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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
    Questioning whether BS 7671:2018 should be used is actually quite valid.


    If the cabling is part of Machinery, as defined in the Supply of Machinery (Safety) Regulations, then it's worth also checking whether the particular correction factors line up with BS EN 60204-1, because:
    • Machinery (as defined in the Regulations) is outside the scope of BS 7671, and BS 7671 accordingly points to BS EN 60204-1; and

    • Whilst the approach in BS EN 60204-1 is based on the approach used in BS 7671 (i.e. based on the IEC 60364-series ), there are some minor differences when it comes to correction factors and ambient temperature effects etc.


    In other words, sometimes, you come up with the same answers using BS EN 60204-1, sometimes there are minor differences.


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  • 0
    Questioning whether BS 7671:2018 should be used is actually quite valid.


    If the cabling is part of Machinery, as defined in the Supply of Machinery (Safety) Regulations, then it's worth also checking whether the particular correction factors line up with BS EN 60204-1, because:
    • Machinery (as defined in the Regulations) is outside the scope of BS 7671, and BS 7671 accordingly points to BS EN 60204-1; and

    • Whilst the approach in BS EN 60204-1 is based on the approach used in BS 7671 (i.e. based on the IEC 60364-series ), there are some minor differences when it comes to correction factors and ambient temperature effects etc.


    In other words, sometimes, you come up with the same answers using BS EN 60204-1, sometimes there are minor differences.


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