150mm Cable Duct fill capacity

[srb41]

In the past i have used 45% fill capacity within cable ducts either burried, enclosed in concrete or both.
The 16th Edition IEE Regulations and guidance notes give tables etc for conduit & trunking but i cannot find any difinitive guidance for cable ducts.
anyone help?

 

[MikeDB]

I have considered cable ducts as conduits and limited fill factors based on:

1. NEC fill factor limitations.
2. Cable "jamming" when 3 cables are pulled in the same conduit.
3. Cable heating considerations when applicable.

I think #2 is often overlooked, but certain ratios of cable diameter to conduit size tend to jam up during pulling.

 

[weh3]

Cable jamming occurs when, in pulling three cables, the sum of cable diameters approaches the ID of the duct.

William

 

[mc5w]

Cable jamming can also occur with 4 cables but is less likely than with 3 cables assuming that all 4 cables ore of the same size. If the 4th cable is rather small such as an equipment grounding conductor then the small cable has negligible effect on jamming or could make jamming worse.

US National Electrical Code chapter 9 Fine Print #2 lists a conduit internal diameter of 2.8 to 3.2 times the average cable diameter as likely to jam. I would not take those as absolute limits. At 2.7 to 3.3 times the average cable diameter you have a smaller probability.

Actually, using an oversize conduit saves on wire pulling labor and it actually can pay to use oversize conduit. If I do not have a place to hook up a wire pulling winch, a forklift truck, or a station wagon to the pulling rope and 4 man nonunion crew costs $120 to $160 per hour, it pays to use a 2.5 inch conduit for a 200 amp single phase electrical service.

So, a 4th criteria for conduit size is how hard do you want to work.

A 5th criteria is that conduit fittings such as LBs have a wire size restriction based on wire bending space. Same goes for the 25% fill restriction of explosionproof seals. Fittings for liquidtight flexible metal and nonmetallic conduits also have a smaller diameter that the conduit itself so you have to account for that as well. The tables in National Electrical Code for liquidtight flexible conduits are wrongo.