Some handy information about ELV or Extra Low Voltage systems and what is important to consider when working with such low voltage systems.
Smaller Off Grid remote power installations normally work with a 12v or 24v DC supply. This supply is often comprised of or powered by Solar PV and / or a small wind turbine. A system with an operating voltage below 50V is often referred to as ‘ELV’ or extra low voltage. An example of this in the regular ‘on grid’ home may be the 12v / 50W halogen down lighters that run from a dedicated transformer (commonly positioned within your fuse box). It may sound rather odd to hear that systems running 240v AC are ‘low voltage’ but this reference to ‘low voltage’ incorporates systems with operating voltages (AC) from 50 to 1000V or for DC from 50V to 1500V - Not such a low voltage!
However, we are only concerned with ELV or Extra Low Voltage here. In the main, systems in this area will be based around a 12v or 24v DC power source. Okay, so what is the main difference between a system working at say 240V (AC) in the home and say 12V DC? A 12V system (in a nutshell) will need a larger size cable to carry the same current over the same distance as the 240v system without serious losses. This sounds trivial but can actually be pretty dramatic, especially when considering using high powered devices on a 12v system. The increase in cable size can be so significant it can render the project unworkable due to cost. Large diameter multicore cabling is very expensive, very heavy and very cumbersome. This is not however, such a big concern when installing fairly lightweight DC lighting systems especially if the power supply is very local to the installation. Why? Well, the problem with a very low voltage as in our case working with 12v, is that the voltage will seriously reduce over a long distance. What is a long distance? A simple example here would be the siting of a small solar PV panel in a garden say 30 meters from the site where the battery or lighting circuits will be installed. You have the PV panel in a very good position to collect maximum sunshine unhindered but you now need a very long cable to reach your battery or lighting installation. Normally, in a higher voltage situation a g 2.5mm regular extension cable would suffice to bring the power across this distance but because we are working with a nominal 12V, it would struggle to reach the site without losing a high percentage of voltage along the way. In this case it would probably deliver under 12v and with heavy loading this may drop to under 11v, perhaps as low as 10v. It would probably be necessary to use 30mm twin core cable in order to maintain a line voltage of say 12.7 to 13v plus in this case. This large section cabling would keep voltage drop to a within a couple per cent (as long as the load / current draw was not too high). If the system was to work with say 1000 watts of solar PV panels at nominal 12v and again travel a similar distance of 30m, we may need to consider an even higher section cable.
So the main difference revolves around cable sizing. Too small a cable size will result in severe voltage drop and poor functionality of the system. We are used to using fairly lightweight power leads to work almost any appliance from our regular household AC supply. Items from power drills to electric mowers work just fine as the voltage loss is minimal over say 10 / 15 meters when using such appliances. It is a different story when working with an extra low voltage ELV system.Our top tip here is to purchase a simple digital multi-meter and use it on its ‘DC voltage’ setting to read the actual voltage present at the terminal (end) points of your various circuits. If you find that the garden light you installed is reading say 10.5 volts at the lamp fitting whilst the battery is fully charged and reading say 12.7 across its terminals, you will need to consider either using a very much lower wattage lamp (to decrease the voltage drop) or alternatively upgrading the cable from perhaps 1.5mm (as may have been installed) to 2.5mm or even 4mm depending on the actual length. In this case swapping a typically installed 12v 11w cfl for say a 12v 3w LED bulb may well suffice as a suitable fix and wont drastically reduce lumen.What will happen if I leave badly undersized cabling and allow say an a string of four 11w CFL lamps to continue running with a severely reduced operating voltage? Sadly, the life expectancy of the lamps will be reduced as the lamp will be operating outside of its ideal design parameters. For compact fluorescent lamps, symptoms such as poor starting, blackening of the tubes early failure and flickering will result as the lamp(s) struggle to run. If your low voltage lighting suffers in this way, we recommend looking at your cabling without delay. It may be that the last lamp in a string is the one that always fails early or perfoms poorly.
Well, another very important consideration relates to safety. 12V and 24V DC battery based systems operate at higher currents than regular AC systems so fuse sizing must be correctly evaluated. For example: a 12watt lamp working at 12v DC has a current of 1 amp. A 12 watt lamp working at 240V has a current of just 0.05amp or a 20th of the current. As an example, a pair of 30w LED floodlights will present a load of around 5 amps. For this we would recomend a 7 to 10 amp fuse and useing high quality 2 core H05 / H07 2.5mm (or even 4mm if the lamps are more than say 10m from supply).
High quality deep cycle lead acid battery can produce hundreds of amps – be sure to use a correctly rated fuse.
See more at the OnSolar Blog
leave a comment below and let us know if you would like to see some working examples to illustrate how cable size can effect an Extra Low Voltage installation