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Cables in ducts - Derating factor for cable grouping

Orion83uk

Hello all. I'm having one of those moments where everything I thought I knew is wrong.

I need some advice on cable derating factor (Cg) when running cables in underground ducts. I'll provide a hypothetical example to help with my question:

I have a switchboard at one end of a set of 4 power cable ducts, and 9 items of equipment requiring power at the other end. 

Lets assume the ducts are:

  • Approx 25m long (voltage drop and Zs are unlikely to be an issue).
  • 150mm diameter.
  • Are in a cluster, 2 ducts on top, 2 on the bottom, seperated 100mm in each direcfion.
  • All circuits will be 'ON' at the same time.
  • We don't need to discuss soil temperature or anything like that - just the grouping factor, Cg

Let assume the 9 circuits are spread evenly across 3 of the ducts, with the 4th duct left spare - so 3 cables in 3 of the ducts.

For my derating factor Cg for each cable, is this based on 3 cables in a group, or 9 cables in a group?

Many thanks in advance for any asistance 

Parents
  • +1

    BS 7671 doesn't really take this installation method into account in Appendix 4, as it only goes up to 6 buried circuits, AND 4C3 only covers 1 multicore cable per duct. You may have to adopt another approach following relevant parts of BS IEC 60287 series, or following ERA guidance.

    It might be reasonable to apply grouping to the cables sharing a duct as if they were touching, and then apply grouping to the ducts per Table 4C3 ... but things might get a little "over-engineered" (much larger cable sizes). You'd have to assume the ducts could affect each other , as whilst opposite corners are further away, with only 100 mm separation, they're likely all to be within the 0.25 m of each other.

    Whichever method you choose, also leaves the question of what you might assume for the spare duct (given it's a spare, some assumption ought to be made regards its potential use, otherwise if you size the cables right to the max, it's no longer spare as it can't be used without overheating the cables in the 3 used ducts we are thinking about).

    All of this perhaps meaning cable sizes double (or more) just on grouping alone ?

Reply
  • +1

    BS 7671 doesn't really take this installation method into account in Appendix 4, as it only goes up to 6 buried circuits, AND 4C3 only covers 1 multicore cable per duct. You may have to adopt another approach following relevant parts of BS IEC 60287 series, or following ERA guidance.

    It might be reasonable to apply grouping to the cables sharing a duct as if they were touching, and then apply grouping to the ducts per Table 4C3 ... but things might get a little "over-engineered" (much larger cable sizes). You'd have to assume the ducts could affect each other , as whilst opposite corners are further away, with only 100 mm separation, they're likely all to be within the 0.25 m of each other.

    Whichever method you choose, also leaves the question of what you might assume for the spare duct (given it's a spare, some assumption ought to be made regards its potential use, otherwise if you size the cables right to the max, it's no longer spare as it can't be used without overheating the cables in the 3 used ducts we are thinking about).

    All of this perhaps meaning cable sizes double (or more) just on grouping alone ?

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