Wiring advice/confirmation needed please

[HamFistedOaf]

Hi everyone,

New guy here; I am competent with basic electrical wiring / conscious of safety. I have completed relatively simple tasks such as wiring extractions fans, installing storage heaters and convectors, and have helped others with more complicated stuff like installing new circuits. I want to build my experience by doing some minor wiring jobs as follows:

please can someone more experienced with leccy confirm the below is correct/ok:

I have two minor wiring projects to do:

- lights in the loft (2 60W bulbs)
- power supply to built-in closet I am converting to hold a PC etc an "office"

For convenience/speed I wish to use radial circuits.
Both cable runs are short (~5m max) and tacked to surfaces
I have spare MCBs in my brand new consumer unit as follows:

-for the loft lights, I will use a 6 amp MCB
- for the power supply, I will use a 32 amp MCB, in an RCD protected bank

There is likely better sized MCBs available, but these are what I have (trying to keep cost down).

Cabling is as follows:

- for loft lights, 1.5 mm2 twin and earth, linked to a switch and then a junction box feeding two batten bulb holders
- for power supply 4mm2 twin and earth, linked to a double socket on other end (I will put an extension cable in for more sockets).

I would appreciate any advice as to whether or not this is acceptable - please do advise on any errors, especially if the cable sizes are OK or not in radial circuits with the stated MCBs.

I have some questions re the connections I will use:

- does the light switch have to be fused (size?) / rated?
- what rating of junction box? (anything above MCB rating?)
- is there a rating for the double socket in the power supply?

Also, I believe it is OK to use 4mm2 cable with a 32 A MCB, but as the cable is rated for 32 A itself, isnt this cutting it a bit fine? I understood a margin between the cable/MCB rating was desirable?

Thank you very much for any advice - sorry for the big post!

 

[skenk]

If you're in a tenement, flat or house with 3 or more floors, you're meant to have a building warrant to do this. As far as i can tell, the nominal capacity for 4mm not in conduit or thermally insulating materials is 37a so you should be fine with a short run. With the 17th, the lighting circuits should be on rcd too if they're buried less than 50mm deep in a wall and not in steel conduit. . . If they're clipped to surfaces all the way, not hidden in walls, then i don't think they need to be on RCD.

Other than that, you need to take into account the extra load on the installation, do you really need 32a for the 'office', what you running that needs all that? If you've got electric cooker, shower, heating or other stuff that uses a lot of current then you could be putting too much load on the system.

The 'fuse' for the lighting circuit is the 6a mcb, you don't need anything else. If you must use a junction box then yeah it should be rated as per the mcb or higher (i think?) Why not stick in, say, 3 doubles in the office, better practice than using extension leads.

- is there a rating for the double socket in the power supply?

eh?

Yeah and it should be tested and installation certificate and test results recorded . . .

 

[streetlighter]

Use a 20 amp 4 terminal junction box as they have more space inside to do a neat job those 5 amp white things are a right pain to install.
Or you could run the feed to the switch and the cable from the lights into the swich box aswell which would do away with a junction box.

 

[HamFistedOaf]

If you're in a tenement, flat or house with 3 or more floors, you're meant to have a building warrant to do this.

Hi there
Thanks a lot for your help. The guy who put in my new consumer unit + wiring is a qualified electrician, though it isnt his main profession - would you advise him doing it rather than I, if regs are strict?

As far as i can tell, the nominal capacity for 4mm not in conduit or thermally insulating materials is 37a so you should be fine with a short run.

Great - take it 5m would be classed as 'short'?

With the 17th, the lighting circuits should be on rcd too if they're buried less than 50mm deep in a wall and not in steel conduit. . . If they're clipped to surfaces all the way, not hidden in walls, then i don't think they need to be on RCD.

These will just be tacked to beams in the loft, so OK here I think.

Other than that, you need to take into account the extra load on the installation, do you really need 32a for the 'office', what you running that needs all that? If you've got electric cooker, shower, heating or other stuff that uses a lot of current then you could be putting too much load on the system.

I see what you mean. It wont use a lot of power - just power supply for monitor, PC, modem, speakers and printer, plus maybe a desk type spot lamp or two. I am building the stuff into the cupboard so I can use the computer room as a 2nd bedroom, its small but you can slide the keyboard tray in/out and close the doors, so the room "finish" is the same.

Ultimately, I am using the 32 A circuit as that is the size of available spares in the consumer unit. I dont want to get into the realms of choosing a new/compatible MCB unit at a different rating lol I do have the shower, oven, electric heating etc - however, use is rarely/never coincidental, and as I say the computer + accessories wont use much power anyway.

The 'fuse' for the lighting circuit is the 6a mcb, you don't need anything else. If you must use a junction box then yeah it should be rated as per the mcb or higher (i think?) Why not stick in, say, 3 doubles in the office, better practice than using extension leads.

Gotcha - thanks, esp re the multiple sockets rather than extension.

eh?

lol take it thats a no!

Yeah and it should be tested and installation certificate and test results recorded . . .

Thanks a lot for your help skenk - much appreciated!
cheers

 

[HamFistedOaf]

Use a 20 amp 4 terminal junction box as they have more space inside to do a neat job those 5 amp white things are a right pain to install.

Thanks for the tip - I do hate cramped fittings!

Or you could run the feed to the switch and the cable from the lights into the swich box aswell which would do away with a junction box.

Ah, so I can run multiple outlets straight off the switch itself - do their enclosures have holes for >1 wire? That would be much simpler - thanks.

Do switches come with ratings, like junction boxes, or is a switch just a switch?

Cheers!

 

[plugwash]

If you are taking the main feed to the switch you will need to get a bit of terminal block in the back box for the neutrals. Given this and the space needed for the extra wires I would use a 25mm box if looping at the switch.

 

[skenk]

No worries

A lightswitch is normally rated at 10a, so more than enough for the 6a circuit.

Re the cable being clipped to beams in the loft, is the consumer unit up there as well? because if the cable is in a wall at any point and not at least 50mm deep or in steel conduit then it would still need rcd protection, however the new rules don't come in until July(?) so you'll be fine if you do it soon..cables are often date-stamped so even if you bought the cable now . . . but then i guess it depends on the date on the test sheets - see further on.

Are you in a flat or 3 story house? or a normal 1-2 storey house? because this could matter if/when you come to sell in the future -

An MCB costs about a fiver. The cost of 4mm, compared to the cost of 2.5mm means you probably wouldn't be much worse off getting the lower rated . . . You can have a 20a mcb and use 2.5mm will be more than enough for pc, printer etc.

Your mate who did the new wiring, did he do a complete re-wire or just a new consumer unit and some new stuff? I was taught that if a circuit has been altered (in your case all of them if you have a new 'board) then the entire circuit that has been altered should comply with current regs - correct heights of switches and sockets etc. But there is some debate on here as to whether or not thats right.

Yeah 5m is short, the 37a rating is subject to various calculations about the conditions of the circuit, and it drops to 32a if installed in thermally insulating material (in addition to the other calculations) but as long as it's not in the thermally insulating material then the other factors wouldn't reduce it by anything like 5a. So you would be fine, but yr still better off with a 16a or 20a mcb and 2.5mm if u ask me.

A double socket is normally rated at 13a, you can still have this on a 32a circuit because the max size of fuse in anything that could be plugged in is 13a (then there is the debate about the double socket being rated at 13a but can be run at 26a due to it being a double - or even more due to the vagaries of fuse breaking capacities, but lets not get into that)

I'm really not too sure about the regs here in Scotland, certainly there is no part P, so any talk of that can be discounted (the wiring regs BS7671 is the same England and Scotland but the building regs are different) As I mentioned above most works in flats and 3 storey houses should have a building warrant but i'm not exactly sure how this is enforced, theres not many scots on here at all but do a search for bald electrician and there is a post about someone who didn't have one for a loft extension and had major hassle when selling, but then that was a loft ext and a rewire is a rewire so how they would know it had been done, or when, i don't know (assuming the absence of certificates).

Certainly in general though any new work should be inspected and tested as a matter of course, and for safety. I just don't know what happens (in Scotland) if you don't do this. Also I don't really know how they define 'competent' in Scotland, i've heard that you MUST be NICEIC or SELECT certified from some, but then i've just done new wiring at a flat i own, subject to a building warrant, and was told (on the phone) by building control that i could inspect and test myself, and sign the certificates. (I am qualified in the wiring regs -C&G 2381) I've sent the forms in so i'm just waiting to see what happens now. To make matters even more confusing it appears that different local authorities have different rules.

If anyone could shed any more light on the rules in Scotland, Glasgow in particular, i'd be very grateful because i'd like to do more work for other people, but i don't even know if i'm considered to be competent to do even minor works where a warrant isn't required.

As far as you're concerned, i wouldn't want to make a call on if you're 'competent' (the official scottish definition or otherwise) to do it yourself but maybe one of the regulars might have an opinion on that.

Good luck anyway mate.

 

[HamFistedOaf]

If you are taking the main feed to the switch you will need to get a bit of terminal block in the back box for the neutrals. Given this and the space needed for the extra wires I would use a 25mm box if looping at the switch.

Hi plugwash.

So the live/earth go through the switch, and a seperate terminal connection for neutrals?

Thanks for the tip!

 

[HamFistedOaf]

A lightswitch is normally rated at 10a, so more than enough for the 6a circuit.

Great, I will double check the rating of any switch used.

Re the cable being clipped to beams in the loft, is the consumer unit up there as well? because if the cable is in a wall at any point and not at least 50mm deep or in steel conduit then it would still need rcd protection, however the new rules don't come in until July(?)

The CU is in a small cupboard off the hallway. The route both cables will take is straight up into the loft directly above the CU. There is a plastic duct (running from CU to ceiling) already installed for this - but it is very spacious and in no way cramped, so no worries about overheating or cramped cables.

Once into the loft, cables are tacked to beams/rafters as necessary. The power supply will descend back through the ceiling above where I aim to relocate my PC etc to. The lighting circuit will just terminate in the loft at the batten lamp holders.

Are you in a flat or 3 story house? or a normal 1-2 storey house? because this could matter if/when you come to sell in the future -

It is a top floor flat - 4rth floor. I am lucky in the sense that having the loft available makes running cables very easy, but I take your point re the regs and things.

An MCB costs about a fiver. The cost of 4mm, compared to the cost of 2.5mm means you probably wouldn't be much worse off getting the lower rated . . . You can have a 20a mcb and use 2.5mm will be more than enough for pc, printer etc.

I didnt realise they would be so inexpensive. Perhaps the 20 A MCB + 2.5mm2 t+e is the way to go. Regardless of who does the job, I will be purchasing the parts. Is essentially "any" 20 A MCB acceptable? The consumer unit and current MCBs are all made by Wylex. I take it any manufacturers MCBs would be acceptable to swap in place of one of the current spares?

Your mate who did the new wiring, did he do a complete re-wire or just a new consumer unit and some new stuff? I was taught that if a circuit has been altered (in your case all of them if you have a new 'board) then the entire circuit that has been altered should comply with current regs - correct heights of switches and sockets etc. But there is some debate on here as to whether or not thats right.

It was almost a full re-wire. All that remained were some existing cabling for double sockets and lighting. The CU, majority of heating wiring (re-used some convector power supplies), much lighting (installed new recessed spots in 3 rooms), shower supply etc is all new. Anything re-used was in good condition, as the flat isnt old in building terms anyway. Im confident the guys work was of a good/safe standard - he is very particular about safety, tested all his work with meters etc.

Yeah 5m is short, the 37a rating is subject to various calculations about the conditions of the circuit, and it drops to 32a if installed in thermally insulating material (in addition to the other calculations) but as long as it's not in the thermally insulating material then the other factors wouldn't reduce it by anything like 5a. So you would be fine, but yr still better off with a 16a or 20a mcb and 2.5mm if u ask me.

Yep - I think I will take your advice, re a lower rated circuit, assuming I can easily get a 20 A MCB, as per my Q above. I would prefer to work with thinner cable anyway for ease.

A double socket is normally rated at 13a, you can still have this on a 32a circuit because the max size of fuse in anything that could be plugged in is 13a (then there is the debate about the double socket being rated at 13a but can be run at 26a due to it being a double - or even more due to the vagaries of fuse breaking capacities, but lets not get into that)

I think I will take your advice and use multiple 13 A sockets on the radial, rather than an extension, however as previously stated the PC etc will not use much power at all (not sure exactly, but well under 1KW anyway).

As far as you're concerned, i wouldn't want to make a call on if you're 'competent' (the official scottish definition or otherwise) to do it yourself but maybe one of the regulars might have an opinion on that.

Thanks - Id appreciate anyones advice on this. I am essentially just a DIYer / hobbyist, so something of a layman with regard to regulations etc. The job does not phase me though - the wiring up is all straight forward; the only thing I havent done before is hook new cable up to an MCB - however, I have experience with old type fuse boxes - both cartiridge fuses and fuse wire, so Im sure with that experience and ensuring the supply is safely isolated, I should have no problem.

I take it the live wire can be connected to the MCB when it is de-mounted from the CU, and then just slotted back in (?) with the earth and neutral going to the relevant terminals inside the CU.

Thanks again for all your advice.
Cheers