Energy Systems — by Chris McLeod June 17, 2011
So far in the series, A Solar Powered Life, we’ve covered most of the components of a solar power system. To have a complete system though, you have to connect all of the separate components using wire and fuses. You wouldn’t think it, but the wire used is one of the most important (and potentially dangerous) components of the entire system. Some people may think that wire and fuses are pretty boring, so I’ve decided to spice it up a bit, myth busters style, by blowing some stuff up for your reading pleasure!
Wire (also known as cable) can be a real hazard because if you draw too much power through it, the wire will melt and catch fire, which can have disastrous consequences. Essentially, all you need to understand in order to determine how much power you can draw through a given cable is a simple rule. The fatter the cable, the more amps (power, energy whatever) you can pull through that cable without overheating it and catching fire. It’s also important to understand that I’m not talking about the plastic sheath on the cable, but the copper strands in that cable.
The amount of power that you can pull through a cable is indicated by the maximum number of amps that the cable is rated at (see an explanation at bottom of the term amps if you’re interested).
The funny thing about electricity is that as the voltage increases, the demands on cables become lower and you can use smaller sized cables to carry a larger amount of power. Low voltage solar power systems can be quite demanding in the amount of power that they require the cables to carry.
For example, the inverter on my system can supply 3,000 watts mains electricity from a 24 volt battery. Remember that watts = amps x volts, so you can work backwards and see that the inverter can draw 125 amps at 24 volts (3,000w = 125A x 24v). Now 125 amps requires a really chunky cable (read expensive) and the ones supplied with it have a surface area of 50mm2. That’s a lot of copper.
For those readers that are more mathematically inclined than myself, they would have immediately noticed something which could save you some money on cables. Remember in A Solar Powered Life – Part 7, we looked at inverters and I noted that they were usually sold for batteries of the following voltages: 12 volts, 24 volts and 48 volts. Well, if you had a 3,000 watt inverter on a 48 volt battery, the amps that the inverter could draw would be halved. As the number of amps is halved, the required cable size can also be reduced.
The photo below shows some different sized cables which gives you an example of how many amps you can pull through them (without the cables melting and catching fire).
From Left to Right:
- Cable 1: 32mm2 rated to a maximum of 110A
- Cable 2: 7.52mm2 rated to a maximum of 56A
- Cable 3: 2.9mm2 rated to a maximum of 25A
- Cable 4: 1.84mm2 rated to a maximum of 15A
I’m always talking about that environmental disaster, the electric fan heater. It draws 2.4kW, but doesn’t require really chunky cables. Why is this? It’s because at 240 volts, 2.4kW is only equivalent to 10 amps (2,400w = 10A x 240v) and 10 amps doesn’t really require massively thick cables. As a comparison at 24 volts, the fan heater would require 100 amps (2,400w = 100A x 24v) which is a pretty thick cable as you can see above. This is why appliances are at much higher voltages than batteries – it is purely to save money on cables.
The electricity grid is subject to the same limitations with cables that a home solar power system is though. Now think of those really high voltage electrical cables that run from the generators to a city and you can start to see why they are run at up to 22 million volts!
If you’ve ever stood anywhere near one of these high voltage power lines on a wet day, you’ll notice something weird. You will hear a buzzing sound and it can be quite loud. This is an example of electricity being converted into sound, which means that power is effectively being lost right at that point in the system.
Also, these high voltage cables get hot if they are required to supply power at or near their maximum amp rating. This can happen on days of extreme weather when a large number of households switch on either heating or cooling appliances. If the cable is getting hot, then electricity is also being lost right there.
When you think how those cables run for upwards of several hundred kilometres, you can imagine how much power is lost between the generator and the end user. A solar power system is no different and you can lose energy to heat and sound at most points in the system, so using the proper cables is advantageous.
If you are still not convinced, you can see the above principles at work by getting back to the example of that environmental disaster, the electrical fan heater, which works through this exact process. A large number of amps are pushed through a small cable (which in this case is designed not to melt or burn) and it converts the electricity into heat. Simple, but wasteful.
So you can see that if you use the wrong cable in a solar power system, it can have potentially disastrous consequences and may indeed burn your house down.
There is another dimension to cables as well. The longer a cable is, the thicker it is required to be. This is because energy in any system is lost to heat (resistance) in cables over longer distances. You can potentially have long cable runs in your system from your solar panels to the batteries and these can sometimes be hard to avoid. The Rainbow Power Company has a chart available on their website which shows what size cable is required to be able to handle the required current for a cable of a certain length. The page can be viewed here (PDF).
Very alert readers will note that, whilst electricity is a wonderful and useful form of energy, all electrical systems are like trying to hold water in a sieve, because energy is lost everywhere. There is no perfect electrical system. It is merely a case of understanding where losses occur in a system and trying to come up with the best compromise so that not too much energy is lost between generating it and using it.
Getting back to the melting cables though. To stop a system or a user pulling too much energy (ie. too many amps) through a cable, a protection system can be installed (this is called a fuse). The fuse is there to set an upper limit on the amount of amps that can be pulled through a cable. It protects the cable, by shutting down the flow of energy through the cable when the number of amps exceeds the rating of that particular fuse. Shutting down the flow of energy stops the cable from melting or catching fire.
There are all sorts of fuses and below are some examples (with photos):
Auto type (inline glass)
HRC type (heavy duty high amp fuses)
As a general rule, it is advisable to have a fuse on every cable between every component in a home solar power system.
Now, to the fun bit. There are always people who will try something dodgy. To show what can go wrong, I’ve setup an experiment to show what happens if cables are not correctly matched to the energy loads in a system.
The experiment involves connecting a small 300w inverter to a 200Ah 12v battery with small 10A cables. Using the formula (watts = amps x volts) you can see that a 10A cable can only carry 120w of electricity at 12v (ie. 120w = 10A x 12v). I deliberately did not include a fuse between the battery and the inverter.
So what happens when you try to pull more energy than the cables 120w limit?
The first image shows the inverter working correctly with a small load of three low watt light globes (2 x 18w and an 11w light globe for a total of 47w). In this picture the globes are lit and the 10A cables to the batteries are OK:
The second image shows the inverter powering a small 380w jigsaw:
The saw operated for about 30 seconds before the cable melted at the battery terminal. You can see in the following photos of the battery and wires, that the red cable has melted some of the plastic duct tape covering the battery terminal. I switched it off at this point as I did not want to either damage the battery or set fire to my shed.
Take this as a warning and use the correct sized cables and include correctly rated fuses. Had a 10A fuse been included in the experiment, the power tool would never have operated and there would have been no risk of overloading the cables.
In the next article, I’ll show how a complete solar power system is connected together.
I’d appreciate it if commenters kept their comments relevant to this particular article or the series in general. If commenters have ideological issues relating to solar power, I’m happy to discuss them and will do so, however this article is not the forum. Please post these types of comments in the article A Solar Powered Life – Part 6 – The ideological debate. Chances are also good that your concerns have already been addressed there.
Below are some explanations of terms for the very technically minded only:
To understand the terms Volts, Amps and Watts it easiest to think about how water is delivered in a pipe:
- Volts – is the pressure in the pipe. A high voltage means a lot of pressure only.
- Amps – is the flow of water in that pipe, but not at one point in the pipe, its the same flow rate at all points in the pipe. High amps equates to a faster flow, not more water in the pipe.
- Watts – is a measure of power ie. an instantaneous measure. Watt hours is an amount of energy, equivalent to the total amount of water that has moved through the pipe.