How much room needed in duct?

I live in a not spot and have recently noticed a contractor laying BT duct in our area. One of the holes just had a bit of plywood thrown over it but can clearly still see into hole. A concrete chamber has been cast with the new duct above the existing duct running to the village. The duct laying stopped about 2 miles from the village in a place that makes little sense for a new cabinet. The existing cable and duct appear to continue out the chamber and continue towards the village.

The existing duct looks to be around 100mm diameter and the cable within it around 40 or 50mm diameter. Would that leave enough space for pushing fiber though it (for an infill cabinet)? Or would they need to extend the new duct further?

Standard grey BT duct can hold 7 of the traditional fibre tubes (25mm diameter) which is then sub divided into small sub ducts that can accommodate blown fibre which will be multiple fibres.

They in some cases have small fibre cables that can be pushed/pulled and support a higher density of fibres (10-15mm diameter) the aim being to allow them to squeeze fibre into ducting that would otherwise be full.

Without seeing what is in the chamber and relationship to other infrastructure hard to say precisely, it may be chamber you've seen will house the aggregation node, and further sub duct/fibre installed heading to final cabinet position.

There's a good study done for Ofcom on capacity within Openreach's ducts and poles:
http://stakeholders.ofcom.org.uk/binaries/consultati...

Section 3.4.4 on page 26 describes the use of the 25mm (inside diameter) sub-duct within standard 90mm duct - enough room for 7 of them.

That document points out that the "unable space" in a duct isn't consistent along a route, which might explain why the extra duct only appears in a part of the route. The restriction could be the way existing cables are twisted in the existing duct.

BT cable specs can be found here:
http://www.btcables.com/products/voice/outside-plant...
Or
http://www.btcables.com/products/voice/outside-plant...

From that, it looks like 100-pair 0.5mm D-side cable is around 23-25mm external diameter. 400-pair 0.5mm E-side cable is around 38mm external diameter, while 800-pair is 52mm.

It sounds, from context, like your cable is one of the larger E-side cables. Is there a cabinet (or more than one) in your village? How many homes?

Now, while the standard subduct has been 25mm, it seems BT are attempting to use narrower cable than previously, which could use narrower subduct...
http://www.ispreview.co.uk/index.php/2016/03/new-mic...

http://blog.thinkbroadband.com/2016/06/a-peek-at-the...

Also have some pictures of 'classroom samples' that I need to sort through and crop up

Edited by MrSaffron (Wed 14-Sep-16 00:13:42)

There is currently no cabinet in our village, we are fed from a PCP around 5km away . From the existing cabinet outwards, there are approx 150 to 160 properties. We are approx 7.5km in total from the exchange and get around 2 to 3Mb down so that suggests we may be on thicker than standard conductors.

Looking at the cable specs link a 100 pair 0.6mm dia conductor is around 62.5mm outer diameter which would make sense and I understand there are two cables feeding our village, one down either side of the road. It would make sense in that if the original duct was 90mm and already had a 62.5mm cable in it, they had to run in a new duct/cable as another 62.5mm cable wouldnt fit in that original duct.

The cable certainly looked like it took up a lot of the existing duct and at 90mm duct ID less 62.5mm OD cable that only leaves ~27.5mm which would be too tight for running one of their "standard" 25mm fibre ducts, hence the new third 90mm duct being laid.

In reply to a post by MrSaffron:

http://blog.thinkbroadband.com/2016/06/a-peek-at-the...

That "splitter chamber" picture gives a really good idea of what can fit into a 90mm duct - though I guess all the small cables there are the "connectorised"-style lines from the splitter out to the DPs. Really good for the final branches out to near homes.

The feed into the splitter from the spine will be different, I guess. In your older blog post:
http://blog.thinkbroadband.com/2015/09/g-fast-and-fo...
the architecture suggested BFT would be used for that feed. IIRC, 7-BFT is about 20mm diameter, and 12-BFT is around 25mm.

That would make sense.

I wonder, therefore, if the thicker cable only runs so far out to the village, before swapping to something thinner. If there were more room in the last leg of the duct, that might explain why they've only overlaid the new duct for part of the run.

On the other hand, you might expect such a joint to be in an existing chamber, and not to need a new one cast.