wiring garage in 20mm conduit - choosing singles?

[GCarnegi]

I've run conduit through my garage for mains and lighting (on different circuits) but am struggling to get the T&E through the conduit (1 x 1.5 + 1 x 2.5 pretty much maxes the conduit out).

As such, I think I'll need to switch to using singles instead of T&E.

Screwfix sells conduit cable but it looks like it comes in a couple of different "grades".

6419X cable seems to be about £28 per reel, whereas 6419B cable is about half that price. I can't see the difference other than the price.

I assume for domestic use, I should use a blue, a brown and a green/yellow?

I'll use 2.5mm for both lights and mains to save buying more reels.

What's the best way to "join" cables in a conduit box - are there "crimps" or something like that available, or should I use screw-terminal-blocks (I guess they might be a bit "heath-robinson" and I want to do things the right way.

Lastly, clutching at straws here, can I strip the singles out of the T&E I've already run and use those instead of buying new rolls (with the addition of an earth wire)?

thanks
Guy

 

[Iggifer]

the difference in the two prices at screwfix are between standard cable and LS0H meaning, Low Smoke, Zero Halogen

http://en.wikipedia.org/wiki/Low_smoke_zero_halogen

Yes, blue, brown and green/yellow

Where will you need to join the cables in the conduit? Surely with a 100m drum you will just be able to pull it through to where it needs termintating? You can get conduit fittings for wiring in things like switches etc, gives you more room to work with

I will let some of the more experienced guys weigh in on the other questions

 

[Space cat]

I'll use 2.5mm for both lights and mains to save buying more reels.

That'll be very confusing to anybody who comes to work on the circuits later - and maybe to you too! You might also have a problem getting 2.5 sq mm cores into the terminals on some light fittings.

can I strip the singles out of the T&E I've already run

Yes, if you've got the time. (One advantage of DIY is that your time isn't money. ) But it'll be a labourious job - and remember that one slip into the core insulation will ruin a whole length. I would buy new wire and save your existing T&E for future use.

 

[holmslaw]

..

 

[Iggifer]

If he's stripping the T+E he can just grab the cpc and pull, hard. As he's not going to be able to use it and it doesn't matter if it breaks. This will avoid damaging any cores

 

[GCarnegi]

Yes, that's what I was going to do, but I've just ordered the singles from Screwfix instead.

Looks like the blue and green/yellow were in the clearance section at £15 a roll, but had to pay full price £30 for the brown. Still cheaper than £42 +VAT per roll from local City Electrical!

6491B 2.5mm LSF Conduit Wiring Green/Yellow 100m
Product Code: 89850
£17.39 1 Next Day

6491B 2.5mm LSF Conduit Wiring Blue 100m
Product Code: 48446
£15.00 1 Next Day

Tower 1-Core Reel Conduit Cable Brown 2.5mm² x 100m
Product Code: 69868
£31.60 1 Next Day

Wierd thing is they're 7-strand, not solid core, but I don't suppose that matters, does it? (I guess i can cancel if it does)

Need to join to T&E at some point as the garage ring also supplies the utility room at the back, which is plasterboarded and does not have conduit. T&E is already run in the uility room, with a few feet of T&E sticking into the garage via the roof. I need to terminate the conduit into the garage ceiling after joining the singles into the T&E.

Mains isn't so bad as the T&E already reaches the first socket in the garage (can just strip off the grey outer inside the conduit) but light wires will need to be directly spliced together. (1.5mm T&E solid to 2.5mm single 7-strand) [x3]

 

[riveralt]

What is the total length of the conduit?
Does it incorporate bends and long straight runs.
If so how many bends are there?
Have you calculated the cable factors and compared that to table 5D in the OSG.
Why aren't you just using Twin & Earth clipped direct?
What is the likelihood of it getting damaged.
Is this a new circuit?
Is it protected by an RCD?

 

[GCarnegi]

What is the total length of the conduit? (Probably about 20 metres)
Does it incorporate bends and long straight runs. (yes, and tee's)
If so how many bends are there? (about a dozen by the time I include the drops to the sockets and cables to the four ceiling lights)
Have you calculated the cable factors and compared that to table 5D in the OSG. (No)
Why aren't you just using Twin & Earth clipped direct? (Can I?, this is a new extension that I'm wiring and the electrician who intends to inspect and sign it off suggested I used conduit)
What is the likelihood of it getting damaged.(It's a workshop / garage - lots of grinding, welding carrying panels around so although damage is still unlikely, it's not AS unlikely as it being purely somewhere to park the car)
Is this a new circuit? (Yes)
Is it protected by an RCD? (Yes)

Read more: //www.diynot.com/forums/electr...hoosing-singles.313365/#2305623#ixzz1mvIxUn7A

Okay, I will, thanks.

 

[riveralt]

The electrician should have done the calculations for you to determine the cable factor given the length of the run, the number of bends, the size of the conduit and the size of the cables.
There is a set ratio that is required for cables fitted inside conduit.
The longer the run the more the bends the lower the cable factor.
So for example - a 2.5mm has a cable factor around 30. So effectively two T&E 2.5mm cables converted to singles i.e. 3 * 2.5mm * 2 = 6*30 =180 (total cable factor) air/cable ratio.

But according to Table 5d OSG for a total run length of 2.5 metres with just four bends in 20mm conduit the max cable factor is 141.
So you will need a larger conduit or you need to consider running them in parallel or consider trunking. But the best course would be to speak to your electrician first.

Also just noticed your potential use of the garage - some welders require their own circuit.

 

[ban-all-sheds]

Have you calculated the cable factors and compared that to table 5D in the OSG. (No)

Why not?

What reason do you think there is why your design does not have to be done properly?

 

[GCarnegi]

Dear readers.
Please see above a couple of beautiful examples of different ways to respond to a simple, well intended question.

One of them is helpful, acknowledges that I'm not a professional, and also that most DIYers (to whom this site is directed) wouldn't know what that was, and explains it in enough detail for a diyer to understand.

The other (what is that font... obnoxious Heavy Bold?) mistakenly thinks that DIYers should know just as much as seasoned professionals who have so much satisfied customers and return business that they have no time to sit around on the forums all day, or else doesn't think that but decides to rub diyers noses in the fact thay they see themselves as somehow "better" than normal people. (And expects them to be respectfully polite in their responses)

Welcome to the priviledged few BAS.

Sorry, I don't have that font installed on my computer, othewise I'd have used it above.

Love and kisses,

 

[GCarnegi]

Okay, so does that mean that a t&e 2.5mm cable, converted to three singles needs 180 times its own cross sectional area as air within the conduit?

2.5 x 180 is 450, plus the 3 x 2.5, means that the total cross sectional area for the ID of the conduit for only these three singles, should be around 24mm (OD larger obviously) - and that's just to replace a SINGLE length of 2.5mm T&E).

All the sockets and switches I have only have 20mm knockouts. Can you point me in the direction of off-the-shelf metal sockets etc with larger knockouts as I can't find any.

As soon as I include a set of lighting cores, that minimum ID probabaly goes up to around 35mm. I think that's more in the realms of waste pipe, isn't it, and not sure how to get it into accessory boxes.

Remember that what was asked was the TOTAL length of conduit in the garage, not the total length of a particular core within the conduit. I have around 5m conduit on the ceiling for the lighting cores (no mains there), and around 5m on each of three walls. A couple of corners or tee's on each wall, and another handful on the ceiling to navigate to the four ceiling lights.

 

[holmslaw]

..

 

[ban-all-sheds]

The other (what is that font... obnoxious Heavy Bold?) mistakenly thinks that DIYers should know just as much as seasoned professionals

I am not mistaken.

If you choose to take on the task of designing electrical installations then you

MUST

(that big enough for you?)

do it properly.

I'll ask again - why do you think that circuits you design and install are not subject to the same constraints as those designed by professionals?

 

[riveralt]

Okay, so does that mean that a t&e 2.5mm cable, converted to three singles needs 180 times its own cross sectional area as air within the conduit
2.5 x 180 is 450, plus the 3 x 2.5, means that the total cross sectional area for the ID of the conduit for only these three singles, should be around 24mm (OD larger obviously) - and that's just to replace a SINGLE length of 2.5mm T&E).
?

The calculation I did was for two 2.5mm2 T&E cable. For one cable converted to 3 *2.5mm singles the figure is 90 which is within the criteria for 2.5metre (20mm) conduit length with four bends. Incidentally the same measurements and bends but with 25mm conduit allows you a cable factor of 260.
You have to remember that when cables are grouped together in confined space or in insulation the cable current carrying capacity is derated because of the potential heat generated.

All the sockets and switches I have only have 20mm knockouts. Can you point me in the direction of off-the-shelf metal sockets etc with larger knockouts as I can't find any.

You might have to drill them or see below.

As soon as I include a set of lighting cores, that minimum ID probabaly goes up to around 35mm. I think that's more in the realms of waste pipe, isn't it, and not sure how to get it into accessory boxes.

Again for the example given I assumed 6 2.5mm2 singles - if you are using lmm cable = 16 cable factor or 22 for 1.5mm.

Remember that what was asked was the TOTAL length of conduit in the garage, not the total length of a particular core within the conduit. I have around 5m conduit on the ceiling for the lighting cores (no mains there), and around 5m on each of three walls. A couple of corners or tee's on each wall, and another handful on the ceiling to navigate to the four ceiling lights.

There is why it is very important to consider all the factors in designing circuit of this type. Such as likely load, cable runs, cable sizes, mechanical protection or not etc.
For example you may want to consider fitting a Garage consumer unit in the garage itself and split the load across three or four circuits. Consider if your lighting actually needs conduit protection?
There is a lot to consider and either you should sit down and do all the calculations (which is what we have to do) or get the electrician back to properly design the circuit(s) and explain to you how he wants the cables run.