Cable calculation

[Mikeymike]

Ok had a go at making a cable calculation up, had a go

A 230v 11KW shower single phase load is run through twin & Earth pvc 70 degrees operating temperature and has a distance of 15metres through joists in a loft extention. the ambient temperature is 30 degrees C and the circuit uses a semi enclosed fuse (BS3036)

Work out!

a) Design current

B) Installation method

c) protective device rating

d) Correction Factor

e) Minimum cable rating

f) Minimum cable size

g) Volt Drop

h) Volt Drop allowed

a: (Ib Design Current) 11000/230 = 47.82A

b: Reference Method C (clipped Direct)

c: (In Protective Device) = 50A BS 3036

d Correction factor from ambient Temperature

30 = 0.94

e: It = 50/0.94 x 0.725 = 73.36A

Ca = 1

Cg = 1

cc = 0.725 semi enclosed bs 3036

ci = 1

f: 16mm cable size is choosen from 87A Reference Method C

g: (mV/A/M)/1000

2.8 x 47.82 x 15 / 1000 = 2.00Volts drop

h: 5% other uses. 11.5volt drop maximum. Which means it's acceptable

If I've made a mistake would anyone be able to point me where I went wrong please.

Cheers

 

[Zeespark]

My only query would be the amount of space between the top of the insulation and the loft flooring.

AndyGuinness

 

[m4tty]

If 'It' is 73.36a after then won't you have to choose a cable size which can carry this current after rating factors have been applied. Ie 16mm cable as 10mm can only carry 65a? So won't 16mm have to be used?

 

[kme]

I`m with the guys:

"clipped direct"? you`ve just said it`ll be in a loft extension; through the joists; so there`ll be thermal insulation around to contend with.....

And as Matty says, if correction factors give you a rating higher than 10mm carries, how can you use 10mm?

 

[Mikeymike]

Yes my bad guys current carrying capacity is to low for 10mm

Factors: 10mm - 65A clipped direct

Factors: 16mm - 87A clipped direct

 

[buddha]

if its a multicore cable,twin and earth,is the 16mm cable rated at 85A rather than 87A? the table showing 87A is for single core cable.

could be wrong,been trying to get this stuff on cable factors in my head,but its beginning to make my brain melt.

for instance it says you dont need to apply a Ci correction factor if methods 100,101,102 and 103 are used,but 103 is the method where the cable is surrounded by insulation,also the amp carrying capacity of a cable of said csa varies between all these methods so surely a correction factor is needed.