The question as to which lets you go further and perform better is especially topical in this season of outdoor Christmas lighting displays, with pretty lights spread far and wide, so this month James Eade explains which will keep Santa’s eyes twinkling brightest during the cold winter nights….
As mentioned in a previous article on cable construction, people rarely pay any attention to the cables used to deliver power. Unfortunately, some ‘professionals’ don’t always pay enough attention to them either and often choose the wrong size for the job. In a future article we will look at the humble fuse and circuit breaker, but the common feature is that they both rely on a high current flowing in a circuit to enable quick operation in the event of something going wrong.
For those less electrically minded, we are interested in three factors when specifying cables for circuits; the voltage, the current and the resistance. These are analogous to water flowing in a hosepipe and you can, in most instances, relate it to water pressure (voltage), volume of water (current) and resistance to flow or obstructions. Generally, protective devices rely on the flow of current to operate – imagine a burst hosepipe with lots of water escaping, a flow valve would see the high volume of water passing through and turn off the tap. So the same is true with electrical circuits; a short circuit or fault to earth should, hopefully, cause a high current to flow, resulting in the operation of the circuit breaker or fuse and thereby disconnecting the supply.
That would be all well and good were it not for resistance. With the hosepipe example, if it bursts, the first thing most people would do is stand on the pipe to reduce the flow of water. In other words, an increase in resistance will result in a reduction of flow. How much of a reduction would depend on the water pressure; and from that we can deduce that current is proportional to resistance – the constant of proportionality being the voltage. This principle gives us the most basic yet most important equation in electrical design – Ohms Law or Voltage = Current x Resistance (V = I x R).
Now, the problem with copper cables is that they have resistance. Basically, current in a cable is the movement of electrons surrounding the nucleus of the individual copper atoms and, as the atoms naturally vibrate, they impede the movement of the electrons, which gives rise to the resistance. So the longer a wire is, the harder it is for electrical energy to go from one end to the other. That can of course be helped by making the copper wire fatter as it gives a wider area for the energy to move through.
So to that end, if we need to get more energy down the electrical ‘pipe’ without a reduction in current flow, it can be seen that we need to make cables fatter – something more technically referred to as the cross-sectional area (or CSA) and measured in square millimeters. A domestic 13A extension lead will typically have conductors with a CSA of 1.5mm2, whereas a 400A single pole cable might have a CSA of 120mm2 for example. You can usually see it printed on good quality cables – they have a code something similar to H05-VVF 3G1.5 (for a domestic flex) or H07-RNF 3G2.5 for a rubber cable. The last digits are the CSA in mm2. The ‘3G’ means that there are three cores, one of which is coloured green/yellow.
In a fixed circuit, the current flow is constant – i.e. if 10 Amps leaves the source of supply, such as a generator, that 10 Amps will find its way back to the source to balance it up. What we lose around the circuit is voltage; we might start at 230V at the generator, lose 10V down the live wire which has a resistance of one Ohm, 220V in the load and another 10V in the neutral, which also has a resistance of one Ohm. We end up at zero volts on the neutral at the source of supply as shown in Figure 1,below. The load ‘sees’ the difference of 220V – 10V which gives 210V; much less than it may actually need.
Evidently, as the resistance of the wire increases, so the voltage drops and the flow of current is reduced. In practical tests, a 3kW load connected to 250m of 4mm2 rubber flex had a voltage drop of 28V, whereas the same load on a 6mm2 cable on the same supply had a voltage drop of 20V. It goes to show that girth is an important factor, but not the only one as length also makes a difference.
So the big killer of current is resistance. The more resistance, the less current flows and the less likely we are do manage a quick disconnection time in the event of a fault. And the most common generator of increased resistance is heat – as cables get hot, the atoms vibrate more and so the movement of electrons is further impeded. Heat is generated in a multitude of ways – coiling of cables, (especially leaving them coiled on a drum under load – a big no-no); grouping, burial, ambient temperature and solar radiation; multicore cables such as Socapex or Lectriflex, (effectively 6 circuits grouped in a single cable); cables in ramps, ducts or dip traps and so on.
The less a cable is able to dissipate heat, the higher its resistance will grow and the hotter it gets in turn – power dissipated in a cable is given by I2 x R. Figure 2 shows what happens when a cable gets too warm under load and coiled on a drum. So it can be seen that we are fundamentally concerned with the resistance of wires and the amount of current flowing – as a brief digression that is why generator outputs are always rated in VA and not Watts, as we are concerned by how much current the alternator can deliver without overheating the windings – and that is a function of the resistance of the windings themselves.
For this reason voltage drop in a cable is measured in millivolts per amp per meter – for example a 1.5mm cable is typically 32 mV/A/m. So for each amp of load current, and each meter length, the circuit will accumulate 32mV of voltage drop caused by the resistance. So for 10 Amps of current over 10 meters, the voltage drop would be 3.2V – quite a lot for such a short cable and a 2.3kW load. In order to reduce the voltage drop, and hence present less resistance to current flow, we can make the cable fatter or shorter. Given that ‘shorter’ is not usually an option as you invariably can’t move the load or the source of supply, we need to increase the CSA to compensate.
So length and girth both effect the performance of a circuit, but in general, for a given length, the greatest possible girth gives the best performance. And on that note, have a joyful festive period!
James Eade is a Chartered Engineer with over 20 years’ experience working in the events industry including theatre, festivals, tours and corporate events. He advises several trade associations and represents the industry on various British Standard Committees including those for BS 7671 and BS 7909. He is also the author of the guidebook on temporary power published by the Institute of Engineering Technology. For more information visit www.eade.uk.com