Polyduct?

[benten69]

Hi all,

New to the forum and looking for some advice. Basically, I have just purchased a new house and the previous owner decided to run some 6mm Twin Earth to the garage, underground in some PVC pipe over a distance of about 20 meters, and only burried about 15cm underground!

As I am getting the drive re-done I am looking to re-do with some 10mm SWA and 32mm polyduct, burried to about 50cm under the new drive.

Now the electrical wiring side of things is no bother what-so-ever. Nice easy job.

However when it comes to the polyduct I have a few queries, so first little background on how the current Twin Earth is laid. When you come into the house through the front door there is a little vestabule where the CU is located. The wall at the side (to the outside world) has been drilled through, then the 6mm Twin Earth has been pulled through the hole, into PVC pipe going vertically down the wall (about 1 foot), underground from the front of the house to the side of the garage, up 1 foot of PVC pipe attached to the side of the garage, then through a hole at the bottom of the garage wall.

So I would like to follow the same route (through the holes in the side of the garage and house that already exist).

I was planning on running the SWA out of the hole in the garage wall, going down the side of the garage wall (clipped direct), into the polyduct which will be laid underground, running it all the way to the front of the house, then exiting the polyduct, going up the wall on the outside of the house (clipped direct), then into the existing hole in the wall and into the house.

My questions are as follows......

1. Do I need to create a proper seal at either end of the polyduct, with some kind of glanding, silicon or something? Basically, I am thinking about moisture getting in.

2. If I need to make a good seal where the SWA exits the polyduct, how do I do it?

3. Can I just leave the polyduct horizontal on the ground and have the cable come out of the polyduct, then make a shallow 90* bend in the cable to bring it vertically up the wall?

4. Would it be better to I make the shallow 90* bend in the polyduct itself so the cable exits the polyduct in an already vertical position?

5. If it is the latter of questions 3 & 4, do I need to seal and prevent dirt, etc getting into the polyduct as it gets burried, and if so, how?

I appreciate this has been a large post, and I would greatly appreciate some advice on the above. I suspect I may be over-thinking it, but would rather do it right the first time, as while the wiring is no issue, I don't tend to do any work with stuff underground, so looking for advice from those who have done it before.

Thanks!

 

[Richard Burns]

Generally moisture will always get into duct, however sealing the duct with a removable seal, so that changes can be made later, is a good idea to prevent rodents, dirt and bugs getting in.
Packing the end with a stable solid such as pvc foam and then covering the end with a sealant would be easiest, keep the end of the duct above ground so it is accessible.

 

[Spoon]

If you are already putting in a new ducting for the power you might want to also put a small ducting in for other cables, like Ethernet, phone or alarm.

 

[benten69]

Thanks guys, but after a bit of deliberation on another forum I have decided to move away from the idea of ducting and opting to bury the SWA directly into a 600mm deep by 300mm wide trench, surrounded by 100mm sand above & below, then backfilled with soil and tape running 150mm below the surface. Purely due to the costs in all honesty, ducting isn't cheap!

If it's not a requirement, then it's money better spent elsewhere on the house. As for ethernet, I have (in the past) had great success with those powerline adapters which use the electrical wiring to transmit ethernet comms, so I can always use them in the future.

However, I have another query.....as the 10mm SWA is pretty thick and bending it 90* to go through the house in the wall while keeping the cable flush against the wall will prove nearly impossible I was thinking of terminating the armour into an IP56 rated external box, then pulling the inner PVC + cores and a seperate earth connection (for the armour) into the house / garage, out of the back of the box & through the wall. The end result will look just like this....



However I have been told it is against regs to do it like that unless it has an RCD on it, which is fine, as I have an RCD in place already for this circuit, which supplies 1 single MCB on the home CU, feeds through to the garage, then the garage CU has its own RCD also, which feeds 2 MCB's, so 2 RCD's in series.

I know it is not ideal, but one guy was saying this would be a bodge and the only proper way to do it is to run the SWA (with armour) into the house/garage then terminate into an MCB (without RCD protection) at the house and only have RCD protection at the garage CU.

Is it really a bodge to do it how I said? Surely having 2 RCD's in series, while not ideal, also gives an additional level of redundancy, should the one in the garage fail to trip in a fault situation. The only thing is if the one in the house trips I need to walk to the house (about 30 seconds) to reset it.

Any thoughts & advice? Can I do it the way I have suggested or do I HAVE to run the full SWA (with armour) into the house / garage and terminate internally?

Thanks!

 

[Spoon]

All cables have a minimum bend radius. Even when you remove the armour from your cable the rest of the cable will still need to adhere to the min bend rad.
Me personally don't mine the idea of having just a RCD in the house. They don't trip very often, so walking back to the house shouldn't be a problem. If you can put a RCBO in your CU for the supply then that would be better.
It all depends on how a cable is installed if it should have RCD protection.

 

[benten69]

I understand the bend radius issue, and with the armour in place there is no way I can adhere to the min bend radius required while keeping the cable tidy. However, if I do an installation like the picture above I can easily adhere to bend radius requirements. Hence I was wanting to terminate the armour into an external IP56 rated junction box.

I can fit an RCBO to the CU in the house no problem, and is actually something I considered doing. So would I be correct in saying that if I have in install like the picture above at both ends (with the cable buried underground) then at the supply it is connected to an RCBO in the house and in the garage is connects to an RCD which then feeds 2 MCB’s (sockets and lights), it would be within the regs?

Obviously assuming proper termination, workmanship, etc.

1 further question. I understand that for ducting, a single cable cannot occupy more than 53% of the available ID. Does the same apply when pulling a cable through a hole in a wall which is about 500mm (50cm) thick? So to pull the inner PVC of the SWA (about 13mm OD) through the wall, does the hole in the wall have to be a minimum of 25mm diameter (cable is 53% diameter of the wall)?

Or does the 53% only apply to actual ducting?

 

[Richard Burns]

No problem with your box terminating armoured and taking another cable through the wall, just maintain earth continuity to the armour.
You do not need RCD protection for a cable passing straight through a wall. (Though you may need it for other areas of the cable run)
Two RCDs in series is not a critical problem in domestic. Generally just costs more and is more inconvenient so if you can avoid it why not but not a big problem if it is there.

 

[bigspark17]

Bury the swa direct and put a duct for ethernet phone tv ect.

 

[Midwest]

Isn't it a bit naughty to be asking advice from two different forums, like you don't trust one or the other? Or is it just me being a grumpy old git!

 

[Spoon]

Isn't it a bit naughty to be asking advice from two different forums, like you don't trust one or the other? Or is it just me being a grumpy old git!

I did think that just for a second as well, then I remembered it's Friday, then I was happy again.........

 

[benten69]

Isn't it a bit naughty to be asking advice from two different forums, like you don't trust one or the other? Or is it just me being a grumpy old git!

Just getting a 2nd opinion is all. Don't see anything wrong with that.

EDIT: Can someone reply to my post #6 above about the hole diameter? Due to me being a new member my posts need approving which means several people have replied before the post has even shown up.

 

[HandySparks]

... I was thinking of terminating the armour into an IP56 rated external box, then pulling the inner PVC + cores and a seperate earth connection (for the armour) into the house / garage, out of the back of the box & through the wall.

OK, providing that the part with the armour removed is properly enclosed, preferably by running it straight into the back of the CU. The inner pvc bedding is not an adequate covering over the inner cores if exposed. It's a good idea to put a bit of plastic conduit through the wall, especially if it's a cavity wall.

...However I have been told it is against regs to do it like that unless it has an RCD on it...

No requirement to RCD protect the cable unless you're going to bury it in the wall.

 

[benten69]

OK, providing that the part with the armour removed is properly enclosed, preferably by running it straight into the back of the CU. The inner pvc bedding is not an adequate covering over the inner cores if exposed. It's a good idea to put a bit of plastic conduit through the wall, especially if it's a cavity wall.




No requirement to RCD protect the cable unless you're going to bury it in the wall.

The part with the armour removed will be coming out of the back of the junction box (via a rubber grommet), straight through a solid brick wall (no external exposure) and into the cupboard where the CU is, then clipped direct for about 1 meter before entering the CU unit itself in the house. In the garage it will be similar, but will need to run for a bit longer (2 meters maybe) before terminating in the CU.

The end result should look identical to the picture I posted above.

Should I still use plastic conduit through the wall seeing it is solid brick right the way through? Also, with the inner PVC bedding having an approx 13mm diameter, how large should I drill the hole in the wall? Do I still need to adhere to the max 53% capacity for single cables? So a 13mm diameter cable will need a 25mm diameter hole? Or is that only for actual ducting and doesn’t apply in this scenario?

 

[Midwest]

Just getting a 2nd opinion is all. Don't see anything wrong with that.

I was always taught that was being very rude to do so, as I previously said. This is forum where people will offer their advice and opinions based on expertise and experience. Sometimes we get it wrong (I know I do), but we are corrected by our peers. You could be getting conflicting advice from the two forums. I would suggest you stick with one or the other IMO

 

[HandySparks]

The part with the armour removed will be coming out of the back of the junction box (via a rubber grommet), straight through a solid brick wall (no external exposure) and into the cupboard where the CU is, then clipped direct for about 1 meter before entering the CU unit itself in the house. In the garage it will be similar, but will need to run for a bit longer (2 meters maybe) before terminating in the CU.

The end result should look identical to the picture I posted above.

Should I still use plastic conduit through the wall seeing it is solid brick right the way through? Also, with the inner PVC bedding having an approx 13mm diameter, how large should I drill the hole in the wall? Do I still need to adhere to the max 53% capacity for single cables? So a 13mm diameter cable will need a 25mm diameter hole? Or is that only for actual ducting and doesn’t apply in this scenario?

Plastic conduit through the wall makes it easier to get the cable through. Don't worry about the size as long as you can poke the cable through without damage.

I would be more concerned about clipping direct the swa cores and bedding. Put it in some properly installed conduit or trunking.

 

[benten69]

Plastic conduit through the wall makes it easier to get the cable through. Don't worry about the size as long as you can poke the cable through without damage.

I would be more concerned about clipping direct the swa cores and bedding. Put it in some properly installed conduit or trunking.

Perfect! Thanks for that! Will run some conduit for the bedding/cores in the house/garage and use some of it to feed through the wall also.

 

[Richard Burns]

The hole in the wall should be close to the cable size, the wall itself will act as a heat sink to allow the cable to cool, this would not be the case in ducts or trunking especially with lots of cables, a single cable is not so much problem.

SWA with the armour removed is not considered as protected by a sheath and so this would constitute exposed insulation which is not permitted.
Therefore it is good that you are enclosing it in conduit.