Flexi plastic conduit for ring

[Del-inquent]

This is probably going to sound like a REALLY dumb question, apologies I have never used flexi conduit before so literally clueless on it.

We're having to do our refurb in little bits, but need to prepare for the GF ring to be replaced eventually. We have been advised as it's being run under the floorboards (which are being replaced a room at a time)  it would be easiest to run flexible plastic conduit in with pull chords fitted as we go which will a: give a bit of added safety as there has been some historic rodent activity and b: leave it ready for them to then pull wires through later once ready. All new socket chases will be done as well (because they're currently at skirting height) so we thought we might as well meet current regs and put them at 450mm, which will help me as I've got a knackered back.

How do you physically route the conduit with a ring? The ring has obviously got to go up to each socket then then back down to continue the ring, so one flexi chased into the wall with both cables in seems the obvious way to do that, but what happens under the floor as (again obviously) the ring splits goes off in two directions from each socket. Can you get Y adapters for it? Or do you have to chase two flexi's in the wall for every socket and have a single cable in a way oversize conduit for the task? Or can you get smaller than 20mm flexible plastic conduit?

Edited April 11 by Del-inquent

[Mattg4321]

In an ideal world don’t run the cables through the floor void where they are more susceptible to rodent damage and damp. Drop them down from the ceiling void above if that’s an option?

 

Easier than messing around with flexicon

[Del-inquent]

Just now, Mattg4321 said:

In an ideal world don’t run the cables through the floor void where they are more susceptible to rodent damage and damp. Drop them down from the ceiling void above if that’s an option?

 

Easier than messing around with flexicon

I asked the same question, we'd have to do it in one hit to do that which isn't possible, we simply can't do it for a number of reasons. (life has a habit of being an a**e at times). The existing ones have been under the floor for 80 odd years and not done too badly, the flexi allows us to get it ready a little at a time. I just have no idea how it is usually put in lol

 

[Mattg4321]

You’ll want 2x flexis right the way up into each socket if you’re doing it that way. 
 

Make sure you can actually pull the cable through it though. Mostly that stuff isn’t smooth on the inside and can be a pig to pull cables through more than a few meters. 
 

If you’re going to all that trouble though you may as well just 1st fix the ring entirely, cables and all. 

[FuerteStu]

In this scenario, I'd question if you actually need it to be done on one ring? 

 

What's wrong with doing this in a couple of radials? 

 You could commission them as you go, extend them if needed. Much more flexibility and easier for testing. 

 

Rings are a hangup from the days when we only had 4 fuses in the board. 

[Mike]

1 hour ago, Del-inquent said:

Or can you get smaller than 20mm flexible plastic conduit?

You can get 16mm, however there are rules that dictate the minimum permissible size depending on the cable size used. I suspect that 20mm would be the minimum for a ring main, but I'm now more familiar with the rules in France.

 

24 minutes ago, FuerteStu said:

What's wrong with doing this in a couple of radials? 

Good idea.

[ProDave]

Flexi conduit is awful for pulling cables.  Use rigid conduit with SWEPT bends not elbows.

 

Any conduit is virtually impossible to pull a second cable through with one or more there already, so you have to plan it that you are pulling all cables together.

 

In a previous house I did such a refurb by starting with two radial circuits adding on as the next room or part room was done, and they only joined and got reconfigured as a ring upon completion of the final room.

[Mike]

24 minutes ago, ProDave said:

Flexi conduit is awful for pulling cables.

This is true, if you use twin & earth cable.

 

However if you duct it all the way then you can wire it in singles (separate individual conductors for live + neutral + earth), which are much easier to pull.

[Del-inquent]

19 minutes ago, ProDave said:

Flexi conduit is awful for pulling cables.  Use rigid conduit with SWEPT bends not elbows.

 

Any conduit is virtually impossible to pull a second cable through with one or more there already, so you have to plan it that you are pulling all cables together.

 

In a previous house I did such a refurb by starting with two radial circuits adding on as the next room or part room was done, and they only joined and got reconfigured as a ring upon completion of the final room.

 

So you basically ended up with a ring made of 4mm2?

Only reasons I've not thought on radials are
a: I've never planned a radial, so was trying to avoid learning something new at a time where I'm already working 15 hour days 7 days a week 😅
b: the house is a small chalet basic box, the layout of sockets looks like a big circle, so it made sense to join the ends up. 

Maybe I should rethink!

 

[Alan Ambrose]

I appreciate you would like this solved quick and without too much fuss ….

 

But … I think it depends on how bad the vermin problem is, how difficult it would be to take up the floor again to repair vermin damage, and how difficult it is to wire all the way back to the CU.

 

If your electrician has suggested plastic conduit then it indicates they think the vermin risk is high and it won’t be so easy to take the floors up again.

 

So, if you’re not so bothered by the vermin risk or the floor is just screwed down and easy-ish to lift - then just run standard T&E without conduit as normal. (Or maybe if you’ll be moving in a couple of years anyway.)

 

If the floor is not coming up again for another 80 years or the vermin risk is high or you’re planning to live there for a long time, then I would go a bit off-piste and run singles in metal flex conduit without any sharp bends. Then (a) it will be easy to pull new cables in 80-years time and (b) the vermin will end up with blunt teeth. Metal flex isn’t crazy expensive either. I’ve used 25mm OD / 20mm ID in a domestic context and would suggest that if you have room. You may find one size down will work, but you’ll want to check ‘pullability’. Check that the metal flex end glands work with your back boxes OK.

 

Sure, if you can get all the way back to the CU without too much difficulty then radial if you prefer, but I don’t think it’s the major issue here. If you go for the non-vermin proof solution you might consider an AFDD in the CU to mitigate fire risk a bit.

 

Oh and a flexi for each run of course - there’s no Y connector that will help unfortunately.

Edited April 12 by Alan Ambrose

[Del-inquent]

21 minutes ago, Alan Ambrose said:

I appreciate you would like this solved quick and without too much fuss ….

 

But … I think it depends on how bad the vermin problem is, how difficult it would be to take up the floor again to repair vermin damage, and how difficult it is to wire all the way back to the CU.

 

If your electrician has suggested plastic conduit then it indicates they think the vermin risk is high and it won’t be so easy to take the floors up again.

 

So, if you’re not so bothered by the vermin risk or the floor is just screwed down and easy-ish to lift - then just run standard T&E without conduit as normal. (Or maybe if you’ll be moving in a couple of years anyway.)

 

If the floor is not coming up again for another 80 years or the vermin risk is high or you’re planning to live there for a long time, then I would go a bit off-piste and run singles in metal flex conduit without any sharp bends. Then (a) it will be easy to pull new cables in 80-years time and (b) the vermin will end up with blunt teeth. Metal flex isn’t crazy expensive either. I’ve used 25mm OD / 20mm ID in a domestic context and would suggest that if you have room. You may find one size down will work, but you’ll want to check ‘pullability’. Check that the metal flex end glands work with your back boxes OK.

 

Sure, if you can get all the way back to the CU without too much difficulty then radial if you prefer, but I don’t think it’s the major issue here. If you go for the non-vermin proof solution you might consider an AFDD in the CU to mitigate fire risk a bit.

 

Oh and a flexi for each run of course - there’s no Y connector that will help unfortunately.

The vermin problem appears very minor and historic, but is going to be addressed during the works - there is only one place they could be getting in / out without running around inside the house. I think the electrician is more worried about the floors, which won't come up easily in future in any way shape or form. Run to CU is definitely easy as that's being replaced and first socket is 4ft from it!

My main issue with the size of conduit though is from CU down, there are already chases around CU and I think it'll start to get a bit "thin" in those parts. Maybe I should just cap from CU to first socket and conduit from there onwards?

[Alan Ambrose]

Well if the vermin risk is low: 99+% of UK houses use pinned T&E only so that's an option. But you'll of course need to agree any method with your sparky. Discuss the many options with him or her and go from there? Be interesting to know what you jointly decide.

[Del-inquent]

On 12/04/2024 at 11:55, Alan Ambrose said:

Well if the vermin risk is low: 99+% of UK houses use pinned T&E only so that's an option. But you'll of course need to agree any method with your sparky. Discuss the many options with him or her and go from there? Be interesting to know what you jointly decide.

Okay so just to throw a curveball in, we went with flexi conduit option for a few reasons, however after getting a load done another problem arose which means we are now looking at ripping up the whole ground floor and insulating / screeding (which I’ve got lots of very helpful advice on here about)

 

is there any reason that the conduit can’t just sit under the insulation and screed? Sparky scratched his head on that one.

[ProDave]

On 11/04/2024 at 23:50, Del-inquent said:

So you basically ended up with a ring made of 4mm2?

No, 2.5mm  And for the period it was an incomplete ring, the two ends were each fed from their own 16A MCB  which was changed to a single 32A MCB when the ring was completed.

[ProDave]

8 minutes ago, Del-inquent said:

is there any reason that the conduit can’t just sit under the insulation and screed? Sparky scratched his head on that one.

Under the insulation is fine.  It's when it is buried within the insulation you need to worry about derating cables.

[Del-inquent]

14 minutes ago, ProDave said:

Under the insulation is fine.  It's when it is buried within the insulation you need to worry about derating cables.

When you say buried… the insulation (190mm thick!) will have to have a chase cut out of the underside, as the conduit would lay on the concrete slab and the insulation going over the top. With my cutting skills probably end up with the chase in the insulation being twice the size of the conduit 😂 but as it’s 190mm thick that doesn’t worry me, does that count as “buried”?

[Del-inquent]

20 minutes ago, ProDave said:

No, 2.5mm  And for the period it was an incomplete ring, the two ends were each fed from their own 16A MCB  which was changed to a single 32A MCB when the ring was completed.

Clever, I like that!