One 240mm¦ or 2x120mm¦ solution ?

[bfuchs]

If you needed to choose, which solution would you go for :<br>&nbsp;&nbsp;- install 2x120mm² cable per phase<br>&nbsp;&nbsp;- install 1x240mm² cable per phase<br><br>We assume of course that both are possible. (2 separate 120s can carry more than 1 240, but we'll assume here that a mutual derating brings the current carrying capacity down to the same level).<br><br>My opinion is that the 120mm² is easier to install and work with(bending radius, bolting to breaker terminals, etc...) but I am sometimes uneasy having multiple conductors in parallel.<br><br>What parameters would influence your final choice (including cable cost, installation cost, risks, maintenance, ...) ?

 

[gmazumder]

your choise is correct and the handling has to be done once while installation only. You need to keep the usual precautions in mind and you are well aware of them.<br><br>My considerations would be<br>1.Maintenance cost.<br>2.Down time cost.<br><br>other parameters are equally important but as they are one time and the above will become important for the opeartion of the plant.<br><br>regards<br><br>gautam<br>

 

[jbartos]

Suggestion:<br>The skin effect may also be considered; especially, if there is a higher frequency involved, e.g. 400Hz. Then, two smaller cross-sectional area conductors are preferable.<br>Another thing is electromechanical forces that may move conductors under short circuit conditions and impact cable insulations.<br>Next, a capacitive charging current of two paralellel cables is larger than one larger cable one. This has an impact on the system neutral grounding scheme(s) and ground relay or alarm relay settings.

 

[khodges]

To add further to the debate - The actual end application will be the determining factor, as the following must be taken into account.<br><br>1. Compare volt drop;<br><br>for 3 & 4 conductor XLPE cable<br><br>120mm2 (Cu) = 0.366mV/A.m<br>120mm2 (Al) = 0.577mV/A.m<br>240mm2 (Cu) = 0.216mV/A.m<br>240mm2 (Al) = 0.307mV/A.m<br><br>Thus for 120mm2 v. 240mm2, Cu = 69% higher volt drop/m, for 120mm2. Equally for Aluminium = 88% higher volt drop for 120mm2<br><br>For motor applications, or any application requiring a reasonable voltage at the load end, the 240mm2 option is preferable.<br><br>Kevin Hodges<br>Electrical Engineer<br>Genesis Power (New Zealand) Limited

 

[bfuchs]

Thanks to all for your feedback.<br><br>Khodges,<br>Are you sure about your calcs ?<br>On the 120mm² solution, there will be roughly half the current flowing through the each cable than there would be on the 240mm².<br>So by using your formulas, we would actually be getting less voltage drop using the 120mm² conductors. Using the same reasoning, we would also be getting a higher short circuit level at the end of the conductor (which is not necessarily bad when looking at the possibility of cascading, and ensuring quick operation of tripping devices to ensure protection against indirect contact = keeping earth fault loop impedance as low as possible under TN systems).<br><br>The impedance of 2 120s in parrallel is less than a 240 :<br>Z(2x120) = (R/120 + X) x L / 2<br>Z(240)&nbsp;&nbsp;&nbsp;= (R/240 + X) x L<br><br>The resistance is the same but the inductance of the 2x120 is less.<br><br>So overall, what would be your conditional choice :<br>&nbsp;- harmonics (high frequencies) are present <br>&nbsp;- long cables <br>&nbsp;- high installation labour costs <br>&nbsp;- high electrical/maintenance labour costs <br>&nbsp;- high product cost<br>&nbsp;- high short circuit currents&nbsp;&nbsp;<br><br>etc...<br>