Diversity on a 20 Way or longer CU with many (low utilisation) ring mains

[axt123]

Hello,

A quick question which I suspect is becoming more commonly asked as we go towards having more ways in our CUs.

Domestic dwelling, nothing too exuberent but during rejigging the large downstairs ring was split into three and some radials added, as well as seperate feeds to allow for a locationally different oven and induction hob.
Hence there are now quite a few ways for a consumer unit to deal with, but the loading will be very typical of a domestic house.

My question is, with regards to the (rather outdated) diversity theories for a consumer unit, how do the more learned feel about this arrangement:

100A DNO fuse. One sub main on 16 T&E to garage via RCD / Switchfuse.

20 Way Main Switch + SPD consumer unit that is to be populated with RCBOs such:

• 40A - Hob 10mm Radial
• 32A - Oven 6mm Radial
• 32A - Kitchen 2.5mm Ring
• 32A - Laundry 2.5mm Ring
• 32A - Downstairs 2.5mm Ring
• 32A - Upstairs 2.5mm Ring
• 32A - Conservatory 2.5mm Ring
• 32A - Immersion 6mm Radial (1x 3kw Locally Switch Fused)
• 25A - Shed 2.5mm SWA
• 20A - Radial Sockets x 2 2.5mm
• 20A - Radial Sockets x 2 2.5mm
• 20A - Radial Socket x 1 2.5mm
• 20A - Fridge x 1 2.5mm
• 20A - Freezer x 1 2.5mm
• 16A - Outside Socket 1.5mm SWA
• 16A - Boiler & Controls 2.5mm (Switchfused Down)
• 6A - Lights Up 1.5mm
• 6A - Lights Down 1.5mm
• 6A - Lights Down 1.5mm
• 6A - Lights Outside 1.5mm

Clearly, if one was to add all of this up you get over 445A, but in reality the loading will be a tiny fraction of this. All LED fittings, Gas Heating etc.

The RCBOs were chosen to protect the wiring downstream of them, as per normal. The DNO fuse should protect upstream (and, in theory, the tails and Main Switch, which are 100A rated. The CU may also be fed via a 100A Time Delay RCD for backstop protection, as the 30mA RCBOs could allow for a fair bit of commultive earth leakage and its a TT property.

Just wondering if there is any advantage into splitting it into two CUs, though that would mean a messier install, confusion on isolation, more tails (pre-RCD) to protect, more cost and woulden't actually change anything.

I could have a 16+14 Way CU with one Main Switch, with all 20A RCBOs, one for every socket on its own radial, and the usage from an end user would not change, so diversity based on MCB / RCBO rating is a bit meaningless. But flipside - you could now load everything up more, at the risk of the DNO fuse blowing...

What is the collective ponderance on the best practice for this sort of sitution?

Many Thanks

 

[mainline]

Just wondering if there is any advantage into splitting it into two CUs, though that would mean a messier install, confusion on isolation, more tails (pre-RCD) to protect, more cost and woulden't actually change anything.

You could use a double stack consumer unit.

 

[SparkySy]

I'd also use a double stack board.
For diversity, depends on how many people are in the property using it.
But If you add up all your circuit protection devices and times by 0.25 you will get a better idea of how much potentially could be drawn.
Unless the oven hob and shower are in use together, the average property I come across with 2-3 people in is generally using around 11-25amps depending on what's being used while I'm there, if nobody is in most will draw 1.5-7amps depending on the boiler fridge and freezer cycles.
So I would say if it's of similar occupancy it should be fine.
Or get a three phase supply and spread the loads over the phases.

 

[littlespark]

The reason we used to add up the mcb ratings was for split rcd boards…. Where there was a chance that, if each mcb took its maximum load on one side on one rcd, then it could, theoretically overload say a 63A main rcd…. A rcd not being an overload protection device.
A 100A mainswitch would be considered to be protected by the main fuse upstream.

With individual RCBO on each circuit, you dont have to add up the ratings.



you could downgrade the rfcs to 20A if they’re not high usage… keep 32 in the kitchen and laundry.

The boiler and controls are fused at the switch…. so don’t need a 16….

And the immersion could be dropped to 16, and just use a dp switch without a fuse.
6mm for the 3kW immersion? Was it an existing circuit?

 

[axt123]

I'd also use a double stack board.
For diversity, depends on how many people are in the property using it.
But If you add up all your circuit protection devices and times by 0.25 you will get a better idea of how much potentially could be drawn.

I generally thought double stack boards were more for situations where a portrait design is better suited to the cupboard - you still generally only have one main switch iirc? I guess you get dual busbars but the loading on the incoming remains the same?

I strongly suspect the actual drawn power in this situation will be very typical, so probably only 5-10 units per day, unless someone cracks out the electric fan heaters...

The reason we used to add up the mcb ratings was for split rcd boards…. Where there was a chance that, if each mcb took its maximum load on one side on one rcd, then it could, theoretically overload say a 63A main rcd…. A rcd not being an overload protection device.
A 100A mainswitch would be considered to be protected by the main fuse upstream.

With individual RCBO on each circuit, you dont have to add up the ratings.



you could downgrade the rfcs to 20A if they’re not high usage… keep 32 in the kitchen and laundry.

The boiler and controls are fused at the switch…. so don’t need a 16….

And the immersion could be dropped to 16, and just use a dp switch without a fuse.
6mm for the 3kW immersion? Was it an existing circuit?

That probably answers where I recall that from, as in that situation I guess it was common to have a couple of 63A RCDs and 100A main switch in same CU, and someone may think it clever to put mostly lights on one and mostly sockets on the other (!).

Good shout on derating some circuits and that was my general advice in this case, but it seems wrong to reduce a rfc to 20 when it can handle the 32 on the basis that nobody would pull that in a conservatory for instance...

Believe the immersion was run in 6mm for future proofing in case a direct heated tank is ever put in, but your right, that could be a 16 for the foreseeable, but would need good notes in the design paperwork to confirm it's rated to 32A.

Thanks for the comments.