3 phase cable calcs

[thomasparry-]

After my last post and having and having to re-learn some stuff can someone check and advise on a few things. Sorry if this is wrong don't hate, will make a good learning exercise as my mentor won't help me and hopefully will help others.


Specification
16 circuits- wired up to caravan hookup boxes, each unit supplied from a sub-main (calculated to worst-case scenario)
Lighting 16A+ Store 6A+ Store 6A+ Store 6A= 40A MCB 16mm single-phase SWA

x3 Circuits
canteen 40A + office 32A = 80A MCB 35mm single phase SWA
toilet+shower 63A + Spare 16a = 80A MCB 35mm single phase SWA
office 32A + spare 32A = 63AMCB 35mm single phase SWA

(cable sizes have been calculated to give less than 2% Vdrop to include 3% sub-main v drop - thus giving 5% within regs)

Total = 679 A
3 phase Split = 226 A

Sub main is to be run approx 150m to a 3 phase dist board and is run underground to supply the above circuits

Derating Factors of Sub Main
IT>In/derating factors
Taking into account 10%diversity (sorry i don't know how to do this properly i get 611.1A/203A Split)

CC= buried in ground =0.9
Ca = Thermosetting in Ambient air - 0.91 7
Other factors are 1 or n/a

IT>611.1/ 0.91*0.9 = 746.15A/249A split

choose cable to fit allowing a volt drop of 5% in total to the end of the distribution
300mm 365A 4 core SWA = 0.185mV/A/m (I'm assing this cable is huge as I've never seen a 300mm 4 core cable lmao)

gives a vd = 0.185mV/A/m*150m*249A = 6.9v

(6.9/230)*100 = 3% thus allowing a v dop of 2% on distribution

11.5V - 6.9 = 4.6V for disribution drop

Can you help with knowing how to apply the following as I have had a look but don't really get what im supposed to be doing

Diversity - I don't know the loading it is assumed worst case, but not all will be on all day every day, how would one go about tackling this? best guess?
Protection devices - for the sub-main (not that important to this application) but for future knowledge
Sub main Earthing - How would I calculate the size of a CSA earth run with the SWA separately.

Thanks all

 

[thomasparry-]

I understand also that people use the SWA sheath for the eath but this is not what I'm looking for to add
i could also carry out this in a cable calculator but this isn't how people learn

thanks again

 

[westward10]

Is this a real life scenario or a college project.

 

[thomasparry-]

Is this a real life scenario or a college project.

kind of a bit of everything i guess, but I've furthered it for my own learning as its practice not installed

like how deep does this rabbit hole go scenario

 

[westward10]

Who is your mentor, work or college.

 

[Lucien Nunes]

My first suggestion would be to re-assess the actual load that needs to be supplied, before going into the detail of cable sizes. At 156 kVA the power you are allocating seems way over the top. Even with your arbitrary 10% knocked off it seems way over the top.

Also have a look at how VD is calculated on a 3-phase circuit. Your existing percentage is too high by an oft-used factor.

 

[thomasparry-]

Who is your mentor, work or college.

mentor irl not here

 

[thomasparry-]

My first suggestion would be to re-assess the actual load that needs to be supplied, before going into the detail of cable sizes. At 156 kVA the power you are allocating seems way over the top. Even with your arbitrary 10% knocked off it seems way over the top.

Also have a look at how VD is calculated on a 3-phase circuit. Your existing percentage is too high by an oft-used factor.

I assumed this but as the load is not apparent I cannot make the best guess to it even

do you mean the 0.86 multiplier

after i search ill pick up a copy of the IEE Electrical Installation Design Guide should help me also , the books i currently have only refer to single phase