you replacing? And do you have a TN-S or TN-C-S supply, or is it TT?

What size cable are you using for the shower circuit?

[ban-all-sheds]

Oscar_Whild wrote:

Do you think that the best way forward would be to take the 50A MCB (shower) from the non RCCB side of the box and use a separate stand alone RCD solely for the shower then ?

Dear God.

You haven't even, or have only just, installed a brand new CU, and already you're having to bodge a solution to a problem caused by bad design and product selection.

Bite the bullet - swap the 63A RCD for an 80A and/or buy a bigger board.

[Oscar_Whild]

Er, yes I have !! Bought but not yet fitted. I had no idea that this was going to be such a difficult job. On reflection I would not have started this one !
I am getting a 80A rcd for it today and will fit that before swapping the boards.
Cable size is 10mm and runs direct from the new shower to the CU. As to the type of supply, I have had a good read at some of the posts in other topics and will have to take a look at that and get back to you.
The intention is to swap the boards one for one.

[Oscar_Whild]

The supply is either TN-C-S or TN-S. I can't see which without taking stuff out.