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AC Conductor and Cable for Commercial Solar Systems
Wednesday, September 2, 2020
What are the main conductor materials?
There are two main types of conductor materials commonly used, copper and aluminium. Copper is typically the most common and the heavier of the two materials. Aluminium becomes more common in larger sized cables and is desirable for its ability to bend more easily than copper - which can be a decided advantage when physically dealing with these materials.
The designer and installer of renewable energy systems has to take into account the characteristics of the available conductor/cable options and also the installation aspects.
Copper has a higher conductivity than aluminium and is resistant to corrosion in non polluted environments but doesn't like ammonia and sulphur fumes.
This fact is incredibly important when installing commercial solar on sites such as poultry farms, piggeries and dairies where ammonia from urine and faeces is a real concern and therefore must be taken into consideration when designing these systems.
The conductivity of aluminium is approximately 60% of copper and it has a lower melting point. But weight-for-weight aluminium is a better conductor. As previously mentioned aluminium’s flexibility lends itself to situations where AC cable runs are long and the physical nature of the cable can affect install costs.
Aluminium and copper cable can be joined but the installer must be aware of the difference in their respective coefficients of expansion and the fact that there are two dissimilar metals coming together.
Courtesy of Klauke
For a durable and secure connection, special Al/Cu cable lugs and connectors should be used.
How are conductors specified?
All conductors can be classified by their CSA (Cross Sectional Area). This means the number of strands that makes up the conductor (this can be a single strand or say seven strands). The larger the CSA the more current it can effectively accept without excessive heating. In Australia and New Zealand the main applicable cable standard is AS/NZ 3008 which contains tables of different types and configurations of conductors.
When looking at general wiring there are only three single strand types which are 1.0mm2, 1.5mm2, 2.5mm2. The rest are multi stranded ranging from 7 to 91 strands. So a conductor of a cable core might be single strand or multi-strand and the cable itself might have a single core or multi-core.
Size of conductors
Copper conductors range from 1 mm2 all the way up to 630mm2 whereas aluminium cables start from 16mm2 up to 630mm2. Depending on the application there are various insulation materials available.
There are various insulation materials which include; PVC, paper, rubber and also any insulation that has an armoured aspect.
Cables are classified by their conductor material, the CSA, insulation material and appropriate standard reference. For example:
PVC CIRCULAR, 2 & 3 core + earth copper conductors, circular 450/750 V-90 insulated to AS/NZ 5000.2. 0.6/kV V-90 insulated to AS/NZ 5000.1
In addition there are specifics on the number of cable cores, the general construction and voltage grade.
Types of insulation
The most common is PVC (Polyvinyl Chloride) and its variations.
- XLPE ( Cross Linked Polyethylene)
- Rubber and rubber type products-
- Oil-based materials
- Glass fibre and magnesium oxides
- Neoprene (wetsuit material)
- Paper (which was popular for underground, submarine and large cables in industry)
Cables on commercial sites
A typical commercial site has various kinds of cables which include:
- PVC single insulated
- PVC orange circular
- TPS (ThermoPlastic Sheathed) double insulated
- XLPE/PVC single core and multi-core
With three phase sites XLPE/PVC is a cable that is used extensively.
XLPE/PVC & PVC Circular
Cross linked Polyethylene PVC comes in single core copper and aluminium up to 630mm2. Also comes in multi-core, copper, up to 300mm2. Another common cable is PVC circular. This cable comes in 2 & 3 core + earth and also 4 core and earth up to 300mm2.
Now both XLPE/PVC and PVC orange circular are common cables but what are the differences?
- XLPE/PVC multicore has higher current rating up to 250 C compared to 160 C
- XLPE/PVC multicore has longer service life compared to PVC cable
- XLPE/PVC is halogen-free and also free of heavy metals such as lead and antimony
- It has low smoke and toxicity when burned
- It is a suitable alternative to PVC
XLPE/PVC is becoming more widely specified where there are environmental concerns (renewable energy systems) due to its halogen and heavy metal free status
The designer and installer, when assessing a commercial site, looks at the existing cabling types and runs and makes informed choices as to which cable to select and where. With commercial solar there are additional factors to take into account such as volt rise and matching the right cable to the output of the inverters involved. At all times cable selection must be based on many factors with safety being the first and foremost priority.