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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
  • Before getting buried in the details of the regs view,  if its not your day job & second nature, it is worth stepping back and seeing that there are a few ways to dice this -

    Firstly how well do you know your load ? Is it constant power 24/7, or is it modulated on and off over time - depending how fast this occurs it may be faster or slower than the cable heating time. If the mains voltage is high does the load draw more current, like a resistor, or less like a building full of electronics with switch mode supplies containing more or less constant power. The regs assume the former, as the worst case, but nowadays it is not always true.

    Secondly the route and space available

    Heavy SWA rapidly gets awkward to install - its worth looking at bend radii carefully not to mention the sizes you are considering weigh in at about 10kg per metre so if it needs to be installed anywhere that it cannot be dropped into or dragged into place by vehicle it can be tricky.  Dividing it down over parallel cables is one solution, - 240mm2 is a lot more flexible than 300mm2 for example, but the terminations rapidly get bulky.  There is a reason that beyond a certain point, ( and if your route is convoluted, or overhead for any great distance you may have reached that ), where installing as AWA singles tied in trefoil or quadrifoil to a tray is the easier method - it is less likely to give the installer a hernia. Killing someone by a poor choice at the drawing board stage is not a good start. Also make sure there is enough space at the ends for termination

    parallel cables heat each other, same circuit or not, and unless you want a fire, this needs to be thought about, and spacing things by a cable diameter or two and allowing air to circulate works wonders.

    finally Length and voltage drop - self explanatory but for a long run, it is sometimes worth having a short (well few metres) tail of one cable, then a joint to something with greater cross-section for the long, and hopefully straighter  run. There is also the economy of losses over the life of the installation - but it depends how that is budgeted relative to construction costs.
    M

Reply
  • Before getting buried in the details of the regs view,  if its not your day job & second nature, it is worth stepping back and seeing that there are a few ways to dice this -

    Firstly how well do you know your load ? Is it constant power 24/7, or is it modulated on and off over time - depending how fast this occurs it may be faster or slower than the cable heating time. If the mains voltage is high does the load draw more current, like a resistor, or less like a building full of electronics with switch mode supplies containing more or less constant power. The regs assume the former, as the worst case, but nowadays it is not always true.

    Secondly the route and space available

    Heavy SWA rapidly gets awkward to install - its worth looking at bend radii carefully not to mention the sizes you are considering weigh in at about 10kg per metre so if it needs to be installed anywhere that it cannot be dropped into or dragged into place by vehicle it can be tricky.  Dividing it down over parallel cables is one solution, - 240mm2 is a lot more flexible than 300mm2 for example, but the terminations rapidly get bulky.  There is a reason that beyond a certain point, ( and if your route is convoluted, or overhead for any great distance you may have reached that ), where installing as AWA singles tied in trefoil or quadrifoil to a tray is the easier method - it is less likely to give the installer a hernia. Killing someone by a poor choice at the drawing board stage is not a good start. Also make sure there is enough space at the ends for termination

    parallel cables heat each other, same circuit or not, and unless you want a fire, this needs to be thought about, and spacing things by a cable diameter or two and allowing air to circulate works wonders.

    finally Length and voltage drop - self explanatory but for a long run, it is sometimes worth having a short (well few metres) tail of one cable, then a joint to something with greater cross-section for the long, and hopefully straighter  run. There is also the economy of losses over the life of the installation - but it depends how that is budgeted relative to construction costs.
    M

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