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

 

[james king]

ok u used a 70mm 5 core cable? that must be huge. so my answer of 16mm is way to small

 

[james king]

so ypu came out the distribution boards in to a lv switch panel to 125a isolator have you got a diagram i am very interested in massive cables and industrial

 

[pushrod]

Is the 50 kW figure total power or power for each phase? Have they said if there is a power factor involved and whether it is connected in star or delta?

 

[james king]

50kw power intotal: No power factor as i know of. No i do not know if its delta or star! If in delta you would not need to carry a nuetrul to the isolator would you?

 

[pushrod]

50kw power intotal: No power factor as i know of. No i do not know if its delta or star! If in delta you would not need to carry a nuetrul to the isolator would you?

correct,no neutral if in delta.

power=√3 x Vl x Il

so rearrange to get Il which will be your design current Ib

 

[james king]

does that mean even though the DB has a N you still do not need to bring a N to the machine??

and say you have 1x 3 phase (100A per phase) CU with say umm 12 ways and you want to run this 50kw load from this supply..

how does switch panel work and does it run of the DB eg

DB - switch panel - from SP to the mccb then from there to the isolator?

 

[pushrod]

No practical expert by any means but if it is a motor connected in delta there will be no neutral - it will be a balanced load with no resultant neutral current.

Forgot to say before that your power per phase willbe a 1/3 of the total power, still use the √3 though.

reckon your line current in each phase is 24A if ihave used this rubbish windows calculator properly.

edit Il (line current) will be 72A as does not need dividing by 3 - reason you are dealing withtotal power, line currents and line voltages, not Ip and Vp and power per phase

 

[james king]

so if each phase is carrying 24A then a 70mm cable will not be needed. i have taped in a cable calculot on the net and it come up with 16mm. im very confused about what to put and how i got 16mm and some one came up with 70mm??

so what cable will i need?

 

[pushrod]

you also need to take account of grouping and ambient temp -you got an OSG with you?

 

[james king]

yes i do mate

 

[bugsy]

With a design current of 50kw how do you get 24a?

 

[pushrod]

Have a look at page 123 temp at 30C has no affect so can be disregard it in this case (table 6A) the other circuit derates it though-look at top line of table 6C (p126) . You will need to divide your current by 0.8

tricky question this one

 

[james king]

some1 help bugsy how did you work it out and what amps per phase do u get? im confused about the incoming tails say they are in 25mm per phase in to the board would u then come from the DB to the switch panel if so what cable size would u use then? can u some how find a diagram of it please mate

 

[bugsy]

could you please demonstrate the calculation

 

[james king]

lol some one demonstate the bloody calc thats all i ever wanted lol

 

[pushrod]

With a design current of 50kw how do you get 24a?

50 kW is a total power according to OP - so 16.67kW per phase then calc Il with formula given in post 25.

 

[pushrod]

16700W/( √3 x 400v) =24.1A

 

[pushrod]

You still hanging in there?

forgot to say √3 is 1.732

 

[james king]

im still watching lol i will top of the class if i get this with all your help lol

 

[james king]

so pushrod what is your cable size chioce?

 

[bugsy]

**** went to the wrong college

 

[james king]

so 70mm is way to big then if each phase only needs to caryy 24amps????????

 

[pushrod]

back to page 123 of osg and read post 32 again to calc It

no point telling you the answer if you can't explain it

 

[james king]

hehe just tell me ok il go get it

 

[pushrod]

so 70mm is way to big then if each phase only needs to caryy 24amps????????

way too big, but not there quite yet

 

[Jimmyb]

I would say 16mm but because of 125amp breaker 25mm

 

[pushrod]

It = In (24A)/Cg (o.8)


you can hit the thanks button if i'm helping

 

[Jimmyb]

so 70mm is way to big then if each phase only needs to caryy 24amps????????

is it not 24.73amps

 

[james king]

yes sryy

 

[james king]

so how would the layout work then can u explain of where the come out from the DB ?