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Posted By Topic: Paralleling conductors

capybara
Feb 06 2020 09:39

Are we allowed to parallel conductors in a Aluminium 3c N/S? for example paralleling the screen and a core. I have seen it in a few older installations however i wonder whether it was before 3.4.3 (i) 'equal impedance is achieved by using the same material' was written in. I have looked at the Nexans catalog, they give the resistance of the cable, and not the difference between the screen and cores, which i assume would be fairly the same.


   

AlecK
Feb 06 2020 10:18

Item (i) is an example of how requirement (c) can be met. Clearly if one core is Cu, and one core is Al, the situation does NOT match the example of "same material".

If you go back to 3000:2000; the "same material" words were a requirement; along with "same cross-sectional area . Noting that with these compound cables, the Al cores are generally larger CSA than the Cu screen, because Cu is a better conductor.

Quick look-up indicates earlier rules appear to be silent on parallelling; even for both conductors = Cu. But since al conductors were uncommon in installations prior to 2000; I think it's more likely that these older instances of Cu in parallel with Al never complied.

Clearly the absolute "same material & size" requirement has been softened; but the guidance in the Example still points that way. And you have to be able to show that the load current is adequately shared. For that you need to know impedance for the frequency (and for harmonics?).
I believe that manufacturers will ensure that the screen is at least as great a CCC as any core - even though for many 3-phase applications the N current is substanially lower. But I wouldn't simply assume it's the same.
Screens have to cover the inner cores more-or-less completely. To get this effect, they're often more-but-smaller strands, even for all-Cu N/S cables. The stranding will affect impedance differently at different frequencies; but while it seems likely the screen's 50Hz CCC will be not less than the cores; it may end up being higher.
And if the CCC is not exactly the same, you can't expect the current to be shared equally.
The question becomes; will the inequality lead to over-heating?"

I certainly wouldn't just assume it'll be OK. Benny Hill once quipped that "when you assume, you make an ass out of U & me".
I prefer another one: "assumption is the mother of all fuck-ups".

Without detailed written confirmation of impedance (note impedance, not just resistance); I wouldn't do it; and I wouldn't accept it as an Inspector . Not even if the cable is over-size to avoid volt drop; and the actual load current is well below CCC.