Calculating the cable size for wiring solar panels
Solar panels are expensive – not as expensive as they used to be, but you are still going to need to budget a few thousand dollars if you plan to power motorhome or caravan from the sun.
Given the cost of purchasing the panels, I am always amazed by the number of people that then want to economise on the cost of the cable used to connect the panels to the battery system. In my opinion this is the textbook definition of “false economy”.
If you don’t want to know the whys and wherefores of this, just skip to the bottom of this article where I have included a simple table telling you the minimum size of cable for a given number of watts and length of cable. If on the other hand you want to know why … read on.
When it comes to cable and its ability to carry the power generated by the panels to the battery system, we have two enemies:
- The cables physical ability to carry the current without excessive heating.
- The voltage drop (or loss) over the length of the cable.
Let’s look at these two factors one at a time…
Power flowing through a cable will cause that cable to heat up. The amount of heat is directly proportional to the size of the cable and the amount of power (amps) we are pushing down it. This is how the old fashioned light bulbs work … current through the tiny wire makes it get hot … white hot!
If you went to an auto-electrician and asked for 10amp cable – he would sell you a length of cable that would not get too hot (well not hot enough to melt) when carrying 10 amps. If you put 20 amps down this cable, it would get very hot indeed.
This heating is due to what is known as “I squared R losses“ because the formula for the amount of heat generated is …
(I squared) times R
Where I = current and R = resistance
As the diameter (and thus the square mm of copper) of a cable decreases, its resistance increases – a cable with a larger cross sectional area has less resistance.
Cable heating is not normally an issue for solar installations. The second factor (voltage drop) normally dictates cables sizes large enough to prevent any cable heating issues.
Just as there are losses involved in pushing water through a pipe – there are losses involved in pushing electrical current through a wire. The longer the wire, the greater the losses.
This increasing loss can be offset by increasing the diameter (and thus the cross sectional area) of the wire. The longer the wire – the greater the diameter of the copper in the wire that is required.
As we increase the current (with say more or larger panels) we must also increase the diameter of the copper in the cable cable to avoid excessive voltage drop.
For a solar installation the maximum advisable voltage drop is 3%. In a 12 volt system this is just 0.42 volts – in a 24 volts system this is 0.84 (based on charge voltage of 14v and 28v respectively).
The formula for calculating voltage drop is …
Voltage drop equals (cable length (in metres) X current (in amps) X 0.017) divided by cable cross-section (in mm.sq).
The following tables give cable size (in square mm of copper) for a given wattage and cable length.
- The table is based on a charge current of 14v for 12v systems and 28v for 24v systems.
- Be sure to round the cable size figure UP to the nearest size cable actually available.
- The length is the length of a twin cable – ie the distance between the panel(s) and the battery (the table doubles this length to obtain the combined length of the positive and negative conductors).
- Be very sure that you are buying cable that is specified in mm square of copper. A 6mm cable when purchased from an auto electrical outlet may NOT have 6mm square of copper.
12 volt systems
|Length of cable in meters|
For 24 volt systems
|Length of cable in meters|
How to use these tables:
- Select the correct table for your system voltage.
- Select your panel wattage(s) from the blue column (either individual panels or the combined wattage of all panels).
- Run across the table until your cable length is shown in green (be sure to include the total length of cable from the panel(s) to the battery bank – not just to the regulator.)
- The figure shown is the minimum area of copper (in mm squared) that will carry the current and keep the voltage drop at or below 3%.
- Buy cable the next size UP from the size shown. Eg if your requirements suggest 4.7 mm sq. cable, buy 6mm cable (NOT 4mm sq).
If you decide to combine all panels together on the roof and bring one single cable down to the battery, simply add all
panel wattage’s together to calculate the total size of cable required to do this. If you wire each panel individually select the correct sized cable for each panel. Either option is quite valid and will have little or no effect on the outcome.
Have you found this discussion and these tables useful? Is there something related you think I should write about? Let us know – we would love to hear from you.
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Thanks and happy Motorhoming and Caravanning - Gavin & Tracey.