Second Consumer Unit

[astroman]

Hi,

I need some advice re fitting a second consumer unit.

I have just built a new extension, kitchen & bedroom with en-suite. My intention is to install a new 8-way consumer unit to supply the neccessary circuits. My current set up is meter to 63A RCD then tails to the consumer unit. Where should I connect my cable for the new consumer unit, the new consumer unit will be about 8m cable run from the first & meter. I cannot connect to the in terminals of the consumer unit as there is no room. Is it acceptable to connect to the out terminals of the RCD, what cable should I use, I thought 10mm T&E with the earth to the main PME terminal block? or do I have to use tails and a seperate earth cable?

Thanks

Astroman

 

[ban-all-sheds]

I hope you've not given this enough thought.

8m of 10mm² T/E protected only by the service fuse?

You must contact your DNO - there will be a limit on the length of tails you can use, typically 3m, before they will require a switchfuse.

So you'll probably end up with
[code:1]
[Meter]==[Henley]==[RCD]==[CU#1]
||
||
[SwFuse]========[CU#2] (with its own RCD protection as required)
[/code:1]

Does Part P need a mention?

 

[Adam_151]

@OP - 10mm² T&E, no chance(as ban said), 16mm² swa / 25mm² swa is closer to what you should be looking at


@ Ban, Wouldn't it make more sense to have the switchfuse before the henlys,

A) can be used to isolate first cu in future, to avoid main fuse hassles again.

b) Single point of isolation.

c) might be able to use descrimination to your advantage to avoid having to call the rec out if you run too much stuff at once (if rec fuse is 100, and you have an 80 in your switchfuse)

 

[user56565]

agree with ban and adam.

you will need a switchfuse if you are considering using two consumer units in this way, as ban explained. as adam said, install it before the henleys. if your supply is TT, hence the RCD before the CU, this will also need to be before the henleys. SWA will be required.

what loads will the new consumer unit be feeding? if under 50amp, one could suggest running T&E/SWA (whatever is suitable for the circumstances) from a suitably rated MCB in the CU, and feeding this onto the second CU. much like how a shed/garage CU is usually fed from main house CU.

if you need one, make sure you buy a proper switchfuse. i have seen it suggested in the past somewhere that one buy an empty CU, put in a 100A incomer switch, and that this would suffice. it doesn't and offers no protection.

i have the switchfuse-henley-single CU setup, and would recommend you get the wylex 110m. make sure you get the 80A fuse too, as adam said, for discrimination.

 

[ban-all-sheds]

@ Ban, Wouldn't it make more sense to have the switchfuse before the henlys,

A) can be used to isolate first cu in future, to avoid main fuse hassles again.

b) Single point of isolation.

True, but then it would need to be a bigger unit. If you want isolation put in an isolation switch before the henley - the switch fuse is there to protect those long tails.

c) might be able to use descrimination to your advantage to avoid having to call the rec out if you run too much stuff at once (if rec fuse is 100, and you have an 80 in your switchfuse)

If the risk of blowing the supply fuse is so great that you feel you have to do this then the answer is an uprated supply, not the introduction of a device which will cause even more nuisance outages.

 

[user56565]

sorry, first time but disagree with you ban..

the switchfuse combines both protection and isolation facilities, so why not use it for both purposes??

edf requested a switchfuse (SWA 'tails' running >3m, to CU) and they also requested a lower rated fuse for discrimination purposes. iirc, the disconnection curves for an 80A fuse are such that it won't blow under continuous operation of 100A, which is also the design current for the entire installation. so nuisance trips shouldn't be a problem. it will take a lot for the 100A service fuse to blow, and the 80A fuse would do a better job of protection (by blowing at lower currents), without affecting full load.

 

[ban-all-sheds]

sorry, first time but disagree with you ban..

the switchfuse combines both protection and isolation facilities, so why not use it for both purposes??

No real reason, I guess. Might need bigger tails/SWA/split-con to supply the 2nd CU if they're on an 80A fuse, but I haven't given that any thought.

edf requested a switchfuse (SWA 'tails' running >3m, to CU) and they also requested a lower rated fuse for discrimination purposes.

Well that's a new one on me.. Thanks for the info.

 

[user56565]

Well that's a new one on me.. Thanks for the info.

no prob. it'll take 3core 25mm2 in no problem, although as you'll know the cable's a pain to work with.
his drawing just happens to be right next to me. so here it is, you won't learn much from it, but just incase you were curious.

http://i7.photobucket.com/albums/y278/user56565/switchfuse.jpg

 

[astroman]

Hi all,

Thanks for your replies. I'm afraid some of it is over my head. I feel quite confident to wire up from the CU, it's just getting the supply to it! What is a "Henley"?. I am in rural Cornwall so this may make difference as to how it's wired here? We are fed by overhead cable from a tranformer on a pole in our drive. There is an earth cable from this transformer to ground. The feed comes from there in a single cable to a large, sealed, company fuse, from there by tails to the meter and then tails to the 63A RCD and tails to the consumer unit (wired fuses marked not to exceed 60A). The earth block is connected to terminal on the company fuse block and the mounting board is marked "connected to a protective multiple earthed" (sic).

The kitchen/bedroom/en-suite won't be drawing large currents, the cooker is oven only, rated to run off a 13a socket (though I intended giving it its own circuit), the shower is a pump. My plan was for 2 X 13A rings in the Kitchen, 1 for sockets the other for FCUs, 1 X 13A ring in the bedroom with a spur for the shower pump and 2 lighting circuits, upstairs & down.

Re part P, My local building regs people tell me that as my plans were approved before part P came in I am exempt. Having said that I fully intend to get it inspected.

Astroman

 

[mapj1]

Henly is also caller a service connector - think giants choc bloc - http://www.tlc-direct.co.uk/Products/MK1101.html

How unusually sensible of the local authority...

It sounds like you have PME, or PNB and may not legally need the RCD, but perhaps it was formerly declared a TT suppply . In any case its not a bad thing to have.

 

[user56565]

based on what you've said, you have a PME supply. see reference and confirm this. is the cross-bonding in your place in correct condition?? very important, and should be dealt with now as you do the other electrical work.

if you do have PME, you won't need the RCD at the beginning anymore. i would personally remove, but as mapj1 says no harm in keeping it.

now i can't tell how big your extension is, but i think it's OTT to have the circuits you've proposed added, just to supply the extension.

why not, instead do this:
extend the upstairs lighting circuit you already have to light the upper floor of the extension.
extend the downstairs lighting circuit you already have to light the lower floor of the extension.
extend the upstairs ring, to the bedroom and bathroom (both upstairs).
supply a new 32A ring/radial for the kitchen, to supply all sockets, FCUs, everything.

this seems far more sensible to me than having the 13A rings you propose, which are unconventional, unneccessary and very likely to nuisance trip. ideally additional factors need to be considered, such as existing loads, on the circuits that i propose you should extend. but it sounds to me as if this shouldn't be a problem. if you can tell us what loads you have on these circuits (ie how many watts in total each lighting circuit, how many sockets upstairs, etc..), we could confirm.

feedback your thoughts..

 

[astroman]

Hi,

I had thought about extending the circuits but my extension will probably take more power than the rest of the house so I thought a completely seperate consumer unit was the best way to go. You ask abou what my loads will be, as always they are intermittent in the kitchen but I intend to put 4 double 13a outlets on the worktops and 5 in the diner area (less than 100m2 just) the fcus were to supply fridge/freezer, dishwasher and cooker hood/extractor fan. A further radial circuit would supply the cooker. Lighting would be a mixture of low energy and low voltage downlighters, no more than 250w I should imagine. Upstairs 5 double 13a outlets, extractor fan, heated towel rail and shower pump. Upstairs lighting low energy & low voltage, no more than 150w.

There are no spare ways in my consumer unit so to add circuits would mean a new consumer unit anyway and if I added a second consumer unit I wouldn't have to disturb the current wiring.

You mention that I may be subject to nuisance trips, why? If I put a switchfuse before the current RCD, take tails from there to a connection block and supply the "old" RCD and the new consumer from the block as suggested by other members wouldn't this stop nuisance trips?

If I did this I take it that I should use a split load consumer unit to gain RCD protection (the old CU has no RCD built in so could this be why its got an external RCD?)

Thanks for your intrest!

Astroman

 

[user56565]

your upstairs lighting circuit, does it run off a 6A MCB?? if so, you can load 1400W in total onto it. how many watts do you currently have on it, and how many do you want to add on?? (you might want to add on the extractor fan here). if it totals less than 1400W, extend the circuit.

your downstairs lighting circuit, does it run off a 6A MCB?? if so, you can load 1400W in total onto it. how many watts do you currently have on it, and how many do you want to add on?? if it total's less than 1400W, extend the circuit.

your upstairs ring. you say that the new extension is just under 100m² in area. so it may then be necessary to put in a new ring here then.

in practice, other factors need to be considered such as voltage drop, etc... BUT, the point im trying to make above is, by extending circuits, you avoid unnecessarilly creating new circuits, when the existing ones will suffice.

You mention that I may be subject to nuisance trips, why? If I put a switchfuse before the current RCD, take tails from there to a connection block and supply the "old" RCD and the new consumer from the block as suggested by other members wouldn't this stop nuisance trips?

the reason i suggested nuisance trips:

The kitchen/bedroom/en-suite won't be drawing large currents, the cooker is oven only, rated to run off a 13a socket (though I intended giving it its own circuit), the shower is a pump. My plan was for 2 X 13A rings in the Kitchen, 1 for sockets the other for FCUs,

but I intend to put 4 double 13a outlets on the worktops and 5 in the diner area (less than 100m2 just) the fcus were to supply fridge/freezer, dishwasher and cooker hood/extractor fan. A further radial circuit would supply the cooker.

plugging in 2 13A appliances into the sockets will overload a 13A ring straight away. using the fridge and dishwasher could overload a 13A ring. etc..etc.. besides you'll find it hard to find a 13A MCB.

at the end of the day, only you can decide what to be done because you are there. but, go along the following guidelines. extend circuits where you can, subject to voltage drop and area covered by a ring. only install 32A rings (i would use just one 32A ring for all the FCUs and sockets and cooker.)

you should replace your CU anyway, with a split-load, so lights etc.. don't go off, and so sockets are protected by a 30mA RCD. if you want to still put in a second CU, then:

Meter->Switchfuse->Henleys->new consumer units.

the 2nd CU must be supplied with SWA, rated correctly in terms of current capability and voltage drop. as you're only going to do it once, you may want to whack in a fatter cable than the minimum you require.

finally it sounds as if your supply is only 60A.

wired fuses marked not to exceed 60A

you could ask the supplier to uprate this. but regarding the switchfuse, buy the lowest rated fuse you can, but be aware that this may still be 80A.

 

[ban-all-sheds]

regarding the switchfuse, buy the lowest rated fuse you can, but be aware that this may still be 80A.

 

[--tom]

Dear all, could you slowly take me through the installation you have established detailing the order of the devices, cables and sizes of things etc?