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Thread: NEC Cable Vs IEC Cable

  1. #1
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    NEC Cable Vs IEC Cable

    Greeting all,
    I have faced one problem in selecting cable type and hope to help me. Let me explain it clearly please

    I have dont all my calculation ( continues, non continues, demand factor, derating factor, breaker selecting size, correction factor and etc) based on NEC book which is recommended in Project Scope of Work.

    Now for selecting cable type.

    We all know that table 310.16 contains THHN, THWN, XHHW, RHW cable types with their maximium operating temperature, ambient temperature as well as current carrying capacity.

    we know that insulation type has very impact on current ampacity. We could not find THHN, THWN, in afghanistan and around countries.

    So, we find one turkish company the cable type is NYY and is based on IEC code. I have also attached the cable catalog. This cable has much more current carrying capacity than the NEC table 310.16.


    I have applied all NEC factors on these cable for selecting a cable and sppose cable 70mm2 which has a current ampacity of 228 ampere.


    I have a load which contains = 214 ampere final with all factors
    I have selected 220 ampere breaker with 70mm2 cable according to cable catalog.

    Now I have got comments to use the NEC cable's current capacity.

    Eventhough this cable is already approved and submitted

    Could anyone check the current capacity of this catalog comparing to NEC cables and advise me.

    Is there any engineering or technical problem since Breaker and cable has no problem per catalog.
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    "Imagination is more important than knowledge."

  2. #2
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    Armored Cable

    You are dealing with an engineered cable with ratings that are not typical for the NEC. The NEC is designed for typical building wire used in construction in the USA. This armored cable is more like what is used in mines or quarries. The operating temperature is non-NEC (70C), the ambient conditions are 20C in ground and 30C in air. No information on ground thermal resistivity or burial conditions is provided. To correctly determine the rating in your application, ampacity calculations are needed for the specific conditions expected. If you need NEC-rating, the manufacturer should be able to provide the adjusted ampacity for your conditions.
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  3. #3
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    NEC 310.15 gives you a choice. You either have to use the tables (310.15(B) directs you to this) or you can use 'engineering supervision' (310.15(C) directs you to do this).

    70mm^2 is slightly larger than 2/0 AWG.

    The temperature rating of the cables that you are looking at is 70C.

    You cannot strictly use the tables because your wire size is not present and your conductor type is not present. However a reasonable approach would be to use the 60C column and 2/0 wire size, both of which are tabulated values that are more conservative than the wire you are actually considering. If you do this you get an ampacity of 145A.

    These tables were pretty clearly created using very conservative values for thermal resistance between the conductors and the surrounding environment. In all but extreme cases, the conductors run cooler than the tables would lead one to belive, because the conductors are cooled more effectively than the tables assume.

    If you use 'engineering supervision', then you will the Neher-McGrath equation given in 310.15(C). This opens up the possibility of adjusting for your non-NEC wire size and insulation temperatue rating, but now you also need to select the 'RCA' (the effective thermal resistance between conductor and environment).

    I think that you would remain consistent with the NEC if you select a value for RCA that matches that used to generate NEC table 310.16, however you would need to figure out what it is, perhaps by solving the Neher-McGrath equation in for RCA and plugging in the values of table 310.16.

    The conductor ampacities given in the catalog that you linked are almost certainly based on much better cooling than assumed by the NEC. I don't think that you would be consistent with the NEC if you use those values, although if you carefully engineer the system and actually know what the thermal resistivity is you can create a safe system with much higher ampacity than given by NEC 310.15 and table 310.16

    -Jon

  4. #4
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    Thanks all for information and one things also in my mind :

    I have two electrical power system software. One is Etap power station 6.0 which is very common in the world in using power system analysis and design.

    I checked its cable catalog which is based on NEC code for THHN, THWN, XHHW type and has same ampacity as NEC code table 310.16

    There is also one thing else which is frequency. If you select 60Hz option all NEC code table 310.16 comes and if you select 50Hz then it does not come

    From this I got that NEC code table 310.16 is for 60Hz whereas in
    Afghanistan system is 50Hz. It is better to use IEC which is for 50Hz.

    I am also using Digsilent Powerfactory Software and I checked its cable catalog which is based on IEC cable for 50Hz.

    Am I right or not please advise me.
    "Imagination is more important than knowledge."

  5. #5
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    If it is required to use NEC with IEC cable types, then I'd suggest using the NEC table column for the temperature rating of the IEC cable. If the temperature rating is between NEC temperature ratings, use the next lower NEC temperature. Convert the mm² area to AWG or kcmil and use the table rating for the next lower AWG/kcmil size. Don't worry about the 50/60 Hz difference. There may be a slight difference because of skin effect, but using the 60 Hz value will be conservative.

  6. #6
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    I am not familiar with either of those software products.

    The difference in physics between 50Hz and 60Hz is slight; so my _guess_ is that your software is automatically selecting different _legal_ requirements based upon frequency rather than calculating different physical results. At 50Hz you will have slightly less dielectric heating, and skin effect will be deeper, but we are talking very small changes.

    -Jon

  7. #7
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    yeah, friends you are right, but there is no problem in ambient temperature and max operating temperature.

    Please read the first question NEC vs IEC that i have mentioned in the beginning

    if you have any idea about that please advise me.
    "Imagination is more important than knowledge."

  8. #8
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    I don't believe that I can answer your question. It seems to me that if you are required to use NEC standards, then you need to use NEC standards for evaluating the cables. The NEC does not recognize the cable that you've described as an approved wiring method...though it almost certainly would be acceptable if it were evaluated per NEC criteria and given a corresponding NEC recognized rating.

    -Jon

  9. #9
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    yes, you are completely right.
    "Imagination is more important than knowledge."

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