Allocation of rings to CU

[Graham.]

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I'd much recommend RCBOs these days, so it's easy to tell what is causing the trip.

Is it? When I asked here, I was told that you can't tell by looking at a tripped RCBO whether it was an over-current or an earth leakage event that caused it to trip.

However if you can't do that (why?), aside from diversity calculations, onethought is to mix up the lighting and socket circuits so that trips onlytake out part of the house at once. eg:RCD 1: Upstairs sockets, downstairs lighting, kitchen socketsRCD 2: Downstairs sockets, upstairs lighting, kitchen lightingThat way you are never in the dark when trying to fix a fault, which is morelikely to be with a socket circuit than a lighting circuit.Theo

[Grumps]

Hi All, It's taking me quite some time to get around to the rewiring of my old house. I've had the building inspector around for the first fix visit, and he said he won't need to come back until it's all finished.

I had it clear in my head where each ring and lighting circuit was to go in the CU, but have misplaced most of my notes. I have a twin RCD CU with an 80Amp and 63Amp rated RCD. (143A total)

The MCBs are:

32A Hob 32A Kitchen ring 32A Downstairs ring 32A Upstairs ring 6A Kitchen lights 6A Downstairs lights 6A Upstairs lights (146A total)

The only sketch I can find has the upstairs ring + kitchen ring + downstairs lights on the 63A RCD. That would be 70A, but can you apply diversity to the upper ring?

[Robin]

You may find some comfort in

(IIRC John Rumm & Adam Wadsworth came up with that in order to shut me up after I made the mistake of thinking the drafters of BS 7671 cared about such things. )

[David Wade]

Why don't you fit RCBOs?

Dave

[Robin]

Sorry for the second bite but I realise I didn't answer your question. I don't see why not if it's a standard ring

[Brian Gaff]

And there was I worrying about a little dimming of the lights in the shed over 100ft of armoured cable on the downstairs ring when the fan heater came on. some years back. Brian

[Brian]

Sensibly, are you likely to be “loading” the kitchen and upstairs sockets at the same time to capacity? (Not to mention the downstairs lighting circuit). I would suspect not.

I note there is no oven - do you have a gas oven?

Also, some CUs have an Immersion heater circuit.

Worth adding an isolation switch and a Henley block. Useful if, for example, you need to have a EV charger installed or a second CU later ( perhaps for a shower, outsider circuits, …)

Tip: check your ‘Main fuses’ if they are under 100A, Power Gen will replace them free provided the ‘tails’ are large enough.

We had 80A ones - which was spotted when we had a charger fitted. I was advised to ask PG to change them. I was gob smacked by how efficient the service was. A phone call, sending a few details electronically, including photos of the tails, and two men turned up a few days later, did the job, and had tea and biscuits I was so pleased.

[John Rumm]

The "by the book" allowances should be more than adequate here, if you follow:

So I would treat the divers loads as:

32A Hob 32A Kitchen ring 13A downstairs ring 13A upstairs ring 4A each for the lighting circuits. (with modern lamps - that is probably way over!)

So ~100A

However to echo other comments, RCBO installs are well worth looking at these days, even if you just have one for the kitchen circuit. (one of the more likely to nuisance trip)

[John Rumm]

That was quite a good discussion I seem to recall... ah yes:

[Grumps]

Thanks. Would it be sensible (or not needed) to use 16A MCBs for the upstairs & downstairs rings?

[Theo]

I'd much recommend RCBOs these days, so it's easy to tell what is causing the trip.

However if you can't do that (why?), aside from diversity calculations, one thought is to mix up the lighting and socket circuits so that trips only take out part of the house at once. eg:

RCD 1: Upstairs sockets, downstairs lighting, kitchen sockets RCD 2: Downstairs sockets, upstairs lighting, kitchen lighting

That way you are never in the dark when trying to fix a fault, which is more likely to be with a socket circuit than a lighting circuit.

Theo

[Michael Chare]

So no lights upstairs!?

[ARW]

Bollocks:-)

It's always going to be the last circuit you check and the fault is going to be in the most inaccessible place.

Please note the smiley.

Adam

[Robin]

No. There are Upstairs lights (the last circuit). And I don't think even the most extreme academics have yet moved on to Critical Case Theory :)

[John Rumm]

Not needed...

This is after all an exercise in statistical loading...

[Grumps]

Thanks. And to all of the other responders too.