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18th Edition Cable Calculations

Steve Brown
Hi guys I need some help calculating cable sizes for a 24v DC supply. I'm a student doing a job report and I want to explain cable sizing basically, and whilst on site my mentor, with his vast experience, knew what cable sizes would work so I just listened to him. Now I want to calculate it properly.


So it's the nitrate monitor requires a 230v supply but 4 cables, 2 pairs each for 2 different alarms, I want to install are on the 24v DC side. (They're just going to a junction box that already has cables feeding back to the PLC).

The cables will be installed in conduit on the wall less than 0.5m away. (distance between monitor and junction box).

The monitor they're being connected to is only 20W.


So the farthest I've got, (unless I'm wrong), using the 18th is this:

Ib, (Design Current), = 20W/24v = 0.83A.


I read in the 18th edition, (correctly I hope), that I will have a rating factor of 0.65 because there will be 4 cables enclosed. (Am I right?) But I've no idea what to do with that figure.


I've really no idea where to go next or even if I'm on the right lines thus far. Can someone explain it to me? I don't necessarily want the answer, but a formula would be brilliant.


Thankyou.
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  • 0
    The OP was concerned with the method of cable sizing according to the 18th edition of BS 7671 as he stated. He was interested to know how to apply correction factors when cables are bunched. This primarily applies to mains' cables  (LV) carrying largish currents as I have already said in my previous posts. There are insufficient details in his post to accurately answer his question regarding his monitor, but I believe that he has been put on the right track to undertake the application of correction factors in the future using BS 7671. I tried to provide simple straightforward answers to help him, and not get too bogged down with too much technical detail as he is a student. I have directed him to the relevant sections of BS 7671 for further information.


    C.
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  • 0
    The OP was concerned with the method of cable sizing according to the 18th edition of BS 7671 as he stated. He was interested to know how to apply correction factors when cables are bunched. This primarily applies to mains' cables  (LV) carrying largish currents as I have already said in my previous posts. There are insufficient details in his post to accurately answer his question regarding his monitor, but I believe that he has been put on the right track to undertake the application of correction factors in the future using BS 7671. I tried to provide simple straightforward answers to help him, and not get too bogged down with too much technical detail as he is a student. I have directed him to the relevant sections of BS 7671 for further information.


    C.
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