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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
    You say the monitor is 20 Watt rated. So what current will the probe/sensor cables carry, not 20 W presumably. If they are connected to some type of sensor they may only carry milli-Amps. The monitor may take 20 W from the 230 Volt mains supply but the extra low Voltage sensor cables may carry much less than that. You need to check. The length of 0.5 m is very short and probably can be ignored. The sensor cables will not get hot in such a short length of conduit and be damaged. If carrying heavy mains' loads they may get a bit warm though.


    If I have got the wrong end of the stick then just use a readily available size of electrical cable above the current drawn by tthe load, say a 1.0mm2 cable for the short run and ignore any factors as that cable will be able to carry much more than you need. It is compact, inexpensive and readily available and nominally rated rated at 13.5 Amps, see B.S. 7671 Table 4D1A. You can ignore Volt drop as well here. Conduit factors etc. apply more to mains cables carrying heavy loads. See Appendix 4 of B.S. 7671. Table 4C1 and notes apply, mainly to mains cables carrying heavy loads.
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  • 0
    You say the monitor is 20 Watt rated. So what current will the probe/sensor cables carry, not 20 W presumably. If they are connected to some type of sensor they may only carry milli-Amps. The monitor may take 20 W from the 230 Volt mains supply but the extra low Voltage sensor cables may carry much less than that. You need to check. The length of 0.5 m is very short and probably can be ignored. The sensor cables will not get hot in such a short length of conduit and be damaged. If carrying heavy mains' loads they may get a bit warm though.


    If I have got the wrong end of the stick then just use a readily available size of electrical cable above the current drawn by tthe load, say a 1.0mm2 cable for the short run and ignore any factors as that cable will be able to carry much more than you need. It is compact, inexpensive and readily available and nominally rated rated at 13.5 Amps, see B.S. 7671 Table 4D1A. You can ignore Volt drop as well here. Conduit factors etc. apply more to mains cables carrying heavy loads. See Appendix 4 of B.S. 7671. Table 4C1 and notes apply, mainly to mains cables carrying heavy loads.
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