Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 7 replies
  • Subscribers 37 subscribers
  • Views 2530 views
  • Users 0 members are here
Options
You may also be interested in
This discussion has been locked.
You can no longer post new replies to this discussion. If you have a question you can start a new discussion

CABLE sizing

James Crook

Hi All 

have a question on cable sizing as I'm calculating it at 16mm SWA  I have 2 circuits that need to be replaced with new cables they are exist 10mm 2 core and 1000 metres total and first point is 350m  the design current is 2.5 amps and is feeding some 12volt transformer, just after some feed back as to weather I can derate the cable some how 

thanks in advance for any feed back 

  • 0

    How have you done the calculations? This is an abnormal situation and I'm interested to see the different approaches to this problem; Over the last 15 years or more, this problem has consistently come up, in its various forms, depending on different loads at the end of the line. 

  • 0

    The usual fix is to move the DNO transformer nearer Relaxed.

    But failing that the next trick is to up the voltage - so you may find loads between 2 phases on a 400V supply (or even 690 in some large factories), rather than one phase  and N to give 230V as you may expect on a shorter run. This is a standard wheeze for football ground lights for example. Of course if your 12V transformers are already wound for 230, this is not an option either.

    If you know the load characteristics will be OK with it, then you can take more voltage drop than the regs appendix suggests, but not so much that a dead short at the end will not promplty fire the ADS - this tends to give an upper limit of 10% or so before special measures are needed to ensure safe fault handling.

    Mike.

  • 0

    If you look to the right of this topic, one useful new feature is called "related"- I assume-  topics could be added to that; and there is one topic there, called "Very long run of LV Cabling". similar problem as your own. 

  • 0 in reply to mapj1

    Thanks Mike that is something to think about 

  • 0 in reply to tattyinengland

    I just used the standard volt drop calculation which just seems overkill 

  • 0

    I just used the standard volt drop calculation

    Did you take into account the lower conductor operating temperature, resulting from the reduced load, which would result in a lower conductor resistance? - e.g. 'Equation 6' under 6.1 of appendix 4 of BS 7671?

       - Andy,

  • 0 in reply to AJJewsbury

    For a modest load of 2.5 amps, moving the DNO transformer nearer the load, or use of 690 volts is most unlikely to be viable.

    Sensible options include accepting the rather large and expensive cable.

    Accepting a larger than normal voltage drop to save on cable size, if the "12 volt transformers" are in fact regulated switched mode power supplies, then as much as 20% voltage drop might be acceptable. You might find that 2.5mm can then be used.

    If a three phase or split phase supply is available at the supply end then use this to reduce the current. If the "12 volt transformers" are available for 400 volt or 460 volt input then that will drastically reduce the cable size needed.

Community Rules & Guidelines