How to Select and Size Cable For Your Off-Grid PV System

Updated: Nov 3, 2019

Choosing the right wire sizes for your DIY Solar system is important for both performance and safety reasons. If the wires are undersized, there will be a significant voltage drop in the wires resulting in excess power loss. In addition, if the wires are undersized, there is a risk that the wires may heat up to the point in which leads to catching of fire.

The current generated from the solar panels should reach the Battery with minimum loss. Each cable has its own Ohmic resistance. The voltage drop due to this resistance is according to Ohm’s law

V = I x R (Here V is the voltage drop across the cable, R is the resistance and I is the current).

The resistance ( R ) of the cable depends on three parameters:

1. Cable Length: Longer the cable, more is the resistance

2. Cable Cross-section Area: Larger the area, smaller is the resistance

3. The material used: Copper or Aluminum. Copper has lesser resistance compared to Aluminium

In this application, copper cable is preferable.Copper wires are sized using the gauge scale: American Wire Gauge (AWG). The lower the gauge number, the less resistance the wire has and therefore the higher current it can handle safely.

You can calculate the cable size by using RENOGY online calculator.

You need to enter the following parameters :

1. Solar Panel Operating Voltage (Vmp)

2. Solar Panel Operating Current (Imp)

3. Cable Length from Solar Panel to Battery

4. The expected loss in percentage

The first two parameters ( Vmp and Imp) can be easily found from the specification sheet on the backside of the solar panel or from the datasheet. One 255W Solar panel specification sheet is shown in the above picture for reference.

The cable length depends on your installation. The loss percentage considered for good design is around 2 to 3%.

From the Solar panel specification sheet Vmp = 36.7V and Imp = 6.94A ( rounded off to next higher number i.e 37V and 7A ) .

Let the distance between the Solar panel and the Battery is 30 feet and the expected loss is 2%.

By using the above values in the online calculator by RENOGY,

The cable size is 12 AWG.

Cable Sizing Chart

To determine the size of solar cable needed, just look at the AMPS in the left column and select the amp rating of your solar system. Next follow the same row over to the approximate length of Solar cable you will be using. Then, follow the column up to the yellow box at the top which will be the AWG number wire you'll need.

You can buy the Solar cables from Amazon or Banggood

The voltage grade of the cable should be matched with the Solar Panel maximum system voltage. In the above picture you can see the voltage grade is written on the cable. Maximum Permitted DC Voltage: 1.8 kV

Images Credit: Banggood

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