New Consumer Unit Config

[mikhailfaradayski]

Smoke alarms,

There's often some confusion about these being a requirement or not.

IF it is ONLY a re-wire, then this is NOT classed as a refurbishment and you are not required to fit them.

If you are providing any new habitable rooms during the works, then it is a refurbishment and you DO require smokes ( i opt for grade D where i can) check BS5839 for further info as there are more point to consider.

All of that aside, given the value of your family and property, the price of replacing the smokes is tiny and should really be a no-brainer for anyone who cares.

 

[MalvernChris]

Would advise you to try to keep the smoke alarms OFF of the RCD side if poss,and not on an RCBO, could you plan it with the other circuits?
Part B of the building regs says smoke alarms are needed for new build & refurbishments. You can consider a rewire a refurbishment.

Hi widdler,

What's the argument for not having the smoke alarms on their own dedicated RCBO? Without any RCD protection, and requiring their own circuit, wouldn't it be much more work to comply with regs whilst routing the cables?

 

[MalvernChris]

Thanks for the comments so far all.

After revising the layout following your advice we get:

Non RCD side:

Bell xfmr
6A RCBO for smoke alarms
40A RCBO for cooker
16A RCBO for immersion heater
16A MCB for 2.5mm radial circuit to big shed (2.5mm SWA external/underground, RCD socket in shed)
16A MCB for 2.5mm radial circuit to little shed (2.5mm SWA external/underground, RCD socket in shed)

RCD #1 (80A)
32A MCB for downstairs sockets
32A MCB for utility sockets
16A MCB for gas boiler (no heating element)
6A MCB for upstairs lights

RCD #2 (63A)
32A MCB for upstairs sockets
32A MCB for kitchen sockets
6A MCB for downstairs lights (inc internal porch light)
6A MCB for outside lights

This is going to require at least a 21 way CU, I haven't seen any 21 way MK Sentry CU's bundled with two RCD's so I guess that I'd need to buy in kit form, a quick costing for the RCBO's, MCB's and RCD's for this config comes out at around 261 inc. VAT (tlc-direct) and leaves just 1 free fuse way. If I go for the same circuits but all RCBO's the price is around £335 and it only requires 14 fuse ways (plus main switch). Thats a price difference of around £74.

I can see that a professional may go with the first option in order to keep costs down and win business but in my situation it seems that £74 is a small price to pay for the advantages of all RCBO's. Am I talking sense or have I missed something?

 

[LiamPope]

The quoted number of ways on a CU is typically in addition to the ways taken up by the main switch and bundled RCDs. For example my 14-way Hager gives me 14 usable ways with the main switch and RCDs fitted. You can tell this from the photos on TLC

So the 15-way MK would be suitable for you. Doesn't give much room for later expansion, but your proposed schedule of circuits is pretty comprehensive already.

Liam

 

[MalvernChris]

The quoted number of ways on a CU is typically in addition to the ways taken up by the main switch and bundled RCDs. For example my 14-way Hager gives me 14 usable ways with the main switch and RCDs fitted. You can tell this from the photos on TLC

So the 15-way MK would be suitable for you. Doesn't give much room for later expansion, but your proposed schedule of circuits is pretty comprehensive already.

Liam

Liam,

The item I was looking at was http://www.tlc-direct.co.uk/Products/MK5621.html, text describes it as:

MK Sentry
21 Way Insulated Enclosure - for up to 19 MCB s

Accepts
21 One Module Products

or

1 Two Module ( Main Switch or RCD ) and 19 One Module Products

The accompanying image is of the "16 way" version populated with a Main Switch and 14 single modules, no free space.
I've just read the tech. spec. for the MK Bell Transformer http://www.tlc-direct.co.uk/Technical/DataSheets/MK_Sentry/5711.pdf and realised that it's 2 modules wide and should be wired in series with an MCB. That means I'll need the capacity for 14 single width modules, a Main Switch and 1 double width module (18 module widths) so I reckon the 21 way CU is probably the way to go.

Thanks

Chris

 

[Wooshzz99]

Is anyone taking any heed to my post ????????????????


Unless its earthed steel conduit OR the wires are more than 50mm into the wall ALL circuits will have to be RCD protected.

WHY is this being ignored !!!!!!!!!!!!!!!!!!

 

[MalvernChris]

Is anyone taking any heed to my post ????????????????


Unless its earthed steel conduit OR the wires are more than 50mm into the wall ALL circuits will have to be RCD protected.

WHY is this being ignored !!!!!!!!!!!!!!!!!!

Calm down Wooshzz99, what is it that I, or anyone else, is propsing that doesn't adhere to this?

The only circuits not on an RCD/RCBO are the two radials to the two sheds, these will exit through the house wall just below the CU (surface mounted) and then SWA to the sheds. What am I missing? Thanks.

Chris

 

[LiamPope]

3 ways for the bl**dy doorbell?

I'd go wireless battery operated!

Out of interest, have you yet determined the angle of crookedness of your LABC regarding testing the notified work?

 

[Wooshzz99]

Fair enough why not make all circuits rcbo then total discrimination with faults

 

[MalvernChris]

Wooshzz99,

Fair enough why not make all circuits rcbo then total discrimination with faults

Yep, I did a quick costing of a modifed layout after listening to the comments from you guys and decided that all RCBO was probably the way to go:

... a quick costing for the RCBO's, MCB's and RCD's for this config comes out at around 261 inc. VAT (tlc-direct) and leaves just 1 free fuse way. If I go for the same circuits but all RCBO's the price is around £335 and it only requires 14 fuse ways (plus main switch). Thats a price difference of around £74.

I can see that a professional may go with the first option in order to keep costs down and win business but in my situation it seems that £74 is a small price to pay for the advantages of all RCBO's. Am I talking sense or have I missed something?

 

[MalvernChris]

3 ways for the bl**dy doorbell?

I'd go wireless battery operated!

Out of interest, have you yet determined the angle of crookedness of your LABC regarding testing the notified work?

Yes, but my darling wife prefers the ring of the Byron Underdome Bell http://lib.store.yahoo.net/lib/myknobs/underdome.mp3

Haven't sounded them (LABC) out yet (pun intended)

 

[MalvernChris]

I was expecting the 'use spurs to stay with 50m' idea to prompt a 'daft' reply but I've read so many times that the max length of a 2.5mm T&E ring with a 32A MCB is 50m and the ring may only serve up to 100 square metres, what's the solution?

Short term solution - get the OSG. Full solution will then present itself with sufficient study of table 7.1 .

I'm no regs guru, but I think the 50m length is outdated (hence my comment on being careful what you read). At best it was only guidance anyway. Even the 100sqm has been relegated to 'historically this was recommended' or words to that effect. I think the 50m may have come from a disconnection time calc assuming no RCD protection, in which case Zs would have constrained the cable length? Table 7.1 has a corresponding value of 41m for a B32 protected 2.5mm ring final with no RCD and a worst case TN-S Ze of 0.8ohms.

Howevever, with RCD protection Zs goes out of contention and volt drop limits the cable length to 106m. You might also design the circuit with a real measured Ze much lower than the max permitted values used in the table which will allow even greater length (I have a TN-S with a respectable 0.06ohm Ze). Worst case is you swap to 4mm, but in reality you are almost certainly OK with a 2.5mm ring much longer than 50m.

If you're going to contemplate this work, you really do need to thoroughly go through the OSG as a bare minimum!

My copy of the OSG guide arrived this AM, hallelujah! I just love table 7.1 This thing is like having all you guys in handy ring-bound form and none of the 'old' forum posts muddying the waters!

 

[RF Lighting]

I bet there isn't as much irreverent humour in the OSG

 

[LiamPope]

My copy of the OSG guide arrived this AM, hallelujah!

It's indispensable! The John Whitfield book is also pretty handy as it goes into much more academic detail (and occasionally philosophical musings). All pale into insignificance compared to this place though in my experience. Not only is it a 24-hour free professional advice centre, it's also (much more usefully) a searchable archive of extensive expert discussion and debate on every contentious, ambiguous or controversial regulation in BS7671, and the sum of all human knowledge related to matters of best electrical practice and general practical tips!

And irreverent humour.

And Picture of the Week™

 

[MalvernChris]

All,

Thanks very much to all who have commented and offered advice. Cheers.

Chris