The type of wire you’ll need for your PV system can be divided:
- Solar panels – Combiner box
- Combiner box – Charge controller – Loads
- Battery cables
Solar Panels – Combiner box
Two types of wires are used for connecting photovoltaic (PV) modules, and according to the UL-4703 standard, they must be connected in a certain way.
When RVing, boat-camping, or installing solar panels on your home it’s important to use the right type of wire. The best choice is called PV Wire and can be used for all three applications without sacrificing quality. The PV wire is an essential component of your solar panels. It connects the individual modules together to form one long string, which makes it easier for charge controllers or inverters that are connected after this point in order to manage power output from each panel’s worth.
These wires meet the high standards of UL-4703, and they’re made from either copper or aluminum or their combination – copper-clad aluminum.
Their insulation cover is made from cross-linked polyethylene (XLPE) or ethylene-propylene rubber, and it can withstand 600V – 2 Kv.
UV-resistant wires allow for a longer life span due to their ability to withstand intense UV radiation.
The copper and aluminum material of a PV wire is designed to withstand high temperatures, so it will not melt or burn up in the sun. These wires also have excellent insulation that protects against damage from exposure – this helps keep your system safe. A key safety feature for any electrical installation should be flame retardant materials like these ones; without them, you could risk fires caused by short-circuits between modules during weather events such as storms.
The wires in this section are used for connecting the solar panels up to a combiner box or charge controller. They can also be attached directly from one device and go through another, depending on what kind of system you have set up.
The second type of wire – the USE-2 – stands for underground service entrance wire.
The USE-2 wire is a less expensive and more environmentally friendly alternative to the UL4703. It’s ideal for solar panels when you can’t buy that specific PVC cable, but it doesn’t have UV protection so be careful about where your installation will go. These wires also get hot under dry conditions at 195°F (90 °C).
The use of a more durable wire is recommended to ensure your wiring remains safe throughout all weather conditions.
It’s important that you use the UL-4703 PV cable which has been designed for resistance against high temperatures, flexibility in different shapes and sizes as well as thick insulation unlike other wires on sale today. USE-2 is the wire of choice for DIYers because it’s more affordable and has thicker insulation.
Combiner box – Charge Controller – Loads
Wire insulation can be simpler in this section because there will no exposure to sunlight. You should use marine-grade wires for mobile and boat applications, which have smaller strands that make them more flexible when the vibration happens – preventing any breaks from happening.
You can use copper wires with an anti-corrosion coating. This will make them even more corrosion-resistant.
The 75°C wires are used for machine tools and appliances with hotter internal wiring. They can work up until 600V, at 195 °F (90 º C) in dry conditions or when wet they must be able to withstand 140° F(60º).
Even though they are thicker and have more strands, this still won’t make them as flexible as marine-grade wires. If you want your wire to be very bendy then get yourself some of those thinner but stronger types.
These wires are perfect for indoors because they won’t get in your way and you can use them without worrying about rain. They’re also safe around batteries, so this is one of the only types that’ll do nicely.
These wires are designed to work with your battery, so they can be used outdoors as well. They’re not just for indoor use though – there’s another type of wire that will do you better if this isn’t where the power goes.
The THW cable will not work for this section. You must use a THWN-2 instead.
THWN-2 is an ideal cable for environments where temperatures can reach up to 195°F (90 °C). hey’re suitable for use in dry and wet locations, meaning they will never short out on you when things get too hot or cold. They also have a flame retardant feature and high resistance against abrasion from oil, and chemical agents thanks to their nylon coating.
If you need a device that can handle large amounts of current, then this charger is perfect for your needs. Some other options that would work well are:
- THHN (Thermoplastic High-Heat Resistant Nylon Coated)
- XHHW-2 (Cross-Linked Polyethylene High-Heat and Water Resistance)
For a safe and secure installation, be sure to use the correct wire types in your battery compartment. The wires must match up with those on offer from charge controllers or inverters as well as other devices that may interconnect between batteries such as series connections or parallel connections.
In order for this system to work properly, the wire from your charge controller should be of a certain type. The wires that go to the inverter, busbar, and DC fuse box should also be this type of wire because they all start or end in the battery compartment.
The thicker the wire, the more flexible it will be. If you need to make your wiring harnesses for an RV or boat then use marine-grade wires which have been proven reliable in harsh environments where other types would break down easily.
Calculating Wire Sizes
Once you have calculated the size of your solar panels, it is important to ensure they can produce enough electricity for all those gadgets and gizmos in your house.
In this chapter, we will go over how much wire you need for each section of your solar system.
PV Modules – Combiner box
To find the short-circuit current of a module, you must look in its datasheet. Once there it’s easy to see that higher irradiance levels and voltage drop will cause resistive losses which reduce power output over time (and increase cost).
The solar cells in this panel can produce a maximum current:
Imax = 1sc x 1.5623
To calculate how much current can flow through a wire, multiply:
Imax = 6.20A x 1.5623 = 9.7 Amps
With a manufacturer’s temperature rating of 90°C, the wire sizes can be as small as #14AWG.
Voltage drop is an important factor to consider when designing a high-performance electrical system. To reduce voltage drop to an acceptable 1% at 20ft (6 meters), we need calculate the wire size. You can find this formula in the chapter ‘sizing factors – voltage drop’.
(0.0171 [Ω x (mm² / m)] x 2 x 6m x 9.7 A) / (0.01 x 17.5 V) = 11.37 mm²
The solar panel needs 11.37mm² or #6AWG to connect it with the combiner box, which is an important part in wiring your home’s electricity system. However, if you want to use the MC-4 connector cables with your project then it will only be available in #10AWG. Let’s do the math again and see what happens when there is a 3% voltage drop.
(0.0171 [Ω x (mm² / m)] x 2 x 6m x 9.7 A) / (0.03 x 17.5 V) = 3.79 mm²
#10AWG wire is a 3.79mm². This is only possible if you wire the cable for 20ft (6 meters) without an extension. You must make sure that you calculate this correctly for optimal results. 3% is the minimum acceptable voltage drop, but it’s not ideal. Reduce your voltage drop to as low a level possible while still keeping it affordable.
This wire will carry electric current up to the fuses in your combiner box or straight out for a charge controller.
Combiner box – Charge Controller
The combiner box will send power to the charge controller, which is responsible for managing how much energy can be put out by all of our batteries in one place at once. The amount of electricity that flows from the solar panels is stored in this section. Wire your panels in series and you won’t need to use this formula anymore. To estimate the wire size, you must use this formula instead:
Ibox = 1sc = number of strings x 1.5625
The number of parallel connections made in the solar array is represented by the number of strings. The 1.5625 factor is associated with security factors and should be taken into account.
For example, 1 array of 2 panels in parallel:
Ibox = 6.2 Amps x 2 strings x 1.5625 = 19.37 Amps
Let’s calculate the voltage drop. From the combiner box to the charge controller the wires travel 5 meters or 16ft:
For 1% voltage drop:
(0.0171 [Ω x (mm²/m)] x 2 x 5m x 19.37A) / (0.01 x 17.5 V) = 18.9mm² or #4AWG
For 3% voltage drop:
(0.0171 [Ω x (mm²/m)] x 2 x 5m x 19.37A) / (0.03 x 17.5 V) = 6.3mm² or #8AWG
When you have your panels connected in series, the voltage will increase but amperage remains constant. This is the preferred way to limit voltage drop is with a series connection.
The same 2 panels in series in this example with a 1% voltage drop:
(0.0171 [Ω x (mm²/m)] x 2 x 5m x 9.7A) / (0.01 x 35V) = 4.73mm² or #10AWG
When wiring a series connection, the wires must be at least of equal size and have smaller diameter. The wire diameter is decided by how many amps of current run through it. Ampacity will be increased as voltage stays the same. Series is when the voltage increases while current remains constant. It’s important to know about this effect later in series chapter, so stay tuned.
Charge Controller – Battery
The charge controller will only deliver the maximum amount of charging current listed in its datasheet to each battery.
It will also recommend the wire size. There’s no need to calculate this cable for voltage drop if it isn’t very long.
The manufacturer’s recommended guideline is always a good starting point. Make sure you calculate this correctly or it can cause problems with the charger and battery. Make certain that your terminals are large enough to accept a wire diameter as well, so there’s no shorting out of either component in use.
Battery – Inverter
In order to size the wires that go from your batteries and inverter, you’ll need an accurate estimate on how much power they can handle. If you have a 1,500 watt inverter and 12 volt battery bank then your power needs can be met by applying this formula:
Current = Power / Voltage
1,500 Watts / 12 Volts = 125 Amps
You need an electric wire with enough power to run 125 Amps.
Finally, you need to choose the temperature rating for your wire. As we are using a different type of wire (THWN-2), the temperature rating will be 194 degrees F or 90 degrees Celsius.
It is important that you decide on the perfect temperature for your battery. The maximum temperature in the battery compartment up to an inverter will be 41 – 45 °C (105-113°F).
The formula to apply when correcting temperature is as follows:
130 Amps x 0.87 = 113.1 Amps
The temperature correction factor means that we only get to 125 Amps ifreecapacity ratings are lower than what is listed on the label. The wire size needs to be increased in order for the current electricity flow. In order to safely carry 150 Amps, we use #1AWG cable instead of #2. With #1AWG wire, we use the same formula:
150 Amps x 0.87 = 130.5 Amps
When we select a #1AWG wire, the maximum current that can be handled by our inverter is maximized. The voltage drop is insignificant because the distance between these two should be almost negligible (a few feet).
The higher voltage of a system means that you can use smaller wires, which will help keep your home safe and neat. With 24 volts of power coming into the inverter, it needs wire that can provide 62.5 Amps in order for everything to work properly- as seen by this calculation:
1,500 Watts / 24 Volts = 62.5 Amps
It is important to note that wires carrying more than 100 Amps are not recommended for use in DIY systems. If you have more than 100 Amps of power wire running through your house, use a system with higher voltage. Not only will this increase safety but it also saves money on the cost for wires.
In this section, we will discuss the maximum current that can flow through your batteries. The discharging current is most likely going to be high so make sure you have an amp or two here.
If you are connecting batteries, they should have the same diameter and length. For more information about this refer to ‘batteries’ in our chapter.
Wire size will be determined by the amount of current that flows through it. For safety, you should size your cables to handle the load current. If that number is 100 Amps at 12 volts then do so.
Wiring for Electrical Loads
Here’s some important information for the section on DC and AC loads.
If you need to power a DC load, we recommend using #14AWG or 16-gauge wire.
The typical AC loads such as lighting, TVs and microwaves will generally use #12AWG or 10 gauge wire. These wires can be found in outlets which provide power to small motors among other things.
If you have an A/C unit, washing machine or refrigerator that uses #8AWG and up to #6AWGs of power then this is the right wire for your home.
The amount of current your appliance requires is typically listed on the product itself or in its technical documentation. If you only have power ratings available, use this formula to calculate your current score:
Current = Wattage / Volts
For a DC appliance:
1,000 Watts / 12 Volts = 83.3 Amps
For an AC appliance:
1,000 Watts / 120 Volts = 8.33 Amps
When you’re installing your off-grid solar system, it’s important to take into account the fact that there will be wires close together. This is because of compact installations and equipment with vibrations like an RV or boat motor, so here are some rules for keeping everything running smoothly:
- When wires are passed through small spaces, they should always be protected with sleeves. This way the insulation isn’t cut and you avoid any shorting happening on your own accord.
- Never make sharp bends with wires because it can damage the wire strands and insulation over time.
- If you have the time and resources, it is best to run your wires through a conduit. It will make adding more connections much easier in future installations or repairs because they won’t be running all over the place.
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