Every electrical installation, no matter the size or proportions, requires cables to transport the energy from its source up to any required point. Likewise, a solar off-grid system requires wires to interconnect all the system components, and to the load itself.
The common wire used on solar installations is fundamentally structured by the following:
- Conductor: the core material of any cable is a conductive metal that transports the electric energy through. The most used ones are Copper and Aluminum, and they can be a solid wire or a stranded wire of multiple core, depending on flexibility needed.
- Insulation: The insulation material prevents the loss and interruption of the energy going through the wire as well as provides electrical insulation between the conductor and the user. This insulation sheath is rated by voltage, and it electrically protects cable from its surroundings. The most common used insulation materials are PVC (Polyvinyl Chloride), XLPE (Cross-linked Polyethylene) and EPR (Ethylene Propylene Rubber).
- Jacket: This sheath of material offers the mechanical protection from exterior to the cable. The most common used materials for the jacket on solar installations are LSZH (Polyolefin, Low Smoke Halogen-Free) and UV rated PVC.
Types of cables for off-grid systems
Normally the cables are classified according to the structure and the insulation materials that describe the applications which such cables can be used for.
There are many more acronyms for the cable types, but the main features to look out for are its application (dry, or wet locations, underground or with sun exposure, etc) and its rated temperature operation.
Commonly used cable types on solar installations are:
|THW||Thermoplastic Heat Water||75°C||-Dry and wet locations |
– Moisture resistant
|Regular electrical installations|
|THHN||Thermoplastic High Heat-resistant Nylon Coated||90°C||-Suitable for dry environments – Nyloan coating protects from oils, gasoline and other chemicals |
-Highly resistant to high temperatures
|Charge controller to battery banks/ Battery to inverter|
|THWN-2||Thermoplastic Heat and Wet resistant||90°C||-Suitable for dry and wet – Nyloan coating integrated protecting from oils and other chemicals -High temperature resistant||Charge controller to battery banks/ Battery to inverter|
|USE-2 RHW-2||Underground Service Entrance-2 stand for high heat Resistant||90°C||-Dry and Wet |
– Used for underground installations
|PV Wire||Photovoltaic Cable||90°C||Wet and dry. Highly resistant to high temperaturesWeather and UV sunlight resistantExcellent moisture resistanceResistant to oils and chemicalsFlame resistant||Connection from PV panels to charge controller|
Sizing a cable
To size a cable for a PV system we need to consider mainly three aspects. For help with any of the confusing jargon surrounding energy ratings and power, we’ve written an entire article explaining these terms for the layperson.
- Voltage Rating: Cables are rated for a specific voltage to which they can provide insulation. Nominal voltage ratings are 600V, 1000V and up to 2000V. For residential PV applications the nominal rating used is 600V always.
- Ampacity: This represents the capacity of a cable to withstand a given electrical current without damage. This varies according to the gauge of the cable. To determine this factor you must first find out the maximum current that will pass through that section. We will dive into these factors later.
- Voltage Drop: The voltage drops as the distance from the source of power to the load increases. To maintain the voltage quality within the reasonable parameters, there is a standard for how much the voltage can drop. For PV applications we must apply the following formula to find out:
- A: Area of the conductor [mm2]
- ρ: Specific resistance,[Ω mm2/m] (for copper wires 0.0171 Ω mm2/m)
- L : Length of the conductor [m]
- I: Nominal Electrical current [A]
- v: Permissible voltage drop
- Vsys: Voltage of the system
Wiring from solar panels to charge controller
In order to evaluate the sizing the wiring of the solar panels up to the charge controller, we need to first know the solar module and charge controller basic parameters. For this purpose let’s assume we are looking to size the wires for a 1kW PV system.
Taking as reference a solar panel of nearly 250W with an open circuit voltage (Voc) of 38V and a short circuit current (Isc) of 9A we can size for a charge controller that has a rated charge current of 40A, a maximum PV input voltage of 100V DC and a battery nominal voltage of 12V@520W and 24V@1040W.
Since we are looking for a 1kW system, we will need four of these 250W panels. We can connect them in series (adding voltage) or in parallel (adding current). There is however a limitation as for the maximum input voltage from the charge controller, which only allows us to connect two panels in series (76V DC) in order to not exceed the 100V DC. Then, since we want to maximize the number of panels in series, we would need to connect two panels in series, then set another string of two panels, and connect these two strings in parallel. This will result in a total 76V DC (Voc) and 18A (Isc).
Once we have decided the layout we must check the ampacity rating and voltage drop parameters.
In the case of ampacity, for PV applications two security factors are used for sizing the PV wire that goes from the panels to the charge controller. The first one is a 25% factor (assuming an excess of irradiance) and the second one is an extra 25% (assuming 3 continued hours of used under high levels of irradiance). This represents a total security factor of 1.56.
Since we have two strings we will need a busbar to connect these two. This means we will have two gauge sections, one for the wires that go from panels to the busbar and another wire that goes from the busbar to the charge controller (this one containing the total amperage of the two strings). Thus, we calculate the rated current for the first section.
And for the second section:
Now we must check the manufacturer’s table for the PV wire. Taking the NEC as reference for this purpose we can notice that with a 12 AWG PV wire would be more than enough to use in both sections since the current carrying capacity is 40A for a 90°C.
On the other hand, as for the voltage drop we need to make sure that the voltage drop in the DC side does not exceed 3%. The target that we will use in this case will be to reach 2.5% voltage drop which can be divided into 1.5% for the first section and 1% for the second one.
Also, the voltage of the system in this case for both sections will be 76V DC as that is the Voc of the series connection. Finally, we must know the distance from the panels to the charge controller. Assuming this distance to be 15m for the first section and 3m for the second section for instance, we can calculate as follows.
For the first section:
And for the second section:
From the results above we notice that voltage drop impact is not big in small PV systems. Using the table below we can convert from mm2 to AWG gauge standards. We can see that with a 12AWG would be enough for voltage drop consideration. Mixing the results from both voltage drop and ampacity we conclude that for this section a 12AWG can be used.
Wiring from a charge controller to a battery bank
Sizing the wire from the charge controller up to the battery bank depends on the voltage of the system, typically it can be 12, 24 or 48 V, and it mainly depends on the size of the load you will feed.
For the previous example, the charger is rated for a 12V or 24V system, but for our case we would need a 24V as the 1kW PV system exceeds the maximum power allowed for 12V. For this section, we use the charge rated current of the controller which in this case would be 40 A, but as in the previous example, a good practice is to oversize the cable amperage by 25%, translating into 50 A. Voltage drop calculations are not needed since the distance between the charge controller and the battery bank in an RV are and should be very short.
In this section, it is recommended to use THHN or THWN-2 cables since they are designed for high heat applications and they have an nylon coated that protects them from chemicals as it is the case of deep cycle batteries. Now, we must refer to the NEC table 310.15 for ampacity values of cable gauges. Based on the table we can go with a 10 AWG.
Wiring from the batteries to inverter
Moreover, the wiring from the batteries to the inverter is based on the voltage for the battery bank (which should be the same as the inverter charger) and the continuous output power of the inverter. For instance in our case, we can go with an inverter of 2200W and 24V. To find out the rated electrical current we apply a security factor of 25% and the following formula.
This rated current can later be used to find the desired gauge using the NEC table as well which in this case using a THHN insulation cable, would be necessary to go with a 4 AWG. The voltage drop calculation is not necessary in this section either since the distance will also be short.
Wiring for DC loads
The DC loads of an off-grid system are typically connected to the charge controller. The wiring of this section mainly depends on the load that you will be connecting. Common DC appliances such as lights, fans or USB charging can be connected to a 12 AWG THW cable (Up to 20A), while DC pumps or motors for fridge will likely require a 10 AWG gauge cable.
Frequently Asking Questions
For any PV system the idea is to maximize the number of panels connected in series. Remember that solar panels connected in series have the same electrical current while the voltage adds-up. It is desirable to keep electrical currents low to minimize the impact on ohmic losses, for safety and also to reduce the wire gauge required. In other words, connect in series the maximum number of panels allowed by the maximum input voltage of the charge controller and then make a second string of panels now connected in parallel.
No. It is highly important that the strings connected in parallel have the same number of panels. This ensures all strings have the same or similar voltage which is important for the charge controller to track the maximum power point.
Using the formula provided can be seen that there are several ways of reducing the voltage drop. One of them is to increase the gauge of the wire. The second one is to reduce the distance between the panels and the charge controller. Finally, the last alternative is to change the voltage of the system (or string), this is done by either by changing the series configuration by adding another module or by selecting another panel with higher voltage.
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