The following article provides worked examples of low voltage cable sizing calculations with an average 75 % time saving and $3,366 cable cost saving using ELEK Cable Pro software compared with performing hand calculations.
Cable sizing calculation requirements
The following main factors involving calculations influence the selection of correct cable sizes:
1) Current-carrying capacity. 2) Derating factor selection. 3) Voltage drop. 4) Short-circuit performance. 5) Fault loop impedance.
The calculations procedures are outlined by the Standards however they may take significant time to perform by hand and finding the optimal cable sizes, usually requiring the use of software, will often save significant project costs.
Software saves 75 % in time
ELEK software saves a significant amount of your time compared with performing hand calculations. Software calculations are more reliable and often more accurate.
Several factors which affect cable size need to be calculated and often re-calculated to arrive at a safe and optimal solution. In the first example below the time it took to perform manual calculations was 38 minutes compared with only 7 minutes using ELEK software. In addition to this ELEK software also produced detailed and well-presented finished reports during this time.
Software saves $3,366 in cable costs
ELEK software cable sizing calculations are very accurate often resulting in optimal solutions which saves you money. ELEK software uses more accurate methods of calculation than is possible by hand such as accurate voltage drop, short-circuit and fault loop impedance calculations.
In the first example below ELEK software used an accurate method of calculating voltage drop which considered cable operating temperature and load power factor resulted in a smaller cable size from 50 mm2 down to 35 mm2 for the 225 m run resulting in a cable costs saving of $3,366.
Example 1 - Six 4 core cables to 22 kV motors
This example calculation is based on Example 4 in Appendix A of AS/NZS 3008.1.1.
Six four-core V-75 insulated and sheathed copper cables are arranged touching in a single horizontal row on a perforated cable tray for the supply of six identical 22 kW motors which have a full-load current of 45 A per phase and are installed at distances of 40 m, 55 m, 90 m, 135 m, 180 m and 225 m from the origin of the cable tray. Determine the minimum conductor size if a voltage drop of 2.4 % (10 V) is permitted for each cable.
SOLUTION – hand calculations (38 minutes)
The same approach used in the worked example by the Standard is as follows. The cable sizes required to satisfy the voltage drop restriction were assessed using the simple voltage drop equation and voltage drop (mV/A.m) values given in Tables 40 and 51 given for various cable types and configurations and limited to maximum operating temperatures which is conservative.