Calculations for cable size

[DavidAP]

I need to show calculations for sizing the cable for a small electrical installaion. We are supplying 2 panels each panel will draw a maximum of 2 amps full load. The feed is 230V single phase and will originate 800m from a substaion to the panel. We will be burying the cable and the location coastal in Oman. I have sized the cable at 16mmsq XLPE/PVC/SWA/PVC from various calculators etc but I need to detail the exact calculations. I also took the load current x length and going off tables in the IEE wiring regs for allowed volt drop of 4% or 9.6V also gave me 16mmsq. Can anyone provide the correct calulations?

 

[crisleon]

Are you asking for the formula for voltage drop?
deltaU% = 200 * l * L * cos? / (gamma * A * U) [%]
deltaU = 2 * l * L * cos? / (gamma * A ) [V]
You have
U=230V, I=4A, cos?=0,8 or 1,0 or something else (you probably know it)
L=800m, gamma =57 [m/Ohm*mm2] for Cu or 33 for Al
A=16mm2
For your circuit the result is about 2,4% for 16mm2 Cu or 3,8% for 10mm2 Cu.
The better result you get with the 3-phase feeding cable (the formulae to find anywhere; you say that these are 2 panels).
Even better solution is to get the solar supply modules particularly efficient in your sunny climate.
Regards

 

[rbulsara]

Voltage drop is only one of the criteria for cable sizing. Typical steps to size a cable will be:

1. Determine the total load, continuous and non-continous.
2. Pick the initial cable size to meet that load (current) from the applicable tables, such as those in Code book or manufacture's published data. Codes may have minimum size requirement.
3. Adjust ampacity according to the ambient temperature.
4. Adjust ampacity per the installation method (buried vs exposed etc).
5. Check for the voltage drop.

There are too many variables. If this sounds like too much, you need to work with experienced people in your firm and learn from them.


Rafiq Bulsara

http://www.srengineersct.com

 

[crisleon]

RBulsara,
well, you are absolutely right.
However please have a look at this case - you have cable-core-CSA=16mm2 and the current load 2*2A=4A in total.
Are you really likely to check dimensioning of this cable on the condition of the withstanding of the total load?
By the way, you should tell also about the selecting in the aspect of the proper electrical shock protection and overcurrent tripping - David tells nothing about that.
It seems to me that he is only interested in checking the voltage drop along this quite a long cable.
Regards

 

[waross]

From the Canadian Electrical code, table D3;
Based on 1% drop at 120 volts.
800 m @4%/ 4% = 200 m @ 4%
200 m @1% x 120V/230V = 104 m @ 230V
Extrapolating from table D3, a number 8 conductor would be adequate for up to 123 m.
2 amps is so far below the maximum rating (about 5%) that most other factors may be ignored, but at such a low relative current, the cable will run cooler than a fully loaded cable and the notes to table D3 allow an extra 10% for a total distance of 135 meters.
Find the appropriate table for your jurisdiction.

Bill
--------------------
"Why not the best?"
Jimmy Carter