Exporting to a second distribution board

[jpadie]

Hello all 

I'm wanting to export from the meter to a second distribution board in a garden room.  Cabling is long since in (SWA) and the building inspector has been around.

 

The BI's dominant issue relates to me taking the second supply from the main distribution board, downstream of the main breaker.  The distribution board has three RCDs in it, two were factory supplied and serve the MCBs for the main house.  The third is a time-delayed RCD taking the SWA cable for the garden room.  Selected with the advice of this forum. 

the issue on the building regs report is that I should have either (i) used a 63A MCB with a bus bar; or (ii) used manufacturer links inside the distribution board (i.e. i've used different wires, same gauge, different flexibility, from the two RCDs that were factory supplied.).  Two challenges I face: Wylex don't do 63A consumer MCBs; and don't sell separate links.  I can find some on eBay. 

I wondered if it were better to put a second small distribution board for which I can get a 63A MCB.  If so what are the rules for exporting to a second DC that's next to the first?  can that be directly from the meter? via a Henley block or maybe direct links to the first DB?  If the latter, do I then have the same problem in needing to match manufacturers?

thanks in advance
Justin

 

[ProDave]

The issue you are facing is you don't have a standard, as supplied from the manufacturer, consumer unit.  As soon as you (you  being anyone other than the manufacturer) alter the configuration of the consumer unit, it's manufacturers certification is void.

 

You would make your life a whole lot simpler if you put the CU back to standard configuration for the house, and added a completely new mini consumer unit for the sub main, teed off the meter tails with Henley blocks.

 

Why is the BI showing an interest in this?  Is he certifying it rather than a Part P electrician?

[jpadie]

16 minutes ago, ProDave said:

The issue you are facing is you don't have a standard, as supplied from the manufacturer, consumer unit.  As soon as you (you  being anyone other than the manufacturer) alter the configuration of the consumer unit, it's manufacturers certification is void.

 

You would make your life a whole lot simpler if you put the CU back to standard configuration for the house, and added a completely new mini consumer unit for the sub main, teed off the meter tails with Henley blocks.

 

Why is the BI showing an interest in this?  Is he certifying it rather than a Part P electrician?

thanks @ProDave
yes - the electrician I had lined up could not do the job with me and recommended that I got the inspection done instead.  

so I am permitted to go Meter -> Henley Blocks -> Distribution Boards * 2?    I thought I might be required to use particular tails to go directly from the meter to the distribution board without a break. 

 

[jpadie]

following up here.  

I spent an informative hour with CEF in Horsham.  bottom line is that they don't sell Wylex links and ... Wylex don't sell flexible links either.  I can get them on eBay.  
I spoke to Wylex directly and they went further by saying that they could not condone adding an RCD to their consumer units (and they don't sell links or 63A MCBs).

the DNO is coming next month to upgrade the fuse to 100A.  British gas is coming in a couple of weeks to repair the smart meter that they've just installed and to add an isolator since they've put a seal on the DNO's fuse (fair enough, but there's not been one there for at least 13 years). 

there's insufficient space in the cabinet to put a second consumer unit there.  Even a wee one.  I've got perhaps 200mm of height.    

the isolator will give me the (safe) ability to install a henley block and split the tails but I think I'm in a bit of a pickle! 

Options seem to be:

1.  swap out the current consumer unit for an 18th edition version from a supplier that has 63A MCBs and use RCBOs to gain some extra ways.   This would be expensive (£400 worth of kit plus it's notifiable so that's another £600 for building inspection and almost certainly there would be insurmountable wiring issues to remedy given the change in regs and the fact that this is a 16th century cottage with a bix of early 60s and early noughties electrics)

2.  abandon the idea of the MCB and instead split the tails into the existing CU and a switched fuse (like this or this).  Ideally I can go straight into something like this with SWA.  Even more ideal would be if horizontal mounting were permitted.

3.  export the wiring a couple of metres and place a new mini CU somewhat out of the way.  Downside is a significant loss of aesthetic.

4.  use a 50A MCB in lieu of the 63A and add an interlock to make sure that both immersion heaters are not on at the same time (or a CT clamp to limit the amount an EV might draw from the circuit).  Just to recap the sub distribution board powers a garden room that has a small ASHP, two 3kW immersion heaters and a 7.5kW EVSE, the two immersion heaters _should_ never be on at the same time and it's a small 120L tank, the ASHP is inverter driven and draws 750W max.  If I used an interlock the max current draw should be 46A.  a bit close to the bone but possible perhaps with a C curve MCB.

 

the possible fifth option of replacing the cabinet altogether for a larger one is not practical due to other constraints.  

Have I missed any obvious options please?