8 Steps to Low Voltage Power Cable Selection and Sizing 

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A recurring theme on our forums is cable sizing.  Now many installations are unique and require special consideration.  However,  a lot of the time things are just repeated.  When looking at low voltage power cables I generally always start with the same basic strategy. 

  1. Default to using XLPE - why bother  with other insulations (PVC, rubber, etc.).  XLPE is well established, cost competitive and doesn't have any of the degradation or fire related issues of other insulations.  You will also end up with a smaller cross sectional area.  Only in special circumstances would you need to look at other installation types.
     
  2. Use armoured - buried cable mechanical protection is essential.  For indoor cables the use of armouring is  not essential, however you have the benefit of using the armouring for the CPC.  On indoor cables, perhaps make the choice of armoured or not  dependant on local practice.
     
  3. Use LSZH (low smoke zero halogen) sheath - smoke and toxic fumes in a fire situation are not good.  Why not just avoid  the issue.
          
  4. Calculate the current rating using an acceptable method.  I tend to use the method given in BS 7671 as this is generally applicable where I work.  Calculate the rating taking into account both the design current and protective device rating and apply the necessary derating factors.
     
  5. Calculate the voltage drop - again I tend to use BS 7671 and check it complies with local regulations.  The voltage drop needs to be the sum of all cables in a circuit (from source to end load).
     
  6. Ensure the cable can take the fault level - for most larger cables this tends not to be a problem, but for smaller cables it can be an issue.
     
  7. Use software - if possible use approved software to do items 4 to 5.  It makes life easier.  Two which come to mind are the myElectrical cable sizing tool and AMTECH.
     
  8. Be practical - make sure your cable size is reasonable.  If you end up with a 120 mm2 cable on a 2 A load due to meeting voltage drops or fault levels start to look a the system design concept itself.

while you cannot say "once you have selected one cable you have selected all cables',  you may be able to get away with saying "once you have selected a few cables you have selected most cables"

Finally we need a disclaimer here.  While the above is good for most situations (low voltage power), it does not cover every case.  There are situations which are different, unique or require some special consideration.   To address these situations, one of the best things is to understand fully the characteristics of the load the cable will be supplying, the environment it is being installed in and be aware of other overriding issues.  If you can do this,  any necessary adjustments to the eight point plan often become obvious.



Steven McFadyen's avatar Steven McFadyen

Steven has over twenty five years experience working on some of the largest construction projects. He has a deep technical understanding of electrical engineering and is keen to share this knowledge. About the author

myElectrical Engineering

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  1. Peter's avatar Peter says:
    9/1/2011 2:11 PM

    When would you not use XLPE for the insulation? Great post by the way.

    • Steven's avatar Steven says:
      9/1/2011 2:11 PM

      Depends on the situation. For example if the cable is to be moved a lot I would investigate more flexible insulations. A cable in an environment subject to chemicals would require different consideration. The key is to understand where and how the cable will be used.

  2. Ivana's avatar Ivana says:
    9/1/2011 2:11 PM

    Thanx Steven..

    I have faced some problem in cable sizing due to voltage drop consideration.

    For 50 A dc load, I had to select 120 sq.mm al cable, pvc insulated, to limit voltage drop within 5%.

    What can be other method apart from increasing cable size to get desired cable within limited voltage drop and what are standard voltage drop limits for different application.

  3. Eng Essam Osman's avatar Eng Essam Osman says:
    9/1/2011 2:11 PM

    Firstly thanks about that summary

    Secondly what about the  Neutral cable (Fourth line at the cable ) configuration ( Diameters  CSA & Material & Current Loading Conditions) in building wiring.

    • Steven's avatar Steven says:
      9/1/2011 2:11 PM

      The neutral is usually part of the same cable (multi core) and hence same insulation/material. If separate it is still normally the same material. Sometimes people look at a reduced neutral CSA, but I tend to make it the same size as the line. In installations with harmonics (particularly triplen), you may need to double the neutral size.

  4. Jansen's avatar Jansen says:
    9/1/2011 2:11 PM

    Steven I have a question also.  When sizing cable in terms of voltage drop what is the latest or useful formula when calculating voltage drop in 3 phase and 1 phase ac system?

    • Steven's avatar Steven says:
      9/1/2011 2:11 PM

      Jansen, mainly because of where I work, I tend to use the IEE Wiring Regulations. These have tables for single and three phase situations and provide the necessary formulae. If you don’t have access to the IEE or don’t want to use them, you can get impedance (resistance and reactance) figures from the cable manufacturer. It’s then just a matter of applying Ohm’s law (V = IR) to get the voltage drop. For three phase circuits the voltage drop relates to the line voltage and for single phase to the phase voltage. One thing to bear in mind is that cable conductor has a positive temperature coefficient of resistance. The resistance of the cable will increase as it heats up due to the load current.

  5. naban's avatar naban says:
    7/3/2012 10:17 AM

    In IEE wiring regulations,latest edition(16th & 17th editions),the voltage drop given for
    a)single phase-should we multiply by 2(phase & neutral),
    b)for 3 phase- should we multiply by 1.732 to get line values

    • Steven's avatar Steven says:
      7/4/2012 7:02 AM

      In appendix 4, section 6 the regulations say that the values in the tables represent the voltage drop in all the circuit conductors. I think this already includes all the multiplying factors.

      You can also look at the following post for more information on calculating voltage drop:
      http://myelectrical.com/notes/entryid/182/cable-sizing-tool

  6. Notes's avatar Notes says:
    2/1/2013 10:30 AM

    Trackback from Notes

    IEC 60287 "Calculation of the continuous current rating of cables (100% load factor)" is the International Standard which defines the procedures and equations to be used in determining the current carry capacity of cable. The standard is applicable... ...

  7. Deepak's avatar Deepak says:
    6/13/2013 1:27 PM

    Firstly Thanks for your information,
    Steven, please give me some information about voltage drop from the indian standard point of view


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