The Risks of Shorting Copper in Cables. Do Not Get Caught Short!

As a supplier of electrical solutions, we take our product quality and compliance very seriously. It is the only way we can sleep at nights. We over-engineer everything we design and have manufactured, even if is to the detriment of our product costs being higher than we would like, while profit is important, compliance is paramount. 

Recently we were speaking with a number of customers regarding our 110°C flexible power cables. We received from a few the following less than perfect feed-back "your cables are much larger in diameter, your cables are less flexible, they strip really nicely, they are well made but the lug is a tight fit over your conductors". Then came the light-bulb moment! To confirm our thinking we bought some samples of the cable they were using and, what was represented to be a 120mm2 conductor, was only 109.85mm2, a 185mm2 was well short at 162.15mm2, and a 300mm2 was a big miss at 280.27mm2 .

 How can these cables be so short on Copper? It certainly explains why they are cheaper, less copper, less insulation, less sheathing material, less manufacturing time, less weight, less volume, less of everything except flexibility, which is better for all the above reasons. Less is bad when it comes to cables, every product recall is because of less.

As a side-note the bending radius's being applied to these cables by the installer, was also far in excess of what the cables were designed to do, creating a point of stress and failure, also any cable lugs being crimped onto the cable, are all under-crimped and the barrel of the lugs severely pinched at the point where the crimp dies meet, creating the potential for electrical hot spots.

We have been in the industry for now 42 years. Up until now we have not seen this scale of missing copper, nor poorly terminated cables. If you buy a cable represented to be 120mm2, you expect the cable to have 120mm2 of copper, the same holds true regardless of conductor size, and especially in cables rated to 110°C, which are designed for carrying high currents. We could allow a tolerance of say +/- 2%, but we would not expect the copper to be short by 8% and more. These cables are going to run hot, resulting in thermal losses, and placing stress on the installation, and components they are connected to. The last thing you want, is the cable over heating, it becomes a risk.

We sought expert advice, not just from our engineers with our manufacturing partner, Keystone Cables based in Singapore, but other experienced cable experts within Australia, as well. The manufacturer of these cables with smaller conductors, have a decent reputation, surely they would not get their conductor sizing wrong? The answer is found on a simple table in AS/NZS1125; Conductors in insulated electric cables and flexible cords.

Standards bodies and cable manufacturers, use the term "nominal cross-sectional area." We even use it, but not in the context of trying to reduce the copper content of a cable.

So a nominal 120mm2, can legitimately contain less than 120mm2 of copper, or any other conductor material for that matter. Conductors have a number of criteria to meet, a most important one being, each size will not exceed a specific Maximum DC resistance at 20°C as per the standard.

Technically you can reduce the conductor size, as long as it does not exceed the stipulated DC resistance. This is not something we would do, as there is no tolerance in case of a fault, or future additional loading, and your watt losses increase dramatically.

Today's engineers are trying to design energy efficient power reticulation systems, one hopes in all installations, presently there is only an optional consideration in AS/NZS 3008.1, Electrical Installations-Selection of Cables, Clause 2.6; Determination of Cable Size Based, on the Economic Optimization Considerations. We believe this should be mandatory, but at least it's now an inclusion, which is a start in the right direction.

With the increasing cost of energy, together, with the high energy losses which follow from the use of modern cables, operating at high temperatures, now raises the value that cable size selection, should be considered on wider economic grounds. The background for economic cable sizing, is provided in IEC 60287-3-2

The optimal economic cable size, is found by minimizing the lifetime costs of the cable, which includes its initial cost, and the operating cost, such as the cost of losses from heat. The initial cost includes the cost of cable and installation. The future costs, are based on the total of the future load, future energy costs, which are continuously rising, and the discount rate. So, Engineers and Electricians also need to become accountants, take a look at the example contained within AS/NZS 3008.1. This is serious. As a default, the cable should always be equal, or larger in cross section, than one selected on the basis of safety.

The issue with cables rated for high temperatures is, they run at high temperatures. When you load them at their full rated current, you get some serious watt losses (I2R). So logically, and ideally, if you want to run high currents through cables, you want as much conductor material as possible, so the resistance is minimised, less copper is not good! Heat is also more rapidly dissipated by larger diameter cables, of which the cable's insulation, and sheath also play their roles.

Now comes the moment of truth. Taking the example of this manufacturers nominal 120mm2 cable, actually 109.85mm2, and we use the lab tested DC resistance of the cable in an example, which is 0.177 Ω/km, higher than the maximum allowable 0.161 Ω/km. Now we can hear many say 0.016Ω/km cannot make much of a difference, but, when the cable is already maxed out, the difference is significant as you will see in the example below, and we give the manufacturer the benefit of doubt, that the insulation and sheath average thickness, complies with AS/NZ standards.

The maximum calculated current rating for a 110°C, rated 120mm2 SDI cable, in trefoil, in free air, with dimensions to AS/NZS5001-1, and insulation to AS/NZS3808.1.1:2017, with class 5 conductors, complying with AS1125:2001, is 419Amps AS/NZS3008.1.1 table 9, gives it as 418 Amps so the calculation is correct.

 If the conductor has a DC resistance of 0.177 Ω/km, the current rating drops to 400Amps. If you tried to pull 419 Amps through the cable, the conductor temperature will rise to 117°C, and outer sheath raised to 110°C.

 In this example, these cables do not comply with AS/NZS1125:2001, and would not comply with the requirements of AS/NZS3008.1.1:2017, (“Limiting Temperatures for Insulated cables” Table 1), if the maximum current ratings included in this standard are used.

We consider these cables would present a burn risk, and could be dangerous if used at the recommended maximum current ratings of AS/NZS3008.1.1:2017. They would overheat, age prematurely, and potentially be a fire risk.

The effect of a short circuit, while the cable is operating at maximum permitted load, would see the conductor temperature, rising above the maximum limit of 250°C, which could actually, initiate a fire.

If that's not problematic enough, the switch gear you connect these cables to, is being thermally challenged, with a 117°C cable connected to it, your switch-gear becomes a heat sink. Further, the heat being dissipated by such cables in an enclosure, or switchboard, would see internal ambient temperatures increase considerably, putting even further thermal stresses on components.

So, be very careful when using 110°C flexible cables, ask the manufacturer, exactly what the cross sectional area of conductors are, and don't just accept the nominal value, you may save a couple of dollar per meter, but you will compromise the electrical power reticulation system, and potentially put at risk, people and property, and the full life-cycle economics, will not stack up. We are working on some case studies on watt losses (I2R). Initial findings based on using Keystone 110°C flexible cables are good, competition cables, not so good. We were a huge advocate of this evolution 110°C flexible cables, as they benefited installers greatly, and it was at a time when electricity costs were low, it made sense way back then.

With electricity costs today, and the requirement for energy efficient optimisation, maybe we need to step back a bit, and have a rethink, or even more simply, change the standards to eliminate the term, "Nominal Cross Sectional Area", and replace it with "Cross Sectional Area". In doing so, risk is mitigated and, as consumers, we will all benefit in longer product life cycles, reduced electrical power bills, and a reduction in national power demand, benefiting climate change. All good things for us and our environment.