LV cable size for 800meter distance

[Muhammad A]

I have to connect 100kW 3phase load at a distance of 800 meter.Aluminium cable size will work? Is it feasible to run multicore aluminium cable as Voltage drop i calculated is too much?Regards

 

[che12345]

" ...I have to connect 100kW 3phase load at a distance of 800 meter. Aluminium cable size will work? Is it feasible to run multicore aluminium cable as Voltage drop i calculated is too much?"
1. Please advise a) what is the voltage (V), b) current (A), c) the % of VD allowed, d) cable buried underground or in open-air on poles?
2. Which std/code i.e. IEC or NEC prevail?
3. There are Copper or Aluminium single-core or 3C , SWA or AWA cables >> 300mm[sup]2[/sup] on the market. Which is your preference in terms of availability, cost and ease of installation?
4. Attention: Aluminium cables of same amperecity are lighter in weight but requires Al gland, Al gland-plate and Al/Cu lug etc., to terminate on Cu terminals.
Che Kuan Yau (Singapore)

 

[Muhammad A]

Thankyou sir for response.
1 ) system volatge is 400V ' 3 phase
2) voltage drop permisible is 5%
3 ) cable can be in open air temperature of air can be about 40C ( in summer)
4) I need to work feasiblity for bth copper and aluminium cables and will choose size and cost for installation

 

[che12345]

1) system voltage 400V is likely to be in the IEC world.
1.1) 100kW 3-ph 400V would be say around 190A* approx. Assuming load factor =1.0** (worst case, at 190A for 24h throughout).
2) Vd permissible is 5% at 400V = 20V *** . Acceptable/usual, fine.
2.1) length 800m given.
3) cable can be in open air temperature of air can be about 40C (in summer). Taken, fine. Attention: for cables, the standard ambient air temperature is taken at 30C. Therefore, at ambient air temperature 40C; a derating factor**** should be taken into consideration.
3.1) Based on ambient air temperature 40C, for (PVC 70C cable, derating factor = 0.87) , (PLXE 90C cable, derating factor = 0.91), approx.
4. Taking into consideration of above major factors (*...****), the design would be reasonably safe. Refer to IEC and BS 7671 for full detail.
Che Kuan Yau (Singapore)

 

[Muhammad A]

Thankyou.For aluminium cable 4 core or single core pvc armoured can you recommend size for above criteria as it seems multiple runs of cable will be required?

 

[jghrist]

can you recommend size for above criteria

You can get technical advise and sources for methods of calculating voltage drop on a forum, but you need to do the calculations yourself or get an experienced engineer to do it for you. You should not base the cable size solely on what someone on this or any other forum tells you.

 

[waross]

I am guessing that the cable will cost more than transformers and a higher voltage.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[7anoter4]

First, if it is about 100 kW induction motor and if it will start D.O.L. [direct online] 6-7 times.
rated current will be at start and 15% [more or less] a limited voltage drop -it depends on GD^2 of the
load and of the motor [including transmission]- in order to facilitate the start.
Second, the voltage drop depends also on the power factor [ cosfi ] and the load factor [usually 75%].
A power factor of 0.85 is usually but let's take 100% load factor and no induction motor involved.
In U.S.A., single core cables are employed, but in Europe more multi core [4 cores] cables are using.
Now let's seek for I=100*1000/sqrt (3)/400/0.85=170 A current carrying capacity [ampacity] in IEC 60364-5-52. Let's take XLPE [90oC] insulation and aluminium conductor.
Multi-core cable in free air [E installation way]. Table A.52-13 (52-C12) – Current-carrying capacities in amperes for 30oC [in free air] 70 mm^2 aluminium 187 A. For 40oC, a reducing factor of 0.91 and we get 170 A.
The Voltage drop for a three phases cable VD=I*sqrt (3)*(R*cos(fi)+X*sin(fi))
Rdc90oC=800/35.7/70*(228+90)/ (228+20) *1.03=0.422801 ohm [for this cross section the skin effect and proximity effect may be neglected]
Conductor diameter 9.3 mm, insulated core 11.5 mm
The reactance X=2*pi () *50*(0.2*ln (2*11.5/9.3)+0.0524)/1000*0.8=0.058683936 ohm
Then VD=170*sqrt (3) *(0.4228*0.85+0.05868*0.5268) =114.92 [28.73%]
So, we have to increase 11 times the cross-section area of the conductor.
If we take 800 mm^2 aluminum [there are only single core cables] in trefoil, we get 4.8% voltage drop.

 

[Muhammad A]

Thankyou very much

 

[che12345]

" ...If we take 800 mm^2 aluminum [there are only single core cables] in trefoil, we get 4.8% voltage drop".
1. I am of the opinion that running 3 1C 800mm[sup]2[/sup] Al cable for 800m would be very costly.
2. A possible alternative is by installing a LV/MV say 0.4/6.6kV step-up transformer on the up-stream and another step-down on the down-stream. A say 500kVA dry or wet trafo may be able to handle the load.
3. Note: a) All switching are done on LV. No MV switchgear except the MV fuses.
b) The MV cable would be of much smaller gauge. Saving in installation cost may pay for two trafo and the MV cable.
Che Kuan Yau (Singapore)