What DC Wire Sizes to use for your Solar PV System?
Choosing the right DC wire sizes in your Solar PV system is essential for both performance and safety reasons. The wires need to be correctly sized for the current and voltages used in your system. Before you install your system, make sure you are up to date with the South African National Standard (SANS) 10142, also known as the “Code of Practice for the Wiring of Premises”. Use SANS 10142-1 for cable sizing and derating factors.
The wires must meet the following characteristics:
1. The voltage rating must be equal or greater than the voltage rating of the system;
2. The current carrying capacity must be equal to, or greater than, the current to be carried;
3. Must be able to withstand the environmental conditions;
4. Special attention to be paid to voltage drop.
It is important to use the correct wire size in a system. The correct cable can only be selected once you know the current in a system. Just like it is easier for water to flow through a thicker pipe, the thicker the wire is - the easier it will be for large electrical currents to flow through it. The same for shorter hoses and wires, they have a better flow than longer hoses and wires, with more resistance.
Generally, cable core thickness is indicated in mm2. This indicates the surface area of the cable core. Common wire sizes used for solar PV installations are: 2.5 - 4 - 6 - 10 - 16 - 25 - 35 - 50 mm2. Sometimes other sizing measurement units are used like AWG (American Wire gauge). The following categories of wires exist:
1. between batteries and to inverter, 50, 35 or 25 mm2
2. from solar panels to charge controller to batteries 10, 6 and 4 mm2
3. from the inverter to the grid, 4 and 2.5 mm2
For each category you will have to use the appropriate amperage, cable length, and accepted voltage (and power) loss. To find out the core diameter of a stranded core cable, look at the cable insulation. There will be markings on the cable that indicate cable core thickness. Be aware that some cables can have very thick insulation and they may appear thicker than they are.
In a solid cable you can calculate the surface area if you measure the diameter of the core, but in a stranded cable this method is not that precise. Please note, however, that solid core cables are not recommended for these connections.
If you know your system’s amperage and maximum voltage drop that you will accept, you can determine the most appropriate wire sizes. The below picture is an example of what cable size belongs to which current, providing that the cable distance is less than 5 meters.
If you cannot find a thick enough cable, double up. Use two cables per connection, rather than one very thick one. But if you do, always make sure that the combined surface area of both cables is equal to the recommended surface area. For example, 2 x 35 mm2 cables equal one 70 mm2 cable. Larger Victron inverter/chargers are equipped with 2 positive and 2 negative battery connections especially for this purpose.
Voltage and wire sizes
In order to avoid very thick cables, the first thing you should consider is to increase the system voltage.
A system with a large inverter will cause large DC currents. If the DC system voltage is increased, the DC current will drop, and the cables can be thinner.
Increase in voltage – cable can be thinner
The preferred upper inverter power limits per system voltage are:
- 12 V: up to 3 kVA
- 24 V: up to 5 kVA
- 48 V: 5kVA and up
If you want to increase the system voltage, but there are DC loads or DC charge sources that only can deal with 12V, you could consider using DC/DC converters, rather than to choose a low voltage for the entire system.
Wire manufacturers publish tables of current ratings applicable to the size and type, so check these ratings before you use them. If the wires are undersized, there will be a significant voltage drop in the wires resulting in substantial power loss. Also, if the wires are undersized, there is a risk that the wires may heat up to the point in which a fire may result. Note that wires have different amperage ratings, based on whether they are e.g. positioned in air or in a duct.
Usually, the longest wire is from the solar panels to the charge controller. Since all PV power runs through this, it is crucial to choose the size correctly to maximize performance and to assure safety. In general, try to stay below 2 - 3% Voltage drop on this run.
The length of the solar wire is essential, use this as a very rough rule of thumb for cables up to 5 metres, and go up to the nearest available cable size:
Current / 3 = cable size in mm2
Example: Current is 200 A – the cable needs to be: 200/3 = 66 mm2 , therefore use 70 mm2
In order to calculate the voltage drop, you can use the Victron Toolkit App (look for "Toolkit App" on this page: Click Here).
Please consult with a certified electrician to verify that you have chosen the right solar wire types for your entire solar power system before connecting any wires.
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Source: Adapted from: Victron Energy, Wiring Unlimited - Rev 06 47