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:

  1. The cables physical ability to carry the current without excessive heating.
  2. The voltage drop (or loss) over the length of the cable.

Let’s look at these two factors one at a time…

Cable heating

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.

Voltage Drop

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.

Notes :

  1. The table is based on a charge current of 14v for 12v systems and 28v for 24v systems.
  2. Be sure to round the cable size figure UP to the nearest size cable actually available.
  3. 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).
  4. 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
Watts Amps 2 3 4 5 6 7 8 9 10 11 12
100 5.88 1.0 1.4 1.9 2.4 2.9 3.3 3.8 4.3 4.8 5.2 5.7
150 8.82 1.4 2.1 2.9 3.6 4.3 5.0 5.7 6.4 7.1 7.9 8.6
200 11.76 1.9 2.9 3.8 4.8 5.7 6.7 7.6 8.6 9.5 10.5 11.4
250 14.71 2.4 3.6 4.8 6.0 7.1 8.3 9.5 10.7 11.9 13.1 14.3
300 17.65 2.9 4.3 5.7 7.1 8.6 10.0 11.4 12.9 14.3 15.7 17.1
350 20.59 3.3 5.0 6.7 8.3 10.0 11.7 13.3 15.0 16.7 18.3 20.0
400 23.53 3.8 5.7 7.6 9.5 11.4 13.3 15.2 17.1 19.0 21.0 22.9
450 26.47 4.3 6.4 8.6 10.7 12.9 15.0 17.1 19.3 21.4 23.6 25.7
500 29.41 4.8 7.1 9.5 11.9 14.3 16.7 19.0 21.4 23.8 26.2 28.6
550 32.35 5.2 7.9 10.5 13.1 15.7 18.3 21.0 23.6 26.2 28.8 31.4
600 35.29 5.7 8.6 11.4 14.3 17.1 20.0 22.9 25.7 28.6 31.4 34.3
650 38.24 6.2 9.3 12.4 15.5 18.6 21.7 24.8 27.9 31.0 34.0 37.1
700 41.18 6.7 10.0 13.3 16.7 20.0 23.3 26.7 30.0 33.3 36.7 40.0
750 44.12 7.1 10.7 14.3 17.9 21.4 25.0 28.6 32.1 35.7 39.3 42.9
800 47.06 7.6 11.4 15.2 19.0 22.9 26.7 30.5 34.3 38.1 41.9 45.7
850 50.00 8.1 12.1 16.2 20.2 24.3 28.3 32.4 36.4 40.5 44.5 48.6
900 52.94 8.6 12.9 17.1 21.4 25.7 30.0 34.3 38.6 42.9 47.1 51.4
950 55.88 9.0 13.6 18.1 22.6 27.1 31.7 36.2 40.7 45.2 49.8 54.3
1000 58.82 9.5 14.3 19.0 23.8 28.6 33.3 38.1 42.9 47.6 52.4 57.1


For 24 volt systems

    Length of cable in meters
Watts Amps 2 3 4 5 6 7 8 9 10 11 12
100 2.94 0.2 0.4 0.5 0.6 0.7 0.8 1.0 1.1 1.2 1.3 1.4
150 4.41 0.4 0.5 0.7 0.9 1.1 1.3 1.4 1.6 1.8 2.0 2.1
200 5.88 0.5 0.7 1.0 1.2 1.4 1.7 1.9 2.1 2.4 2.6 2.9
250 7.35 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3.0 3.3 3.6
300 8.82 0.7 1.1 1.4 1.8 2.1 2.5 2.9 3.2 3.6 3.9 4.3
350 10.29 0.8 1.3 1.7 2.1 2.5 2.9 3.3 3.8 4.2 4.6 5.0
400 11.76 1.0 1.4 1.9 2.4 2.9 3.3 3.8 4.3 4.8 5.2 5.7
450 13.24 1.1 1.6 2.1 2.7 3.2 3.8 4.3 4.8 5.4 5.9 6.4
500 14.71 1.2 1.8 2.4 3.0 3.6 4.2 4.8 5.4 6.0 6.5 7.1
550 16.18 1.3 2.0 2.6 3.3 3.9 4.6 5.2 5.9 6.5 7.2 7.9
600 17.65 1.4 2.1 2.9 3.6 4.3 5.0 5.7 6.4 7.1 7.9 8.6
650 19.12 1.5 2.3 3.1 3.9 4.6 5.4 6.2 7.0 7.7 8.5 9.3
700 20.59 1.7 2.5 3.3 4.2 5.0 5.8 6.7 7.5 8.3 9.2 10.0
750 22.06 1.8 2.7 3.6 4.5 5.4 6.3 7.1 8.0 8.9 9.8 10.7
800 23.53 1.9 2.9 3.8 4.8 5.7 6.7 7.6 8.6 9.5 10.5 11.4
850 25.00 2.0 3.0 4.0 5.1 6.1 7.1 8.1 9.1 10.1 11.1 12.1
900 26.47 2.1 3.2 4.3 5.4 6.4 7.5 8.6 9.6 10.7 11.8 12.9
950 27.94 2.3 3.4 4.5 5.7 6.8 7.9 9.0 10.2 11.3 12.4 13.6
1000 29.41 2.4 3.6 4.8 6.0 7.1 8.3 9.5 10.7 11.9 13.1 14.3

How to use these tables:

  1. Select the correct table for your system voltage.
  2. Select your panel wattage(s) from the blue column (either individual panels or the combined wattage of all panels).
  3. 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.)
  4. 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%.
  5. 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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32 Responses to “Calculating the cable size for wiring solar panels”

  1. Mark Doolan Says:

    Hi Gavin, just wondering if there is a formula for not jamming ones fingers either under busses or in spear gun rubbers? Cheers Mark. BTW the 1.5Kw solar on my ECO house has my electricity bill at – $366.20

  2. Hobo Says:

    The formuler most people use is “take more care”. It is a wonder I have any fingers at all.
    Just been looking at a 5kw system over here. They are getting big credits each month.

  3. Solar Panel Fixing - MotorhomeFun Says:

    […] roof) and s/s bolts (to panel) and of course Sikaflex type adhesive. A good site for info is: http://hobohome.com/news/?p=581 http://hobohome.com/news/?p=491 […]

  4. Robert Dickson Says:

    Hi Hobo,

    I am confused!!

    In the above article 12v for a 200watt panel you say the Amps are 11.76 and if length of cable is 9mtrs (18 mtrs return used in your calc) the size of wire is 4.6mm2.

    In your other article “Basic electrics…Part1” you give the following example…
    “Example – you have 2 X 200w panels and want to connect them to the battery bank that is 9 meters from the panels – you are considering a cable that is 4mm sq of copper.
    200w divided by 12v = 16.66amps – this is about the maximum current for each of the two panels.

    18 (two times 9m) X 16.66 X 0.017 = 5.09 now divide that by 4 (mm sq copper) – this tells us that we will have a voltage drop of 1.27 volts. The highest acceptable voltage drop is 3% of the supply voltage (so in a 12v system, 0.36v). Clearly our 4mm cable is far too small.
    For this installation we are going to need length of cable that has at least 16mm sq of copper in each conductor for each panel. That is a sizable lump of cable!”

    The correct size of wire is 16mm2!!

    What am I miss understanding, which is correct??
    Thanks
    Robert

  5. Hobo Says:

    Hi Robert,
    You are partly correct – there was an error in the table. This has been corrected. There are a couple of other things going on here that need to be taken into consideration …
    A 200w panel will not produce 16.66 amps. This is because the wattage rating of the panel is stated at pmax (the maximum power point – where V x I = the greatest value – (look at the label on the back of the panel)). This is normally around 17 volts on most 12v panels. To work out what current to expect from a panel you must divide the rated watts by V@pmax (~17) . Thus a 200w panel will produce about 11.76amps (200 / 17 = 11.76).
    Furthermore you can not charge a 12v battery at 12 volts – you will need a minimum of 14v. Thus the percentage of voltage drop is calculated at the higher charging voltage. So a maximum of 3% of 14v is 0.42v (NOT 3% of 12v which is 0.36).

    Let me know what you think.

  6. richard Says:

    Thanx at last after days of google-ing and digging out my shades to protect eyes from the bright lights i found myself halfway round the world for the answer to what i thought was a simple question – cable size from 160w of solar panel you did indeed make it easier to calculate thanx.

  7. Scott Sutcliffe Says:

    Hi Gavin and Tracey,
    Have followed your travels and crash’s(sorry) for the last couple of years. Am getting our bus completed at the moment and looking at end Feb 2013 for completion.
    Can you check my cable calculation:

    Cable Length 30 l/m
    24V System
    3x250W panels 24v & 8.2A MaxPower
    1v190W Panel 24v & 5.11 A Max Power

    Based on a Voltage drop of your recommended 3% I came up with 36.5mm2 so go up 1 size to 50mm2

  8. Hobo Says:

    Hi Scott,
    That is a very long length of cable (and consequently a very large cable). However my calculations suggest that 25mm of copper would give a VD of 0.67v or 2.4%. Can I suggest two other options that may save some money (and weight).
    Option one – wire each panel individually – measure the actual length of run for that panel and use the correct sized cable for each panel – that way the panels that are closer to the controller will have a smaller cable.
    Option two – use an MPPT controller that is able to convert high voltages to the correct charging voltage. This will allow you to connect the panels in series and use a much (much) smaller cable. This will also result in a considerable increase in the total charge available from the array. Sadly these controllers are not a cheap option.

    In the calculations use the charge voltage and not the system voltage (eg 28v and not 24v)

    Cheers

    Gavin

  9. Scott Sutcliffe Says:

    Thanks Gavin,
    I am somewhat confused as I calculate using Collyn Rivers Formula L x I X .0164 / .4 and get 41mm2

    Where am I going wrong please.

    Other calculators give sizes from 50-70mm2 but none near the 25mm2

    Cheers Scott

  10. Hobo Says:

    The formula is …
    Voltage drop equals (cable length (in metres) X current (in amps) X 0.017) divided by cable cross-section (in mm.sq). (assuming standard copper cable – don’t forget that this is the total length of the conductor so if your panels are 30m from the battery this is 60m of cable – positive and negitive)

    Solar current is 940w divided by 28 (the charging voltage) = 33.57amps (the reality is that it will very rarely produce this current as the MaxPP of the array is not matched to the battery unless you are using an MPPT controller)

    For 25mm cable carrying 33.57amps over 30m the voltage drop will be 0.6732v or 2.4% of 28v

    (30 X 33 X 0.017) / 25 = 0.6732

    Cheers

  11. Scott Sutcliffe Says:

    Thanks Gavin thats a very clear explanation.
    Much appreciated and safe travels

    Cheers Scott

  12. fran gray Says:

    Hi Gavin, we are adding solar panels to our current system on a bus.. At present all the panels run into a junction box the furthest away from the junction box is the largest a 200w panel 3mtrs from the junction box all are running 6mm cable.. our problem is from the junction box to the controller is 5mtrs.. the total watts are 690.. The regulator has just been upgraded to a 60Amp wellsee.. The regulator only takes 6mm cable but the calculation says we need 16.7 for the distance.. What is our best option 1… to extend all the cables from the panels and bring the junction box to within 2mtrs of the regulator or 2 ?? any suggestions.. Have only just found your site (awesome) as it has taken us ages to work all this out..thanks Fran

  13. Hobo Says:

    This is always a difficult situation. The best option is to reduce the conductor size from the required 16mm to 6mm as close to the regulator as practical. 100mm of 6mm cable will not introduce any significant voltage drop into the system.
    So just to clarify – the 16mm cable runs from the junction box to a connector (or junction box) that is located just 100mm from the regulator. At this point the cable reduces to the 6mm required to fit into the Wellsee regulator.

    BTW – shame on the Wellsee people for producing a 60amp controller that only accepts 6mm cables – Dah!

  14. fran gray Says:

    Thankyou .. Your right there when I purchased the controller I never thought to check what size cable it took.. All good thankyou for that info..

  15. fran gray Says:

    Just a quick question . Do we need to change the cable size from the controller to the batteries they are only a metre apart and run 6mm..

  16. Hobo Says:

    I would double the size of the cable (ie two times 6mm). I think you said 680w total?? at 12v that is potentially 56 amps.

  17. fran gray Says:

    Awesome thanks so much for that info..Ah and in easy to understand english…

  18. Thomas Says:

    I have just completed a 9400 k trip with nonstop 12v electrical problems. I have been ripped off by auto electricians and motor home specialists fixing the problems. No more. I am home and want to do the work myself. I drive a Toyota Coaster bus with a 24v system and a 24v to 12v converter for the 12v items I use in the bus. What is the best 24v to 12v converter I should use. What is the best solar controller that gives me the readings to show battery condition? Thank you for your articles. I am learning a great deal from what you write.

  19. Hobo Says:

    I agree – learning to DIY is the best solution.
    depending on a number of factors, I would look at Plasmatronics regulators. As for 24-12v converter – the Jaycar ones are fine.

    I hope this helps

  20. jim Says:

    Hi Hobo
    I have two 130w 17.5v 7.5amps panels .= 260w 35v 15amps I need 6mts of cable from the panels to controller and 500mm to Battery I have 8mm cable for that . Your table says I need 7.1 cable from panels to controller . My question is . Can I run two 4mm cables for both pos and neg to the controller . As I have a 100Mts of 4mm solar cable . I have ordered a 30Amp MPPT Controller .
    Hope you under stand my question Thanks

  21. Hobo Says:

    Hi,
    The short answer is “yes”. with some minor exceptions, two 4mm cables are (from a current carrying / voltage drop point of view) identical to one 8mm cable.

  22. jim Says:

    Hi
    Thanks for that.
    Cheers Jim

  23. Chris Says:

    I’m a bit concerned about the units people are using here. Some refer to 6 mm or 8mm cable for example, as if they mean the cable diameter. Millimetres are not the same as square millimetres, which are the units specified in the tables (often abbreviated to mm2.)

    The difference is important. A 6 mm diameter conductor has a cross sectional area of 28 square mm (mm2)

    You cannot replace an 8 mm cable with two cables of 4mm, you actually need four of them to handle the same number of amps.

    4mm cable = 12.5 mm2
    8mm cable = 50 mm2

    Sorry if this sounds like a nit pick, but it could result in costly mistakes if people don’t understand the difference.

  24. Hobo Says:

    I think this point is well made in the article “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.”
    When an auto electrician talks about 6mm cable – this is NOT 6mm diameter of copper and it is NOT 6mm squared of copper – it is an old industry standard that give some indication of the size of hole you would have to drill to pass the cable through (including the insulation). Thus the examples you provide here are not correct.
    Industry term 6mm cable has a cross-sectional area of 4.58mm2 (square mm) of copper and it is “about” 6mm in diameter INCLUDING the plastic insulation.
    That said – your point is well taken – I have no idea why the “industry term” continues – it tells us little about the current carrying ability of the cable.

    Thanks for your comments

  25. Chris Says:

    Many thanks for the information. I did not know about the old standard that includes the thickness of the insulation. Another trap for new players to be aware of.

    I was not criticising your article by the way, but only noting that some people in the comments were referring to mm instead of square mm, as if they had not understood your point.

    I would still be concerned about someone wanting to substitute two wires of “industry term 4mm” in place of one wire of “industry term 8mm.” I don’t know what the actual conductor sizes are, but it seems unlikely that the two smaller wires would be enough.

    Of course if they are referring to 8 square mm and 4 square mm conductors, that would be fine.

  26. Hobo Says:

    Thanks Chris. Yes, it is definitely a trap. We should ALWAYS talk in mm2.
    You may be interested in a phenomenon called “the skin effect”. This is only evident in conductors carrying alternating current (so not really applicable here), but is results in current traveling predominantly in the outer layer of the conductor (the skin). As the cross sectional area increases the amount of skin reduces compared to the cross sectional area – this results in a higher resistance. In this case two 4mm2 cables WILL carry MORE current than one 8mm2 cable. The effect is proportional to the frequency.

    Cheers and thanks for your comments.

  27. mick Says:

    hi hobo n guys , was talking to a guy with a bus , had a heap of solar , i think it was around 1000watts made into 24v to charge a 12v battery bank , that i can understand running into a mppt , but his wires were big jumper leads , the type ya start a car with , the wires/copper part looked like it was 5-8mm round, would this not be to large to use , seems a to cheap way to run solar power around were u need it. ie i looked on ebay and 5m 1200amp is around $40

    conductor spec:8mm2
    cable outer diameter:10mm

    regards mick

  28. Hobo Says:

    Hi Mick,
    I don’t know of any way of charging a 12v battery bank from a 24v set of panels without using a step-down MPPT controller.

    A cable that is 10mm in dia (assuming 2mm of plastic) has a conductor size of 8mm dia. Using the formula – area = PI*R2 (Pi times radius squared) gives a cross sectional area of 50mm.

    A cable is given two different current ratings for two different reasons…
    1. The amount of current it can carry without damage (ie melting). Clearly this can change depending on where the cable is run (ie in conduit or in air etc)
    2. The voltage drop (or losses) over the length of the cable. Solar panels are very inefficient (about 17% at best) – so introducing additional losses is like burning money. Thus the acceptable voltage drop for solar is very low.

    There is never any harm in running cables larger than they need to be (other than cost and the hassle of running and terminating them).

    Cheers

  29. ryan Says:

    G’day,
    Is the length of cable the run from the charge controller to the battery or the solar panels to the charge controller?
    Has me a bit confused what cable length i need to add up.
    Regards

  30. Hobo Says:

    Hi Ryan,

    Both cable runs should be considered. The total voltage drop is what is important. The section that runs from the controller to the battery bank is MOST critical as any voltage drop in this cable will not be considered by the controller and the batteries could be being damaged by undercharging.

  31. Perrie iles Says:

    Interesting, I find the cost, fitting and physical size of decent cables beyond the pale,
    I guess using the chassis as a return path offers a reduction in cost (albeit at the possible cost of electrolysis in metal, especially where dis-similar ones make contact, rivets, sheeting, chassis etc).

    I am intending to use small self designed 12 to 48V dc and 48V to 12V convertor boards
    (check ti.com for webench) – over 98% efficiency at 50% load – enabling use of 2mm squared copper instead of 29mm squared for a 0.2V drop over a 10m run. saving a lot of effort/time/money/space

    Always wanted a main backbone running into a series of drops (like ribs of the spine)
    some of these boards can handle over 25A at 12V, and if needed can use multiple smaller ones.
    and small enough to keep spares, XT60 or similar connectors (designed to handle 60Amp)
    have used these and smaller 20A connectors for my modified NERF (TM) guns.

    I chose 48V as a multiple of 12 and not to high to run standard electronics, insulation and cabling
    without requiring special licenses (although I do have an electrical license)

    Currently building a 1.3Kw battery for camping (equivalent to a 100Ah 12V battery) except I can drain it 100% with no ill effects as it all reclaimed 18650 li-ion 2.1-2.2Ah, run in 2P7S config giving around 4.1Ah per pair (have an Arduino based battery tester, controls a fet on a heat sink , with controllable current load and measures resistance of battery + wires). the 7S gives a peak of 29.4V and then this passes into a high current 24 to 12V convertor (which has a peak limit of 35V – might up that to 50V though) Every cell is checked for capacity and ability to handle a load at 0.5 and 5A per cell

    Also thinking about using micro-inverters on the solar cells to reduce wiring losses and size
    and increasing power generation under partial illumination. needs to be done with more care and higher quality

    My next nerf will use 26650 li-ion 40A peak cells and 2 arduinos to control fire rate, speed etc.
    high speed motors, balanced flywheels, machined motor container, 0.8″ oled display, Teflon slotted barrel,
    time of flight sensors (to determine muzzle velocity) and Perspex covers to show off the controllers

    Getting an UDOO x86 based controller with Arduino pinouts but multi GHz so great for
    controlling a lot at once with time precision… (check kickstarter)

    Till later..
    Perrie iles

  32. Hobo Says:

    Hi Perrie,
    This sounds really interesting. I like the sound of the very efficient dc converters. I wonder why you don’t series connect the panels on the roof to get your higher voltage. This would be 100% efficient and have the advantage of effectively producing a longer solar day (voltages above minimum charge voltage for longer). Most people do this then feed the result into an MPPT controller near the batteries that finds the most efficient charge point and produces the maximum charging current at the optimum voltage. (That said – I do like the DIY approach).
    I have used a number of Arduino controllers to manage the systems around the shouse, but don’t have any in the bus. Years ago I built an engine monitor for the bus from a large picaxe – this has been serving us very well for about 8 years now. It warns of issues that are a potential indicator of a problem – long before a normal gauge would detect any problem.

    I’m very interested to hear how you lithium battery bank goes over the long term.

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