Guidance on sizing cable ducts for LV armoured cables

Are these the sort of tube like duct to be threaded by pulling ropes through (example)  the sort of thing you may poke a 10mm SWA through, or the concrete channel kind that you lay the cable in from the top ( example) These tend to be biggeer cables or longer runs..

In either case ‘larger than you think’ makes threading up easier ;-)  Usually while you are digging, it is getting the kit there rather than the exact size of the groove that sets costs, and it is the ease of threading or the number of access points that makes or breaks the job.

 I suggest that anything more than ⅓ full is under sized, but I cannot point to a standard figure form anywhere to justify that.

Are you grouping cables that are running near the thermal limit, or is it long enough that voltage drop is the limiting factor ? 

Mike.

The rigid ducts (plastic one) and the length for the duct run will be fairly small , as the cable needs to under the gate. However, the overall length of the cable run is fairly long and will be use cable trays.

No grouping for cable.

The cable will be fairly big in size.

I came across this but not sure if its accurate.

 I guess I could ask the seller for rigid ducts for some guidance in terms of sizing.

Regards,

J

There is another point worth considering, that is if there are any bends you will need a much bigger duct than expected or the pull force will be immense or impossible. The telephone system is interesting to copy, it uses 100mm ducts for large cables but the runs are always straight with manholes at any change in direction. Ducts used to only have one multipair cable each of perhaps 40mm diameter, but they were lead sheathed types. Modern practice is to use thinner cables and therefore some constraints have gone. There is little point in using ducts less than 100mm, the cost is in digging, not the duct itself. The big question is do you need a duct? Direct burial is often perfectly satisfactory and reduces the chance of damage as the ducts usually become full of water, and sheath defects are not unknown. Correct backfill will mean that immersion changes to damp conditions, which is probably more satisfactory.

A single  cable in one pipe the width of a driveway sounds quite tame. Even so, put something like a length of poly rope through the duct while straight before you bend it into position - as Dave comments the pulling forces can stack up quiet fast and pulling is easier then pushing.  (I have seen cables that had to be pulled in by forklift truck and it is not pretty sight.)

Sometimes duct is supplied with cord inside, - you can use it to pull in something bigger.

Even then the yellow greasy stuff is worth applying with a rag as the cable goes in (example)  Do not pull the rag in !! Note also you will get nothing like the bend radius from the cable you would be able to do if you were laying it in ,all bends need to be very gently swept. (if it is a single width domestic gate of a metre or so, it will end up being all bends and no flat section pretty much if you have no access holes.)

35 or 50mm ID duct and a cables of ~ 18-20mm OD is  about right for a moderately comfortable straight run. If the cable is more like 25-30 mm OD, then go up  a pipe size. Larger duct never hurt as much as too small. ;-)

I think that website you linked to uses a 40% CSA fill  and that should be for straight run ducts with access pits.

Mike.