New Consumer Unit Config

[MalvernChris]

Hello All,

I'm trying to determine a sensible consumer unit config for my victorian property, the old wiring was a real dogs dinner and has been completely removed for peace of mind. Being a fairly competent type I intend to do most of the rewiring myself, get my electricity supplier to move the meter from the kitchen to a cabinet outside and have the installation inspected for Part P compliance by the local Building Control's agent.
Having read literally hundreds of posts in various forums my first stab at a sensible config for the new (yet to be purchased) consumer unit is:

On the non RCD side:

Bell xfmr
40A MCB for cooker
16A MCB for immersion heater
16A MCB for 2.5mm radial circuit to shed #1 (2.5mm SWA external/underground, RCD socket in shed)
16A MCB for 2.5mm radial circuit to shed #2 (2.5mm SWA external/underground, RCD socket in shed)

RCD #1
32A MCB for downstairs sockets
32A MCB for kitchen sockets
6A MCB for upstairs lights

RCD #2
32A MCB for upstairs sockets
32A MCB for utility room sockets (with fused switched spur for gas boiler)
6A MCB for downstairs lights (inc internal porch light)

Does this sound sensible?

I'd also like to provide for a socket on the outside rear wall and outside security lights (not of the 500W halogen variety). Should I provide a third RCD with a 16A MCB for the socket and an 6A MCB for the security lights or would it be perfectly OK to put one on RCD #1 and one on RCD #2?

With regard to the downstairs ring, the front two rooms have suspended timber floors, the back room has a solid floor and due to high ceilings the cable length would exceed 50m if I were to drop down from the ceiling void to a socket and back up again on each wall. I know horizontal runs are permitted but I'm not keen on this approach so I'm thinking about taking spurs from junction boxes in the ceiling void for each of the four sockets, thereby staying within the 50m limit, is this reasonable or daft? any other solutions?

Finally, I'm considering going for one of the larger (plastic) MK CU's which seem be amongst the favorites of most sparks whose posts I've seen.

Any constructive comments would be most welcome. Thanks.

 

[SparkyNinja]

16A MCB for 2.5mm radial circuit to shed #1 (2.5mm SWA external/underground, RCD socket in shed)
16A MCB for 2.5mm radial circuit to shed #2 (2.5mm SWA external/underground, RCD socket in shed)

Do you really need 2 radials? What is the load for the shed?
How are you planning on installing the SWA?
How are you planning on wiring the non-RCD circuits?

RCD #1
32A MCB for downstairs sockets
32A MCB for kitchen sockets
6A MCB for upstairs lights

I would put the kitchen and downstairs sockets on seperate RCD's (assuming these are on the same floor)

RCD #2
32A MCB for upstairs sockets
32A MCB for utility room sockets (with fused switched spur for gas boiler)
6A MCB for downstairs lights (inc internal porch light)

Possible to wire designated boiler circuit? If so could you wire this to non-RCD?

I'd also like to provide for a socket on the outside rear wall and outside security lights (not of the 500W halogen variety). Should I provide a third RCD with a 16A MCB for the socket and an 6A MCB for the security lights or would it be perfectly OK to put one on RCD #1 and one on RCD #2?

On the RCD circuits will be fine, just consider your cabling methods.

With regard to the downstairs ring, the front two rooms have suspended timber floors, the back room has a solid floor and due to high ceilings the cable length would exceed 50m if I were to drop down from the ceiling void to a socket and back up again on each wall. I know horizontal runs are permitted but I'm not keen on this approach so I'm thinking about taking spurs from junction boxes in the ceiling void for each of the four sockets, thereby staying within the 50m limit, is this reasonable or daft? any other solutions?

Daft.

Finally, I'm considering going for one of the larger (plastic) MK CU's which seem be amongst the favorites of most sparks whose posts I've seen.

I wouldn't use anything else.

How about smoke detectors? Do you already have an installation?

If you have read the forums you would have come accross main bonding discussions a lot. What is your current understanding with your existing?
Do you have the correct test equipment in order for you to comlete the necessary paperwork or will get it tested by a third party?


Oh, and do you know any electricians (just in case)

 

[nozspark]

If you are mounting the bell transformer in the CU then the outgoing cables need to have the same level of insulation as any other cable in the enclosure

 

[LiamPope]

No arbitrary 50m length limit for your downstairs sockets. Calculate or refer to table 7.1 of OSG.

Do your main switch-only protected circuits meet requirements for omitting RCDs?

Generally, when 'researching' on this or similar sites, pay attention to what regulations were in effect at the time those posts were written. I hope you have at the very least the 17th OSG.

Good luck.

Liam

 

[Wooshzz99]

Earthed metal conduit or more than 50mm into wall OR ALL CIRCUITS RCD protected

 

[MalvernChris]

Thanks for the reply widdler,

Do you really need 2 radials? What is the load for the shed?
How are you planning on installing the SWA?
How are you planning on wiring the non-RCD circuits?

My wording may be a little misleading here, I have two sheds and am proposing one radial for each.

Possible to wire designated boiler circuit? If so could you wire this to non-RCD?

Could do, it's a gas boiler with no electrical heating element, from what I've read it's the heating elements e.g. electric cookers, immersion heaters etc that usually have inherent earth leakage due to moisture penetration of the heating element, is this so? If not, what's the advantage of putting the boiler on a separate circuit / non RCD circuit?

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?

How about smoke detectors? Do you already have an installation?

There are a couple of 'battery only' smoke detectors already installed.

Do you have the correct test equipment in order for you to comlete the necessary paperwork or will get it tested by a third party?

As I understand it, I pay the local council Building Control dept. to have the work accredited, they sub the job out to a professional who will perform whatever tests are required and complete the necessary paperwork?

 

[MalvernChris]

Thanks nozspark,

If you are mounting the bell transformer in the CU then the outgoing cables need to have the same level of insulation as any other cable in the enclosure

I'd noted that one

 

[ColJack]

if you need more length, run a 6mm radial round the sockets instead of a 2.5 ring..
you can get further with 6mm than 4mm..

if you're re-wiring, you are supposed to put mains powered smoke alarms in.. interlinked ones..

 

[MalvernChris]

Thanks Liam,

No arbitrary 50m length limit for your downstairs sockets. Calculate or refer to table 7.1 of OSG.

Do your main switch-only protected circuits meet requirements for omitting RCDs?

Generally, when 'researching' on this or similar sites, pay attention to what regulations were in effect at the time those posts were written. I hope you have at the very least the 17th OSG.

Good luck.

Liam

As I understand it, the prescribed max lengths / floor areas are to enable installations to be carried out without the need for all the calculations, i.e. if you stay within these values you are sure to be within the safe limits without having to do the calculations? I always try to ensure that any forum posts I take head of are specific to the 17th edition.
I had planned to use earthed metal trunking/conduit for the two non RCD circuits (cooker & immersion heater) but I'm beginging to think that it may be easier to accept a little extra cost and use an RCBO for each?

 

[MalvernChris]

Thanks ColJack

if you need more length, run a 6mm radial round the sockets instead of a 2.5 ring..
you can get further with 6mm than 4mm..

if you're re-wiring, you are supposed to put mains powered smoke alarms in.. interlinked ones..

I'd been told by a work colleague that that requirment only applied to new builds, not rewires, so I hadn't looked into it, I will now. Thanks

 

[flameport]

I'd been told by a work colleague that that requirment only applied to new builds, not rewires, so I hadn't looked into it, I will now. Thanks

Even if it isn't a requirement, why would you NOT want them?

 

[MalvernChris]

Thanks flameport,

I'd been told by a work colleague that that requirment only applied to new builds, not rewires, so I hadn't looked into it, I will now. Thanks

Even if it isn't a requirement, why would you NOT want them?

It's not that I don't want smoke alarms, there are currently two battery powered alarms installed, I didn't realise that I needed to replace them with mains powered/battery backup versions during a re-wire, thought that requirement only applied to new build having taken a work colleagues' opinion as gospel. Have modified my plan to include a RCBO soley for smoke alarms.

 

[SparkyNinja]

Thanks flameport,

I'd been told by a work colleague that that requirment only applied to new builds, not rewires, so I hadn't looked into it, I will now. Thanks

Even if it isn't a requirement, why would you NOT want them?

It's not that I don't want smoke alarms, there are currently two battery powered alarms installed, I didn't realise that I needed to replace them with mains powered/battery backup versions during a re-wire, thought that requirement only applied to new build having taken a work colleagues' opinion as gospel. Have modified my plan to include a RCBO soley for smoke alarms.

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.

 

[LiamPope]

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!

As I understand it, I pay the local council Building Control dept. to have the work accredited, they sub the job out to a professional who will perform whatever tests are required and complete the necessary paperwork?

Your understanding of how it should work is correct. However your LABC may well have different ideas! Some of them (mine included - I'm a notifying DIYer like yourself) will refuse to test (mine will at least inspect). They can get away with it too unless (or maybe, even if) you have the stomach to take them to court. So you're then left with having to pay a spark to come and do a PIR on your new installation. I'm possibly facing this too, though as luck would have it I now have access to an MFT1552 to play with so have carried out full tests on the circuits I've finished so far before energising. Weeeeell, maybe slightly after energising . So I'm trying to get my LABC to accept a full EIC from myself. We'll see. Do check with yours what their take on the matter is (not that approved doc P or more than one letter from central government to LABCs isn't clear enough).


Cooker circuit - if you add up the cost of the earthed conduit plus fittings etc etc, the RCBO is probably not really an extra expense! Not to mention the time. 30mA leakage should be plenty enough for a big double electric oven to play with. If not it's faulty.


Smoke alarms - there seems to have been a recent shift in manufacturers recommendations towards putting these on a dedicated circuit rather than on another well used circuit as was often recommended. That's both alarm/detector manufacturers and CU/device manufacturers. There's a reg that states circuits for 'safety services' should be seperate from others, and an earlier sentence that includes smoke detector circuits as an example of a safety service. This seems to be what the manufacturers are picking up on, and even quoting directly. With battery backup devices that tell you if the mains has tripped, it's hardly an issue anyway. My smoke/heat/co alarms are going on an RCBO all by themselves.

Good luck,

Liam

 

[Wooshzz99]

Earthed metal conduit or more than 50mm into wall OR ALL CIRCUITS RCD protected