cable calculation

[alextrician]

hi to everyone.

i got a question regarding cable calc. I am just about to finish my first year at college and i've got my last assignment to complete.I am running 4 multi core cables on a perforated tray from CU to the workshop throughout the ceiling for 3 phase equipment. from the ceiling to each of equipment, cable runs through PVC conduits. How is this gonna have a influence on the corrections factors(temperature)or other factors, or how should I go about it?. At the moment i've got just 2 correction factors for temperature and grouping. I am using a bs en 60898 type c rated at 10A as means of fault protection, the consumer is a 5kW/400V machine (no pf given)the cable i chose is 70 deg C Thermoplastic with 25 deg C ambient temperature giving a temperature factor of 1.03 . The numbers of cables on the tray are 4 giving a grouping factor 0.77

 

[alextrician]

hello there...

i wander if someone could help me please. i've done my cable calc for my 5kW/400V 3 phase machine and according to that result i've got i have to use a 1.5 mm cable. my common sense tells me that the cable is to small for the job. I haven't got enough experience as I am still 1 year at college.I am doing this for my last assesement. Please help.

 

[Zeespark]

Remember 3 phase wiring is generally smaller than 1 phase wiring sizes.

AndyGuinness

 

[NozSpark]

Alex,

You'd be better off starting a new thread off your own in the Student and learning section of the forum - I see if I an get a mod to split it out for you.

Anyway, give us the question that you've been given and your workings and we'll try and help you out.

 

[alextrician]

Remember 3 phase wiring is generally smaller than 1 phase wiring sizes.AndyGuinness

I though i've done something wrong the current drawn is 7.21A and I've got just 2 correction factors (temp and grouping). Also I am not to familiar with 3 phase as we studied more single phase and household work...

than you very much.

---------- Post Auto-Merged at 16:36 ---------- Previous post was made at 16:31 ----------

sorry for any inconvenience caused. i did not know how this things works. I will start a new thread .

cheers alot anyway guys.

 

[Doc Hudson]

Alex,You'd be better off starting a new thread off your own in the Student and learning section of the forum - I see if I an get a mod to split it out for you.

Moved and merged with duplicate bit.

Doc H.

 

[alextrician]

hello there...

right the question sounds like this:

"determine using drawing 2357-c-01/02/03/04 the minimum csa of live conductors suitable for current carrying capacity and volt drop in the following circuits:

-5kW/400V 3 phase

-12kW/400V 3 phase

-11kW/400V MIG welder

-3kW/230V water heater

The rating factor for grouping are based on the above 4 circuits within the same containment system with an ambient temp of 25 deg C "

So...what I've done so far is :

I found Ib=7.21A therefore I "installed" the protective device a MCB type C rated 10A

Method of instalation is B

The correction factors are: 1.03(70 deg C Thermoplastic cable @25deg C),0.65 (grouping of 4 cables/bunched in the air, on a surface, enbeded or enclosed)

Iz = 14.93 (table 4D1A ref meth.B 3-4 cables , 3 phase) - 1.5mm (csa of live wires)

 

[Zeespark]

What is your total circuit length Alex? you will have to factor in your voltage drop as well.

AndyGuinness

 

[alextrician]

yes, i figure it...the total leinght of the circuit is 20M and the volt drop is 3.61V .Actual value of Zs is o.6 ohms. Now comes the tricky part, minimum size of cpc is 2.5mm( calculated with the adiabatic equation

According with table 41.1 of bs7671, 230v<Uo</=400v is 0.2s a.c circuit. The notes I've got send me to appendix 3 the check that "actual disconnection time of protective device in sec correspond to the fault current", but I haven't got a clue how to work that out...