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Cable Derating Factors - Single circuit, multiple cables in duct

I've been asked to size a cable for a large load - 630A.
With BS7671 in one hand and IET Electrical Installation Design Guide: Calculations for Electricians and Designers, 5th Edition in the other I'm working through the different derating factors.

The cable is not close to any other circuits so I'm working on the assumption that the Grouping Factor Cg is 1.0

Most of the other factors appear straightforward but there are two points that I'm not clear on...

"Buried Circuit Rating Factor" Cc 

Regulation 433.1.203 states
For direct buried cables or cables in buried ducts where the tabulated current-carrying capacity is based on an ambient temperature of 20 °C compliance with condition (iii) of Regulation 433.1.1 is afforded if the rated current or current setting of the protective device (In) does not exceed 0.9 times the current-carrying capacity (Iz) of the lowest rated conductor in the circuit protected.
Which suggests I need to derate the cable by 0.9

I've read the discussion (on this forum and elsewhere) and can see how this relates to the way that fuses operate, however, this circuit will be protected by a Schneider NSX630N MCCB which will trip when the load current >1.2 In after 100 seconds.

Consulting 433.1.1 (iii)
The current (I2) causing effective operation of the protective device does not exceed 1.45 times the lowest of the current-carrying capacities (Iz) of any of the conductors of the circuit
Am I correct that I can set Cc to 1.0 for this installation?
Effect of running multiple conductors per phase
If Cc =0.9, then then for my 630A load, It > 700A
Based on four-core, copper, SWA cable, then we're looking at 3no 185mm2

As these are all feeding the same load am I correct that the Grouping Factor Cg is still 1.0?
Any issues with running the three cables in the same duct? (Apart from physically pulling them in of course!)
Many thanks
Parents
  • As these are all feeding the same load am I correct that the Grouping Factor Cg is still 1.0?

    Not quite (as Mike as already mentioned from a physical point of view) - from a regs point of view it's worth looking at the descriptions that go along with Cg - e.g. the heading for table 4C1 - 'Number of circuits or multicore cables'  (OK while the wording still isn't 100% especially for the case of single core cables in parallel, it does follow the general logic that what you're counting is how many multiples of the (loaded) conductors described in the installation method you have e.g. if you have 6 and the installation method says 3, then Cg is for "2 circuits".

    Which suggests I need to derate the cable by 0.9

    The way I think of it is that tables for buried in the ground reference methods (e.g method D etc) have already been increased to take advantage of the lower ambient temperature - the 0.9 factor puts them back closer to "normal conditions" (which makes the assumptions used for overload calculations valid again).

    If you want to argue that your overload protective devices have a smaller than the usual 1.45x margin, so you can safely run the cables a little bit harder, than that might well be valid - but arguably it should be equally applied equally to above ground cables as underground ones. So maybe more a matter of applying 0.9 and then applying some other factor to adjust for your choice of protective device (which may or may not come out to be 1/0.9).

        - Andy.

Reply
  • As these are all feeding the same load am I correct that the Grouping Factor Cg is still 1.0?

    Not quite (as Mike as already mentioned from a physical point of view) - from a regs point of view it's worth looking at the descriptions that go along with Cg - e.g. the heading for table 4C1 - 'Number of circuits or multicore cables'  (OK while the wording still isn't 100% especially for the case of single core cables in parallel, it does follow the general logic that what you're counting is how many multiples of the (loaded) conductors described in the installation method you have e.g. if you have 6 and the installation method says 3, then Cg is for "2 circuits".

    Which suggests I need to derate the cable by 0.9

    The way I think of it is that tables for buried in the ground reference methods (e.g method D etc) have already been increased to take advantage of the lower ambient temperature - the 0.9 factor puts them back closer to "normal conditions" (which makes the assumptions used for overload calculations valid again).

    If you want to argue that your overload protective devices have a smaller than the usual 1.45x margin, so you can safely run the cables a little bit harder, than that might well be valid - but arguably it should be equally applied equally to above ground cables as underground ones. So maybe more a matter of applying 0.9 and then applying some other factor to adjust for your choice of protective device (which may or may not come out to be 1/0.9).

        - Andy.

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