This presentation leads on from our Cable Derating presentation where we look at the various factors when it comes to selecting and assessing what cables are needed from a Current Carrying Capacity Perspective and how location and method of installation plays a huge role.
Now we delve deeper into this area and look at the actual differences between the cables and the variations.
Remember with all electrical standards there is a shall and should component but it seems with AS3008 it is all shall!
In this presentation we are going to look again at the actual case study of a 300 kW + system.
So what inverters are we using on this site?
We know that when we compare Al to Cu that for the same size cable Cu has a greater current rating even though per weight Aluminium is the better conductor.
If we look at the different insulation types, thermoplastic versus XLPE 90 deg we see variations in CCC. In addition there are differences between multicore and single core and further in the case of copper flexible versus solid stranded.
And how the cable is installed and in what, plays a role and the proximity to other cables, circuits etc. The list goes on!
So, how big are these differences?
Let’s first look at the difference between Al and Cu
As an example we will look at:
Al XLPE 90 deg 4C+E Wiring Enclosure in Air Enclosed 88A
Cu XLPE 90 deg 4C+E Solid Wiring Enclosure in Air Enclosed 114A
*Most common method of installing cable between inverter and PVDB, in perforated tray, against wall with lid.
There is a substantial difference between the two and in the case of the 44 kVA inverter we have referenced it requires a current of 63.51A
Can we still use 35mm2 aluminium in this case? What about Al single core XLPE?
Al XLPE 90 deg Single Core Wiring Enclosure in Air Enclosed 93A X 0.65 = 60.4A close but no cigar*
*Table 8, column 17 AS3008
In many cases tray lid is used for aesthetics but if the inverters and PVDB are in an authorised area it may be an option to install in perforated tray on the wall using single core Al with NO TRAY LID.
If we look at AS3008, Table 8 column 7 Unenclosed spaced from the surface we see the CCC of 35mm2 Al single core XLPE 90 deg cable is 119A
If we look at clause 3.5.2.2 (c)it says: Groups of circuits in free air. Groups of circuits installed unenclosed under the conditions and circuit arrangements depicted in Figure 1
See part of Figure 1 below:
The reality is we can use Al XLPE single core 35mm2 cable now ‘cos we changed the method of install from full enclosed tray affixed to wall with lid to tray slightly off the wall with no lid!
In fact if we look at Table 8, column 7 we see that we can even possibly use 25mm2 from a CCC perspective (97A)
Obviously we have to also calculate the Voltage rise and this may result in having to use a larger capacity cable or swap to copper.
So we now see how important it is to really assess the situation completely!!
This is the question?
From an installation point of view installing a tray without lid takes less time and material BUT we know we have to use extra components to space the tray from the wall.
Now, using multicore with the 4 x inverters under consideration, install time is less as there are less cables than using a single core option and what about the cost of the different cables under consideration?
It’s a question of material: install cost ratios. Does the extra install time potentially offset the gains from using smaller diameter cheaper cable?
Also if using Al, there are specially cable lug requirements etc.
Here goes!
So it can be seen that the amount of copper or aluminium material can be reduced considerably if the method of installation is more conducive to heat dissipation as this is what determines the CCC of the cable in question.
We have seen that if the installation is in perforated enclosed tray directly on the wall, not only do we require a thicker cable but also we have to derate the cable as well . In this example, there are four ‘circuits’ ( read inverters) so derating is required.
So from 16mm2 Cu XLPE 90 deg Single Core Solid Touching Unenclosed @ 86A best case to 35mm2 Cu XLPE 90 deg 4C+E Flexible Wiring Enclosure in Air Enclosed 111A derated to 72.15
The reality is that if the cable runs from inverters to PVDB are fairly short the cost savings may be negligible if selecting an unenclosed cable option ( if allowed) but if the runs are long. . . . .
Real savings can be made with the run between the PVDB to the DB or the MSB.
Note: even though the cable may satisfy the current carrying capacity there are still Vrise considerations that must be taken into account.
The whole area of cable selection according to AS3008 is a complicated one. Correctly assessing what installation options are available may save the installer a considerable amount of money but of course this may be tempered by the material:install cost ratio and the overall amount of money spent to design, supply and install cable that satisfies the Standards, CCC and Vrise.
Good luck on your next project.
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