Cable size required

grousedogtom

Hi
Could someone advise me on an swa cable size for the following
3 phase supply
Total load 21kw
cable in duct in ground
PF 0.9
Distance 200 meters

Thanks

zega

10sq

grousedogtom

OK , i was reckoning at least 16sq over this distance,can you tell me how you calculated this out ,thanks

Bruthal

grousedogtom wrote: »

OK , i was reckoning at least 16sq over this distance,can you tell me how you calculated this out ,thanks

I would say 10 sq is too small at that distance. Without any calculating id imagine 25 to 35 square is needed there.

grousedogtom

I had 25 sq in mind ,what is the formula?

Bruthal

grousedogtom wrote: »

I had 25 sq in mind ,what is the formula?

Well for the amps per phase its 21000/(400 x 1.732 x 0.9)

Randyleprechaun

In my opinion you'll need 35mm.

That should combat volt drop and also your fault loop impedance should be ok, although that will depend on the impedance where you're taking the supply from and the type and rating of the protective device protecting the cable

2011

You need to start with a few basics:

1) Is the load perfectly balanced? If not how far out of balance is it?
If balanced it is about 34 amps per phase.

2) What is the maximum permissible volt drop on the cable? Remember this cable is only part of a circuit.

3) What type of load is it?

4) What protective device do you intent to use and why is it rated at?

grousedogtom

2011 wrote: »

You need to start with a few basics:

1) Is the load perfectly balanced? If not how far out of balance is it?
If balanced it is about 34 amps per phase.

2) What is the maximum permissible volt drop on the cable? Remember this cable is only part of a circuit.

3) What type of load is it?

4) What protective device do you intent to use and why is it rated at?

1)Yes the load is balanced at about 32 amps per phase max

2)4%

3)2 Sewage pumps which will be run on vsds, there 16 amps each and will rarely run together as duty assist

4)open to options, maybe a switch fuse unit

THANKS

2011

Where is the VSD placed? The reason I ask is that VSDs correct the power factor to unity (you had said 0.9 PF). Also it is best not to insert a VSD 200m from the motor it is feeding. Check the manual, because sometimes a VSD installed far from the load will require a choke. What type of cable are installing between the motors and the VSDs ?

grousedogtom

The vsd s will be placed in a kiosk a few meters from the pumps, the cables that are on the pumps will be used to connect straight to vsd output

2011

grousedogtom wrote: »

The vsd s will be placed in a kiosk a few meters from the pumps, the cables that are on the pumps will be used to connect straight to vsd output

Ok, power factor is = 1, the will reduce the current in the 200m cable by 10%.

2011

What we know:

1) Installation method D applies
2) Maximum volt drop = 4 %
3) The design current = 30.31 A
4) Length of run = 200m

mV per amp per meter values are provided for installation methods C & E for 3 phase loads wired in XLPE multicore cables in table A52-J6.


This works out at:

10 sq. volt drop of 24.25 V
16 sq. volt drop of 15.16 V
25 sq. volt drop of 10.00 V
35 sq. volt drop of 6.97 V

A more accurate volt drop calculation can be achieved using the manufacturers values (this is permitted) and the correct installation method. Earth fault loop impedance values should be checked, see annex 61C.

At work we have a young genius using Amtec to size all cables.

Steve

Yeah, as 2011 said you need to check if you need to derate for short circuit, at a guess in this case it'll be around 3 or 6 kA so won't really factor but it should be accounted for.

grousedogtom

Thanks for all the advice, really appreciated, great help

Steve

I've used this a few times, it's pretty accurate if you want to comply with 60364:2001

http://myelectrical.com/tools/cable-sizing-calculator

solargain

Using the regs its just (mVxAxM) / 1000

2011

solargain wrote: »

Using the regs its just (mVxAxM) / 1000

Yes, that is exactly what I did.