the calculations are not as hard as you fear mate, once you learn the logical approach to them.
1) find your design current, (in laymansd terms, how many amps do you need). take diversity into account (will every bit of equipment on this circuit be on at the same time or not)
formulas for this= Ib= Kw/Uo x pf Ib= Kva/Uo Ib= (Kw/Uo)/1.732x pf
for discharge lighting, multiply calculated value by 1.8 to get Ib, this allows for the power factor, and inductive nature of the lights.
Ib= design current, Kw= appliance rating, Kva= appliance rating with power factor already taken into account, pf= power factor, Uo= line to earth voltage, usually 230v.
2) select a protective device that is at least the same value as Ib. This is called In.
3) derate the cable due to external influences. the value your looking for is now called It.
formula= It= In
Ca,Ci,Cg,Cc,Cr
Ca= ambient temperature
Ci= If the cable is covered in insulation
Cg, if the cable is grouped with other cables
Cc= if the cable is buried in the ground
Cr= if the cable is protected by a rewireable fues, then use 0.725
4) find a suitable cable.
to find a suitable cable, you need to select a cable that satisfies the It, and also volt drop.
decide on the volt drop limit that is applicable (3% for lighting, 5% for power or other client specifications in industry ect)
so for example, 3% of 400v is 12v
maximum mV/A/m rating of cable = vd
length of cable x Ib
once you know this, you then select a cable that meets this, and is also rated to carry It.
5) select or calculate a CPC
to select use table 54.7 of bs7671, on page 162,
to calculate use S= sq root (I sq xt)
k
I is calculated by Uo/Zs
Zs can be measured or calculated by finding the conductor resistance figures in appendix 9 of the OSG, and adding this to Ze
t is the time it will take the protective device to operate using I as calculated above. this info can be found in the time/current graphs in appendix 8 of BS7671.
k is found by looking at page 161 of Bs7671.
6) short circuit withstand.
you can either use the calcutae method above to work out the resitance for the line and neutral conductor. to work out the supply line to neutral resistance( Zp-n), divide PSCC into 230v for 1ph, or 400 fpr 3ph. add this resitance to your R1+Rn. divide this into either 230, or 400 (as applicable), this will give you max short circuit current.
rearrange the adiabatic formula, so it reads t squared = k squared x S squared
I squared
if the result is more than the time taken by the protective device to operate , then the cable will withstand a short circuit.
7) wil the circuit disconnect in time.
find the max Zs on page 53-57 of Bs7671.
using the table of conductor resitance in appendix9 of the OSG, find the reistance of your line and CPC (R1+R2). multiply this by 1.2 if your conductor hasd a max operating temp of 70deg C, and by 1.28 if its 90deg C.
add the above value to your Ze, and if its less than the max Zs as stated in BS7671, then your circuit is good to go.
