Dual Tarrif consumer unit

[SJD]

I have a domestic customer with an Economy 7 supply, where both the main supply and the off-peak supply are brought into the one fuse box, with two separate main switches in the centre. The customer wants the fuse box replacing with just one new consumer unit. The property is a small 1-bed flat, total of just 7 circuits (two are E7 – water heater, storage heater).

I typically install Wylex CUs, and Wylex do a Dual Tarrif range (prefix NHIIX, e.g. NHIIX7504 for 7+5 ways). Because these dual tarrif CUs are less commonly sold, it seems I’d pay about as much for one of these – a bare CU with just the two main switches – as I can buy a Dual RCD CU with 10 MCBs already installed.

I am thinking, can I take a standard Wylex Dual RCD 10xMCB CU and reconfigure it by removing two MCBs and adding a second main switch. I’d use one RCD for the main supply, and one RCD for the E7 supply. And since the lighting comprises two circuits, perhaps put the larger lighting circuit on a separate RCBO. The CU has 3 separate neutral bars to support this configuration.

Am I missing something, is there some reason why I can’t do this, either practically or regs-wise?

 

[Mork]

Ideally split your 7 N/T circuits equally over 2 rcds (reg 314.1 &314.2) not 6 on one rcd and 1 on a rcbo.

 

[SJD]

Not sure if I explained it very well - intent is 4 circuits on main supply RCD (cooker, sockets RFC, immersion top-up, lights B), 1 circuit on RCBO (lights A), and 2 circuits on E7 RCD (E7 immersion, E7 storage heater).

 

[Richard Burns]

Always a minefield to start modifying a CU since the manufacturers may then say it is not up to scratch.
That said so long as you have suitable supports for all your components and you are keeping the supplies separate and ensuring that they are no going to inadvertently connect somewhere then you should be OK.

 

[guest119]

If Im reading this correctly you are descsibing having an rcbo supplied by an rcd?
Rcd main switch would need to match the main supply/tails rating.......100a?

 

[Had8Lives]

You also need to consider what will happen if the immersion element causes the RCD to trip overnight in winter. The customer will have no hot water and a chilly flat! I'd go the RCBO route for the Immersion and the Storage Heater.:smiley2:

 

[SJD]

Thanks for the replies.

Richard - I agree about being wary of modifying anything, but I think I'm probably just reconfiguring the CU to look like it would be if I bought a CU designated as a dual tarrif model, and using standard components.

Boydy - Intent was the main 24hr supply has a main switch that feeds one RCD (plus MCBs) and one RCBO - I wouldn't have the RCBO supplied via the RCD.

Had8Lives - Probably it is better to use 2 RCBOs for the immersion and storage heater, instead of one RCD - I'll give the customer that option. Actually the immersion element has already failed and needs changing. Curiously, there is though also a gas fire in the same room, despite it being a flat.