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Temperature rating of C.B's, cables and conduits

Former Community Member
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Question 1: All PVC conduits in market to BS:61386 are rated for 60C operating temp while CU/PVC single core wires to IEC 60228 are rated for 70C. what is the BS 7671 point of view.
Question 2: BS 7671 section 512.1.5 states that :Switchgear, protective devices, accessories and other types of equipment shall not be connected to conductors

intended to operate at a temperature exceeding 70°C at the equipment in normal service unless the equipment manufacturer has confirmed that the equipment is suitable for such conditions, or the conductor size shall be chosen based on the current ratings for 70° C cables of a similar construction. where as NEC 110.14(c): “Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment,correction, or both.” Derating factors may be required because of the number of conductors in a conduit, higher ambient temperatures, or internal design requirements for a facility. By beginning the derating process at the ampacity of the conductor based on the higher insulation value, you may not be required to upsize the conductor to compensate for the derating.is there a similar way to apply the exception of NEC using the BS 7671.
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    Kindly check pages 5 and 6 of attached file and give me your opinion.



    Thanks for that - an interesting read. It does seem that they just mean applying correction factors.


    Curiously they have a concept of a minimum cable temperature rating to suit MCB terminals - e,g, if you're running a conductor at 60 degrees you still need to use a cable with 75 degree insulation to cope with the heat generated by the MCB itself. Maybe we just side-step the issue as all our normal cables have an insulation rating of at least 70 degrees.


       - Andy.
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    I'm not entirely sure what the yanks mean by 'ampacity adjustment' etc.


    Hmmmm. Just yanks being yanks, invent a stupid name or saying for summat.


    Sames as this phrase "Be advised, at this time, we have a situation"  over here in proper speak land the bloke would come home, take off flat cap and say "there`s been trouble at t`mill" and everbody knows what he means. Nobody here ever says "Be advised, at this time, we have a situation" we would be looked upon as stark raving bonkers
  • Former Community Member
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    . (I'm not entirely sure what the yanks mean by 'ampacity adjustment' etc.)


     


     

    Kindly check pages 5 and 6 of attached file and give me your opinion.
  • Former Community Member
    0 Former Community Member
    I tried to apply Thomas Cook's Equation for NEC tables but it useless there.
  • Former Community Member
    0 Former Community Member

    AJJewsbury:




    You mean Formulas such as Neher-Magrath or that which are found in IEC-60287 or is there another formula in BS-7671.

    I usually use ETAP to find actual operating temperatures of cables for Under ground systems but I can't find easy equation for another mode of installations. 



    I've (to my shame) no idea what the formula is called or where it's originally from. I'm pretty sure it doesn't appear in BS 7671 in this form, although it does have some similarity to equation 6 in appendix 4 of BS 7671 if you strip out all the correction factors.


    I've dug it out - this is an extract from the IET's Commentary on the Wiring Regulations, 16th Ed which probably explains it better than I could ever paraphrase. (it's way out of date now of course, but the underlying physics won't have changed I'm sure):

    8d464cf8fce7f0b9103de4ff7b0e720e-huge-extractfromcommentary16thed_conductortemp.jpg


       - Andy.

     



    So Interesting equation I've been searching for such equation for long time.I like the idea to estimate the operating temp of a conductor before even loading it. and the idea is that it's so simple and no need for a software. But Thomas Cook didn't mention a normative or engineering reference or how did he derive this equation.It is vague.

     

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    Andy the  Neher & McGrath forumula is explained here 


    It includes terms for the temperature gradient across the insulation, and the change of resistance when in use relative to a cold DC reading.  If you allow both if these effects to tend to zero, what is left can be re-arranged from "what is the current at X temperature?", to "what is the temperature at Y current?" , this becomes the same as the forumla used in the IET analysis.
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    You mean Formulas such as Neher-Magrath or that which are found in IEC-60287 or is there another formula in BS-7671.

    I usually use ETAP to find actual operating temperatures of cables for Under ground systems but I can't find easy equation for another mode of installations. 



    I've (to my shame) no idea what the formula is called or where it's originally from. I'm pretty sure it doesn't appear in BS 7671 in this form, although it does have some similarity to equation 6 in appendix 4 of BS 7671 if you strip out all the correction factors.


    I've dug it out - this is an extract from the IET's Commentary on the Wiring Regulations, 16th Ed which probably explains it better than I could ever paraphrase. (it's way out of date now of course, but the underlying physics won't have changed I'm sure):

    8d464cf8fce7f0b9103de4ff7b0e720e-huge-extractfromcommentary16thed_conductortemp.jpg


       - Andy.
  • Former Community Member
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    AJJewsbury:

    There's a formula relating the derating of a cable to its conductor temperature (I don't have it to hand but should be able to dig it out if needs be) - from memory it's similar to the one for adjusting voltage drop when a cable isn't fully loaded. I'm sure I've seen it used to justify the use of 1.5mm² MICC cable on ring circuits in one of the IET books - showing that the 70 degree terminal temperature isn't exceeded.



    You mean Formulas such as Neher-Magrath or that which are found in IEC-60287 or is there another formula in BS-7671.

    I usually use ETAP to find actual operating temperatures of cables for Under ground systems but I can't find easy equation for another mode of installations.
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    You are making a big assumption here - that there is logic behind the cable rating figures.



    I'm sure there's some logic. Maybe not a single consistent approach though, I agree.


    At one point I tried re-scaling the OSG's table of R1+R2 values (given at 20 degrees) to various other temperatures (just to see the difference temperature could make to test results) - and realised that some of values for 70 degrees should match the mV/A/m tables (at least for smaller sizes where reactance is ignored) - which of course they didn't quite. The official reply seemed to be that both were right .... each in their own way.


       - Andy.
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    AJJewsbury:


    On the face of it that sounds inconsistent, but I think the difference is accounted for by PVC  having a higher thermal insulation value than XLPE 


    Andy,

    You are making a big assumption here - that there is logic behind the cable rating figures. I know of various standards where the cable current ratings would not stand up to a rigorous scrutiny as to their basis. However having been around for decades and no-one questioning them does mean that they are accepted as valid, and after all if they have not given any problems then who is to say that they shouldn't continue to be used.

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