Anyone able to help?

[kajdean]

Hello all,

Im currently training to become an electrician, for my theory ive been asked this question, its taken me a while to get my head round what i think is right, could anyone tell me if im working this out right? much appreciated.


Q. A single-phase circuit is to be wired in 70 degrees pvc insulated and sheathed single core cables to BS6004 having copper conductors. The cables are to be installed in free air, horizontal, flat spaced on vertical cable tray system.

If the Ib=135A the ambient temperature 50oc and protection is provided by a BS88 device, what is the minimum current rating for the fuse and the minimum csa of cable conductors that can be used.

A.
Ib=135A
Ca=0.71 (table 4B1)
In=160A
Cg=1.00 (table 4C1
It=225.4
Ci=N/A
Iz=N/A
Cf=N/a
Mi=N/A


160
___________ =225.4 Minimum current rating for fuse=225.4
0.71x1.00



Minimum CSA of cable =25mm2 (table 4D1A, column 11)

 

[telectrix]

from table 4D1A, for Ib =160A, i get 50mm cable, with a fuse rating of 190A. (Iz>In>Ib)

 

[kajdean]

Ib is 135A, so im looking at table 4D1A column 11? Is that wrong then? The explanation in these books aren't very clear

 

[telectrix]

as i read it, your Ib is 135A but this needs to be corrected for ambient temp by 0.71 thus giving a corrected CCC of 160A. there fore your Iz of the cable needs to be > 160A. 35mm is 162A ( table 4D1A ) which is borderline so next up is 50mm @ Iz = 197A, which gives a good margin. your MCB rating needs to be below 197A so as to open before the cable os overloaded, but above 160A, so as not to open at this level of current.

 

[kajdean]

Thank you Think i get it now. Is the question worded badly? it says horizontal cable on vertical cable tray.

 

[telectrix]

i think the question should read horizontal and vertical tray

 

[kajdean]

Ah right ok, thanks again