So you've bought yourself a panel. You've done the research and made sure it is actually the number of watts stated and not some fabrication by the manufacturer. That's it right? Well...unfortunately not. There is more to solar performance than just the panel.
Have you ever tried to drink a thick shake through a skinny straw? Doesn't work very well does it? No matter how much you suck you can only get a certain amount through the straw at a time. Put in a large diameter and away you go! You get the flow you expect. This is what I pictured when Jason explained what B&S/AWG cable sizing and voltage drop was for the first time.
Put simply, the B&S/AWG standard gauge or size of a cable. But strangely, the lower the number the thicker the cable. Go figure. I'm sure there's a technical reason for this but its not really important.
Trying to run a panel with an undersized cable can result in significant voltage drop. The amps produced won't really change, but the longer the cable to more the voltage drops. As you increase cable length, you need to increase the cross sectional area of the cable (mm2). Is doesn't matter how big or small the panel is, how well it performs or how efficient it is when it comes to voltage drop and small cables. If the cables are too small, you will get voltage drop. Whilst amps will stay mostly the same in a DC application, the more the voltage drops away, the lower the watts you get going into your battery. It is very important to have your cables sized correctly.
Unfortunately, cables can be expensive (as anyone who has hooked up an inverter can attest, a longer cable can often work out to be the same, or more, than the cost of the unit itself!). Because of this, there is a huge problem with the cheaper panels on the market, even if they are honest about the size of their panels (which so many budget brand sellers aren't), they more often than not have undersized/flimsy cables which means you won't be getting the maximum power out of the panel. There are some brands/panels out there that have 17% voltage drop due to such a small cable. You therefore waste 17%-20% of your panel output with the wrong size cable.
Unless you sell the stuff, install it or have regularly used it, being able to tell from a glance, work out the correct cable size needed, or even just deciphering what it is you're being told (which is rare to begin with, often times the seller has no clue what size cable their own panels come with or if they are really rated to what the stickers says).
Cable makers produce "4mm, 5mm & 6mm - Industry equivalent sizes". They neither describe the diameter, gauge or cross sectional surface area of the cable and are just an indicative number applied by cable makers rather than an accurate descriptor of the cable size as you can see from the table below. 6mm cable is where cable sizes become the most confusing for most people, as it's the most common size called for or used by 12v accessory installers for running sockets for fridges and power sockets from front to rear of vehicles.
Here's a table showing each of the different cable sizes commonly used in 12v applications in Australia:
How do you know what guage is correct for what size panel?
When sizing cables there is two requirements to take into account - current carrying capacity (amps it can handle) and length of cable in relation to voltage drop. This is relevent not just to solar but for any cables for any electrical load. Take an example of a 200w solar panel that has a 5m cable. 10AWG cable is sufficient for this application as it can handle up to a maximum of 55 Amps and with only 4% voltage drop. This is what we use in our 2019 AllSpark ETFE 200w folding solar blankets. If you wanted to power the same blanket but with a 10m cable, if you still use 10AWG cable you would be getting 7-8% voltage drop. To avoid this voltage drop, you need to increase to the size of the whole cable to 8AWG which simply has more copper inside and can handle the power being transmitted down the cable. Many panels on the market use 12AWG or even 14AWG which they have sized because it can handle the current, but given no thought to voltage drop. A 14AWG cable can still handle 30A (way more than the panel produces), but over 10m produce 17-18% voltage drop. Thats the equivalent of a panel that produces 18v at the junction box, but only 14.9v getting out the other end or a loss of 33+ watts.
So if you have a panel already, and are wondering why it isn't producing enough power to keep you running, first check that it is the number of watts stated, then have a look at the cable. Same goes for if you are looking at purchasing one, ask the question of the seller, what is the AWG or B&S size, not the mm. If you can't tell, or aren't sure what to look for, pop up a post over on the Offroad Living Tribe Facebook Group (questions like these often times prove helpful to others), or send through a message to Jas directly to firstname.lastname@example.org.