Does this secondary CU need a main RCBO as well?

[TonyRPH]

I have an electric boiler with a 6mm2 feed, and this boiler is about to be decommissioned.

In addition to this, the boiler cupboard has two 2.5mm2 feeds for the immersion heaters.

I want to repurpose these 2.5mm2 feeds for electric heaters and split the 6mm2 into two separate feeds for the immersion heaters.

My plan is to fit a secondary CU in the boiler cupboard to facilitate this, and my question is; Does the secondary CU need a primary RCBO along with the two 16A RCBOs?

The 6mm2 feed is fed via a 40A RCBO on the main CU, and the cable run is about 10m (it's hidden in an inaccessible ceiling void, hence my desire to do this).

[Belle427]

Its not advised to have multiple RCDs on a circuit as it can cause issues.
I would think you only need the one at the main CU end, the secondary CU would then have a main switch and 2 x Mcbs.
May be someone else can confirm, im not up to speed on latest regs.

[TonyRPH]

Thanks, I believe you are correct (I got the same answer elsewhere).

[V8 Stang]

Does the Submain require RCD protection?

Unless its buried in a wall or a TT system, it does not require it.
So you can just have overload (MCB) at the main board, then RCBO’s on the second.

[Regbuser]

Tony, I don't think there's enough information to provide a recommendation.
Apart from this being new distribution and circuits, so needing notification and certification under ADP.

[TonyRPH]

Regbuser said:

Tony, I don't think there's enough information to provide a recommendation.
Apart from this being new distribution and circuits, so needing notification and certification under ADP.

There is an existing CU with all the usual gear (all RCBO's with main switch etc.).

One of the RCBO's (40A) feeds the main boiler circuit via a 6mm2 cable.

It is this cable that I wish to repurpose by fitting a secondary CU in what is currently the boiler cupboard, equip this secondary CU with a main switch and 2x 16A MCBs for the two 3kW immersion heaters.

[Regbuser]

You're not considering:

Characteristics of the supply
Existing EICR information, readings, and notes
Method of installation of existing cables
Adverse thermal effects of proposed CU location
Requirement for selectivity of cascading protective devices

Whereas a competent electrician would factor those into the new distribution circuit design.

[TonyRPH]

Regbuser said:

You're not considering:

Characteristics of the supply
Existing EICR information, readings, and notes
Method of installation of existing cables
Adverse thermal effects of proposed CU location
Requirement for selectivity of cascading protective devices

Whereas a competent electrician would factor those into the new distribution circuit design.

All cables are already present, and will only be moved a small amount. The CU will be located on the wall adjacent to the cupboard, so no thermal issues.

As the preceding device is a 40A RCBO, only MCBs are required.

The 6mm2 cable has ample capacity (even after derating based on location**) to supply 6kW, and both immersions are highly unlikely to be on at the same time anyway, therefore based on selectivity (or diversity for the old school!) ample capacity is available.

• this cable forms part of the existing installation.

[Regbuser]

Well, as you're obviously competent, crack on old chap, and don't forget the ADP notification and certification!

[OutInTheShed]

Belle427 said:

Its not advised to have multiple RCDs on a circuit as it can cause issues.
I would think you only need the one at the main CU end, the secondary CU would then have a main switch and 2 x Mcbs.
May be someone else can confirm, im not up to speed on latest regs.

What 'issues'?

As far as I can see, the only 'issue' is that, in the event of a fault, either RC breaker might trip, so everything might go down, or just the 'sub box circuit'.