Are you getting the most out of AS/NZS 3008 Selection of Cables?
With a new revision to AS/NZS 3008 just around the corner, Peter Vandenheuvel recommends now as the best time for electricians to brush up on how it’ll impact their business.
All who are regularly involved in the installation of cables use AS/NZS 3008 Selection of Cables as the go-to for sizing-compliant cabling installation. But are we getting the most from it?
With some additional understanding of the Standard, the smart designer or installer can cut costs significantly in one of the very few things they can actually have some control over.
This Standard sets out the Current Carrying Capacity (CCC) of cables for a wide range of installation arrangement methods in common use in both Australian and New Zealand conditions.
But it’s not just about defining CCC to limit the temperature rise on cables. It also deals with the other important factors that must be taken into account. These include:
- Voltage drop – as an excessive voltage drop will increase the current flow so potentially overheating the cable during normal operating conditions; and
- Short circuit temperature rise – where a prolonged short circuit may raise the conductor temperature rise above 200oC!
It is listed in AS/NZS 3000 as a ‘Normative’ reference, meaning it must also be complied with where applicable. So, because the rules are used by some 100,000 to 150,000 users in Australia, New Zealand and a number of other countries in the South Pacific, many of those users must also ensure they have access to it as and when needed.
The AS/NZS 3008 series of Standards originally came out of the Wiring Rules back in 1984. Before then, there was a limited set of basic tables in the rules, but that was quite adequate before that time. With all the new developments in cable types, insulation materials and installation methods, it was then recognised additional information would make it more effective, safer and with better economical outcomes.
The original 1984 publication was fine-tuned in the 1989 second edition, further revised and redesignated in 1998 as a combined Australian and New Zealand Standard. It was then again revised in 2009 and 2017. During that time, it has grown from around 60 pages to possibly 190-plus pages for the now-in-the-pipeline 2023 edition.
A critical hazard warning
Over the years since those first electrical installations, even preceding the initial publishing of the wiring rules, there have been a number of cable types such as bare copper, lead sheathed (late 1800 to 1910), vulcanised India rubber (VIR) (1910s to 1960s), tough rubber sheathed (TRS) (1940s to 1960s) and then thermo-plastic sheathed (TPS) (from 1960s onward).
In addition, there have been many wiring systems ranging from bare wires and insulated cables in the open, on insulators, in capping and casing (in cedar wood channels) and the most hazardous; in split ‘closed joint’ metal conduit (CJ).
Then since the advent of TPS cable, there have also been several instances where ‘on the edge’ installers wanting to be more competitive (or just making extra profit) were sourcing cables not up to standard.
Many jurisdictions have publisher warnings about these risks because in all these situations, there can be an extreme risk of a fire burning the premises down as well as electrocution of anyone touching it; especially the latter with CJ conduit that can become ‘live’ in service.
If found, electricians and contractors must be aware that no work whatsoever should be started until the whole installation (including the incoming supply) has been isolated. Also, all such wiring should be replaced in its entirety before the installation is placed back into service.
The revision procedure so far
Standards Australia and Standards New Zealand, like their counterparts worldwide, regularly revise standards to ensure they are kept up to date with new technologies, products, processes, work practices, safe work obligations, industry expectations and safety to the community; to name but a few. To do this requires regular review and publication of new editions of all the standards under their control.
As part of this process, AS/NZS 3008 is being reviewed. That has so far included the consideration of the many different suggestions, recommendations and proposals for change from the large number of organisations and other stakeholders reliant on it.
These proposals have all been recorded, discussed and judged on their merit, resulting in a refined list considered as requiring action for inclusion in the revision redraft. That draft has also now been scrutinised and reworked by both the working group and the overriding committee – Standards Committee EL-001 – and issued for public comment.
The public comment draft gives an opportunity for users to comment and has provided them with a glimpse of what it is likely to include. It should be noted that there may still be minor changes arising out of the public comment review.
All these public comments are now in the final stages of being dealt with by Committee EL-001 for any further changes deemed to be required.
This is where the process is now basically up to, so there could be some further changes. If any of these are major, they may need to be put to a second public comment, adding another two or three-month delay in publication.
But it is unlikely such high-level changes will be made unless deemed critical, as they can otherwise be carried over to a future amendment or the next revision. So, what we see in the PC document is most probably (in the main) what we will get.
Users to not just focus on the tables
Because there are so many tables, time-poor users are often tempted to just refer to the tables, skipping the most important information. But this can be a real trap. It would be very unwise for the user to gloss over it. Why? Because it is in the fine print and the detail where some of the real cost savings, as well as the avoiding having to do rework, are hidden. It’s also where you may find your competitive edge, as detailed later.
The tables only tell you what you can do, you must decide how best to use them as it’s the other details that can give you the smarts on how you can do it smarter, better or cheaper.
Here is a quick summary of the different sections, just in case you don’t have a copy handy nearby.
- Preface; – Among other things, this includes the list of major changes, some of which are listed here later. Additionally, until now, the Standard has only dealt with 0.6/1kV AC voltages, but it will now also include DC voltages up to 1.5kV to align with the Wiring Rules.
- Section 1; Scope and General – the Standard applies to Australian conditions – with AS/NZS 3008.2.1 for NZ (and possibly also Tasmania) in the pipeline.
Also, the normative references in the Wiring Rules to other standards also apply to this Standard, so these must be considered and complied with where applicable.
A word of caution, the ambient temperatures are noted as those in the immediate cable neighbourhood including materials to which the cables are or will be connected, implying this may be higher than the normally accepted ambient for the overall location.
- Section 2; Cable Selection – selection must satisfy all three criteria, CCC, Voltage Drop and Short Circuit Capacity. It also refers to economic optimisation, which is in part dealt with under that heading later.
- Section 3; CCC – ratings, insulation temperatures, types and method of installation; this includes much information both in the lead-up to the tables but also in the tables themselves and requires careful reading of both so nothing is overlooked.
- Section 4; Voltage Drop – calculations based on millivolts per amp/m for AC & DC, impedance, operating temperature and power factor. As these can impact the selection of cables, especially those carrying higher current, as well as those of considerable route length, they should be referenced if there is any doubt.
- Section 5; Short Circuit Performance – temperature limits, calculation of short-circuit currents and installation method influences. Although these are generally considered as ‘being covered’ by the earlier sections, they should also be referenced for particularly onerous short circuit fault levels.
- Appendix A; Different examples and comparison of installation methods – a ‘must read’ later in this article for those wanting to find the lowest cost method of installation or best owner ‘pay back’ on larger CCC cable selection.
- Appendix B; Examples of correction factors for harmonic currents – should be referenced in any situation where harmonic currents fall outside of the ‘normal’ range.
- Appendix C; Recommendation for circuit configuration for parallel cables in AC circuits – a helpful guide in finding the best and most cost-effective installation configurations, especially for high current and or extended route length situations.
- List of Figures; Users to become familiar with those figures relating to your applications to ensure you optimise the outcomes and assist in finding the best arrangements to apply.
- List of Tables; There are 140-plus tables, so always read the heading carefully to ensure you use the best-fit table for your application.
The proposed changes
The following changes are detailed under ‘Preface’ in the public comment draft as likely to be of interest or included;
- DC ratings including some worked examples; these will include both AC and DC ratings in the appropriate table.
- Some amalgamation for flexible vs rigid conductor types.
- In many tables there is minimal difference between AC and DC ratings except where cables approach or exceed 95mm.
- CCC ratings for 110oC aluminium conductor will now be for X-HF-110 instead of R-HF-110 cables as these are now more common in the industry use.
- Dimension alignment with flexible cables to AS/NZS 5001.1.
- Values for 10mm cables have been reassigned with their IEC equivalents in direct buried installations.
- All cables including 630mm (previously omitted) are now included.
There are other changes listed under Preface in the PC document items (j) to (t).
Optimising Cable Selection.
Cable selection, especially for larger amperage cable runs, is not just about selecting a cable rating that will do the job, it is as much to do with how the cabling is installed. As an example, I have summarised part of Appendix A1 here as it is very much worth studying:
The example in Appendix A1 shows four options for installation for a 1,450A three-phase circuit underground, all using 400mm V75 single core insulated and sheathed cable, here is a comparison;
- Option 1; using a single conduit or duct, it requires five route lengths of three cables, with a CCC of 1476 Amps, so it has 15 cables in one enclosure.
- Option 2; using four conduits or ducts, it requires only four route lengths of three cables, with a CCC of 1554 Amps, so 12 cables installed in four enclosures.
- Option 3; using twelve conduits or ducts, it also requires four route lengths of three cables, with a CCC of 1636 Amps, so still 12 cables but installed in 12 enclosures.
- Option 4; using no conduit or duct requires, just three route lengths of three cables, with a CCC of 1547 Amps, so nine cables with no enclosures.
See the comparison table below;
|AS/NZS 3008 Appendix A1
Installation Cabling Configuration Comparison
|Number of Cables
As noted, all have a CCC of 1,450A or greater, and based on the option four cable as 100% of the cable required and its cost, option two and three need 33% extra in cable cost alone and option one an eye-watering 66% extra.
The above example shows the considerable savings in using one option over another and although the savings may be smaller in an above-ground installation, the careful selection of the configuration, environment, cable support type and arrangement can make considerable savings in cable size, length, support requirements, installation hours and – for the customer – even a similar reduction in cable power losses as these are directly related to the square of the current.
For this reason alone, it is worth doing the exercise in every situation where cable, cable support systems and installation costs are high.
Don’t skip the fine print
As mentioned earlier, when selecting cables, there may be a tendency by many users to jump to the first table that seems to describe the installation criteria, then just look at the numbers and run with them.
However, unless the information at the start of the section is also studied in detail it will most likely result in the cable being either undersized and so ending up as rework (or a fire) or oversized, adding cost for the installer both in larger cables and supports as well as the extra labour for its installation, all for no gain.
So, it is critically important when using any table in Sections 2, 3, 4 and 5 to start at the start of the section and to understand on what the section tables applying to your installation are based, before finding the table best suited, so all issues needing to be considered are complied with.
And a further word of warning, because many of the tables have similar titles, great care must be taken in double-checking that the table being used is the one needed.
Cable selection software
One way to avoid having to refer to tables in Standards is to use an industry-recognised or in-house developed software program designed specifically for AS/NZS 3008 cable selection. It should take much of the actual work out of the selection process. So, choose the right one, make sure it is to the current standard and you will have a high degree of confidence in the selection of cabling options.
However, in order to have that degree of confidence, the user of the program must also have sound knowledge of the standard’s fine print to ensure the selection is based on the correct installation situation, as it will otherwise be a matter of rubbish in equals rubbish out.
Further, given there are considerable changes in the values within the tables, and additional tables, as well as the introduction of DC supplies, its upgrading and testing may take some time.
Also, as there will be the “normal” requirement that the new standard be complied within six months from the date of publication, it is assumed software developers, including any using an in-house program, will need to be up to speed in that time.
So, the advice is to place a priority emphasis on ensuring the software you are using will comply with the new edition.
Optimising cable routing
Although this is not a specific focus in AS/NZS 3008, if we are talking about selecting cables for an installation, it may also be worth making a special effort to keep the lengths of cable routes as short as practicable, as this will not only save cost for the installer but can also reduce ongoing cable power losses to the user.
Having been a judge of excellence award entries, this sometimes threw up some conflicts, especially in cable routings. After all, the best practice would be to minimise the cable length and the I2R power losses within the cables. But in many instances cables were run following the ‘squareness’ of the structure, even when the cabling could have been installed diagonally, not only saving considerably on the cable itself but also on the cable supports and installation cost, for no gain to the installer and to the detriment of the owner. A total waste all round.
Now, installing cables diagonally may not always be practical but cabling installed underground, in walls, in ceiling spaces, where the aesthetics are not important or when the cables can be buried or camouflaged, could make both substantial installation and ‘whole of life’ savings. Both the installer and the owner get a benefit.
As an example, say, in a 20m square structure, with the route length along two sides of 20m each the ‘square’ route will be 40m, whereas diagonally it would only be just over 28m. In this example, the cost of installation and supports would be reduced by 12m, or some 30%. And the same would apply for vertical risers and anywhere else a direct point-to-point route is possible.
In short, keeping the cables short, installing them in the most economical way and also protecting them from extreme environments will save heaps.
Economic cable selection optimisation
This may present an opportunity for a marketing ‘edge’ for entrepreneurial installers.
In a nutshell, it’s about being ‘green’ and selecting the best or optimum whole-of-life installation and operational cost combination by way of cable type, cable size, installation arrangement, installation method and cable route length for those circuits in the installation where overall savings would be of interest to the owner or user.
This is referenced briefly in the Standard in Section 2 (at 2.3.4).
In looking at this however, it is fair to say, that when installing wiring for the typical domestic, residential, small commercial and light industrial installation owner, there is absolutely no incentive for the electrician or electrical contractor to select anything more than the minimum size cable.
But, moving up the customer ladder, that may not be the case for larger or more complicated installations, especially for environmentally focused customers and those with very high-power usage, large cables, long route lengths and large HVAC or similar industrial loads, as well as for substantial DC wiring from solar panels or for EV charging.
In those situations, although you, the installer, may not benefit over the long term, you may have the edge on your competition by offering a second or alternative proposal that minimises the cable power losses. All you need do is prove the pay back from the reduced power losses over time. This could especially appeal to customers sensitive about their environmental image but not able to reduce power use or not wanting to forego the comforts their installation provides.
There was an investigation done in 2012 by the Copper Development Centre on residential buildings. Unfortunately, it suggested that the savings for residential and other small users, although not insignificant if widely adopted, were only marginal and not likely to appeal to enough individual potential customers.
Now however, with electricity prices having gone through the roof and – contrary to our politicians’ promises – not likely to go down any time soon, this may at least make the larger and or greener customers, and even the greener medium customers, more interested.
To work out what the likely savings in a particular installation could be, start with the simple formula; Power Loss = I2R, where I is the current and R is the conductor resistance in the tables in Section 4, to compare the cost of the larger cables in installs with a potential saving.
You never know, you may want to consider becoming a potential cabling guru and make it a point of difference for your enterprise.
What does the future hold for AS/NZS 3008?
It has grown considerably over its lifetime and may continue to do so in years to come, although most common installation situations are now fairly well covered. But during the time it has been in use, the world has moved from paper books with tables in them to smartphones, now with ready access to real-time and interactive software programs and apps.
However, the need for the Standard in book format will probably remain as it is a legal document in many jurisdictions, in use as a formative companion to AS/NZS 3000.
Based on this, my guess is that as time progresses, it will also be available for web access or download in a digital format. As well, no doubt, the publishers of the dedicated AS/NZS 3008 software will continue with updates and enhancements to make the need for users to own a paper copy seem less important.
However, it will still be essential for those selecting cables wisely to have the information as touched on under-read the fine print above at hand to ensure full compliance, although many will be tempted to ‘wing it’ using just the software. This will no doubt need some smart thinking on Standards’ part.
For information, according to the Standards Australia website, the current AS/NZS 3008 – 2017 edition is being sold for $289.97. Out of those 100,000-plus AS/NZS 3000 users mentioned earlier, a substantial number would probably also need to access it from time to time. So, some combination package of the two may make some sense. In addition, many of the users may also opt for accessing AS/NZS 3008 software which could also be part of such a package.
The questions for the future would include, can the Standard be;
- Made available as software including all the text, notes, calculation, features etc.
- Simplified, perhaps with rounded numbers where there are only small differences in identical situations have the same value?
- Rationalised by combining tables that are repetitive.
- Focused more on information on economic optimisation including for climate change.
- Guides, possibly with software backup in finding or recommending the lowest cost installation options.