cable sizes

[james king]

hi in collage today we were doing cable sizes and i am stuk on one question can any one help.

the question is

the load is 50Kw
3phase
the total lengh is 50m
its ran in metal trunking with 1 other circuit
30 degrees room temp

what will be the cablke size required and what mcb should be used?

i have had a look and i think it should be 16mm 5core and 125A mcb but i think im mising some thing. any one help

 

[bugsy]

Is the load resistive inductive or your not sure?

 

[james king]

doesnt say

 

[bugsy]

what value are you using for vdpm

 

[james king]

what do u mean all new to this stuff im just following the book haha

 

[bugsy]

What caluculations do you think you should be doing , and what cousre are you on

 

[james king]

im doing my level 3. cant get my head around it tbh what does vdpm (volt drop per metre)??

 

[bugsy]

It does, not being awkward but if you get the answer without knowing how you got there its is about as much use as a choclate teapot

 

[james king]

could you talk me through it step by step? its only the 3rd Q i have done on them only trying to learn and get my head around it, maybe it was a fluke lol

 

[james king]

was it correct?

 

[james king]

am i wright in saying the amps are 50,000 / 400 = 125 so thats the mcb i should be using

 

[i=p/u]

50kw/415= Ib 121A (Design current)
In= 125A (mcb)
125/.80= 157A what your cable should carry
It, tabulated current says you should go with 95mm squared

volt drop= 0.48(mV/A/m) x 121(Ib) x 50(length)/1000=2.9V

thats what i get,maybe make some sence to you

 

[james king]

ye thats how i did it i think! so what is the min cable size? i did did not know of this 95mm squared? what does this mean

 

[i=p/u]

cables size your reccomended to go with, i cud be a bit wrong, im only on level 2

page 123-124 on site guide.

 

[james king]

no i dnt think its 95mm cable that i 1 massive cable

 

[bugsy]

Havent got regs with cable tables to hand but we installed a 50kw induction oven about a month ago and it was protected by 125a mccb .

The reason I asked about the load is because if you look at the cable tables for current / VDPM will see that it's different depending on the load.

 

[james king]

i take the cable you used was not 95mm lol? yes i relaise that but the Q does not state it im sorry! it says the machine is a bio fuel machine!!!

did you install the mccb into the 3phase CU then run SWA from there to a isolation point and connect the flex up in the out going of the isolator? just wondering how much big loads work

 

[bugsy]

I think you will find 70mm will fit the bill and should you not be calcing @ 400v

 

[i=p/u]

aye maybe , multicore says 95 squared and single core 70

 

[bugsy]

L.V switch panel MCCB to 125a switch disconnector and no the final connection was not in flex