Choosing the Right Wire: A practical Guide to Electrical Cable Size Selection and Voltage Drop Calculation.

Engineers must take into account a number of variables, including current carrying capacity, voltage rating, short-circuit rating, and environmental concerns to select the best cable for electrical systems to be reliable and efficient for a particular application.

Voltage drop calculations and proper electrical wire sizing are essential because undersized cables increase the risk cable overheating, melting and even causing fires. On the other hand, an oversized cable can needlessly eat into the budget causing cost inefficiencies and potentially interfere with circuit operation by adding unnecessary inductance and capacitance.

PROBLEM STATEMENT
An NPK plant is under construction within the industrial complex, during this construction a temporary fabrication workshop was erected nearby. The Workshop would contain cutting machines, welding machine and other hand tools for cutting and grinding. Considering the power requirements of the temporary fabrication workshop, we are to carry out cable sizing and voltage drop calculation. 

  • The Power source (Substation) is 200 meters away.
  • The system voltage 440V,
  • The permissible voltage drop is 5%
  •  The power factor is 0.86,  
  • The demand factor is 1

 

Step 1 - Determine the power requirements of each piece of equipment in the workshop and add 15% tolerance for possible overloads.

Step 2 - Take into consideration the cable laying detail i.e. if it is going to be buried or overhead. This is crucial because the current rating of a cable is affected significantly by the installation conditions and the external environment. In our case since the workshop is temporary the cable is going to run overhead on an existing cable tray. The ambient temp around the cable should be approximately 40 ° C worse case.

IEC 60364-5-52 standard provides cable derating factor tables to be applied to the cable current ratings when the actual installation conditions for your project differ from the standard conditions. There are four (4) factors typically used to calculate the overall cable derating factor namely ambient or soil temperature (k1), buried depth in meters, thermal resistivity of soil, and cable arrangement / grouping on cable tray.
When the cable is in open air, the Temperature correction factor(K1) is 0.87 for PVC and 0.91 for XLPE, K1 will be used to calculate the derating factor of the cable. 

CALCULATIONS

V = 440Vac

PF = 0.86
Workshop Power Requirement = 80KW, Overload tolerance = 15%, Demand factor = 1

Load in KW = 80 * 1.15 * 1 = 92KW
Load in KVA = KW /PF = 92/0.86 = 106.98KVA

Total load current/FLA = (KVA *1000)/ (1.732 * V) = 140.38A

 



                             Cable Ampacity Chart

If we look at Table 14, column 8 and we find that 70mm2 cable will be sufficient for us because it has an ampacity rating of around 177A rating which comfortably accommodates the load current of 140.38A that 106.98KVA power would demand

There is more ...

We have to look at the derating factor, acceptable voltage drops, and other factors and the following conditions must be met.

  • The derating amp must be higher than Total load current.
  • The cable voltage drop must be within permissible limits.
  • Cable Short-circuit capacity must be higher than system short circuit current.

 

DERATING FACTOR

Like we stated above the cable derating factor table for an ambient temp of 40 is 0.87 for PVC and 0.91 for XLPE, the cable is also going to run in a cable tray with other cables. In this case a cable on a perforated cable tray with up to 6 other cables have a derating factor on 0.91.

 

Total Derating Factor = Temp Correction Derating x Cable tray Derating x Ambient Temp Derating

                                   = 0.91 x 0.91 x 1 = 0.8281  

Total Derating Current of selected cable 70. sq.mm = 177 x 0.8281   = 146.6A

146.6A is greater than total load amp of 140.38A

See Applicable Derating Tables Below




VOLTAGE DROP

Voltage drop = (Total load current x Distance (m) x Resistance per unit length) / (System voltage x 100)

Voltage drop = (140.38 A x 200 m x 0.098 Ω/m) / (440 V x 100) = 6.14 V

Percentage voltage drop = (Voltage drop / System voltage) x 100 = (6.14 V / 440 V) x 100 = 1.4% (within the permissible limit of 5%)

 

FINAL DECISION
Reviewing the requirements, the 3C 70mm² Copper/XLPE/SWA/PVC cable selected would be a suitable choice for the temporary fabrication workshop.

If it wasn't within requirements, we would have selected the next size up in the cable ampacity chart

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