302 question help

[lofty84]

Hi there

if anyone is able to show me how to go about working out this question it would be most appreciated.

I think that it will require haveing to look at either the on site guide bs7671:2008 or the IEE wiring regulations seventeenth edition thats the only thing.


A pvc trunking is used to enclose single core PVC insulated distribution cables (copper conductors) for a supply to a new 400 V TP&N distribution board. The balanced load consists of a 24 KW of discharge lighting. The fuses at the main switchboard are to BS88. The voltage drop must not exceed 6V. The ambient temperature is anticipated to be 35 oC. The declared value of IP is 20 KA and the ze is 0.3 ohms. Assume that the requirements of BS 7671 434 are satisfied by the use of BS88 fuses. For the distribution cable determine the

i. design current
ii. the maximum rateing of the fuse in the main swith fuse In
iii. maximum mV/A/m value
iv. minimum current rateing It
v. minimum cross sectional area of live conductors
vi. actual voltage drop int he cables

Ive had ago at finding the design current and the rateing of the fuse (In) which I think is a s follows

Ib = 24000/400x1 = 60 Amps

In = greater then or equal to Ib = 63 amps

but then I get stuck

 

[i=p/u]

appendix 6 on site guide should help everything there.......

 

[lofty84]

thanks mate it time to have a go then


Hopefully the first 2 answers are correct

iii. Voltage drop = (mV/A/m) x Ib x L / 1000

how do I know what figures to put in for (mV/A/m) tho ?

 

[telectrix]

mV/A/m for all sizes of cable is in nbrb appendix 4

 

[lofty84]

nbrb ?

 

[lofty84]

nbrb ?

sorry but is question 1 worked out by saying

IB= Power / Uo x pf

or

Ib = power / 1.732 x UL x PF

 

[lofty84]

is the minimum current rateing

It ------ > greater then or equal to In / ca x cg x ci x cc

im not sure if its right but if anyone can say whether it is right or wrong it would again be appreciated

ca = 0.94
cg = 1
ci = 1
cc = 1

 

[i=p/u]

find (ib) then pick (in) then (it) has to be equal or greater (in) and yes you use 1 in factors if no change. you should get your current carrying capicty from this sum and pick cable size accordinly from table in on site guide

 

[i=p/u]

once you shown once to do this , should be no swet

 

[i=p/u]

you would then use your (in) say 60A/ca x ci x cg x cc

 

[lofty84]

the problem is Im not sure is my ca cg ci and cc values are correct if there not right then the equation will be wrong.

 

[lofty84]

would just like to take the opotunity to say thank you for all the help you have given me in working this out