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Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578
Posted: 10:10am 11 Mar 2021
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I'm trying to work out the size cable needed for an install but every damn thing I look at says something Different as far as current capacity and gauge I need. All of the calculators I have looked at are basicly suggesting welding cable and I know that's not right. Must be sponsored by cable companies!
I want to have 3 pairs of 250w Panels, around 60V,1500W and run the cable 30M. Loss would ideally be under 5%.
My belief is 6mm should do this but I would like confirmation. I was also thinking 6 MM would have more capacity than 2 runs of 4mm pair which would be one cable on each pole, 8mm effectively.
Perhaps the smart people here could give me some more accurate info working out the resistance and give me an accurate figure and if 6mm ( or 2x 4mm pair) would do the trick?
Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1138
Posted: 10:33am 11 Mar 2021
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Copper cable resistance approx 0.0172 ohms per Meter per 1mm^2 area.
So 30M run = 60M total length R = 0.0172 X 60 = 1.032 ohms @ 1mm^2
Your 6mm cable has a R = 1.032/6 = 0.172
Current in the wire 1500W @60V = 25 amps
Loss @25 amps 6mm^2 = 25 x 25 X 0.172 = 107 watts % loss is 107/1500 = 7.1%
Wiring regs spec is loss must be < 3% loss, so 6mm doesnt meet standard.
If you use 2 x 4mm^2 cables: R = 1.032/8 = 0.129 Loss = 25 x 25 x 0.129 = 80.6 watts % loss = 80.6/1500 = 5.3%, not quite spec, but you could get away with it.
Cheers Mike
Murphy's friend Guru Joined: 04/10/2019 Location: AustraliaPosts: 648
Posted: 10:44am 11 Mar 2021
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Davo,you are looking at 25 Amps (1500/60). For a 30m distance that is 60m (there & back) for this current. Then use the cable location (in free air, inside conduit , etc.), that current and total cable length when you look up the cable size in your favored cable size table. Always err on the bigger size unless stinginess prevents you .
Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406
Posted: 09:23pm 11 Mar 2021
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Mike is spot on.
Dave, there are basically two factors that determine the current ratings of a wire, temperature rise, and voltage drop.
The allowable temperature rise will be different for different types of insulation. So brand "A" says their wire can carry X max amps, and brand "B" says Y max amps, for the same diameter copper. So yes, it can be a bit confusing.
Allowable voltage drop is a very different thing, and of course the further the distance the larger cable size you require for any given current. Rules and regulations are another aspect again.
So it pays to work it all out just as Mike has demonstrated, and the whole thing then becomes a lot clearer.Cheers, Tony.
brucedownunder2 Guru Joined: 14/09/2005 Location: AustraliaPosts: 1548
Posted: 11:06pm 11 Mar 2021
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Hi Dave ,
I have just finished my 10 panels with a similar configuration to yours. I used the tinned single cables of 6mm up under the panels to the disconnect breakers at chest height on the outside wall for accessibility ,( and to satisfy the code for disconnection)..
From there to under my house into my MPPT controller and associated switching/monitoring gear/breakers, I used 10mm cable ,because of the 20-30 mtr run.
Now , a point I have observed is ,very infrequently do I get ,even in full sun , the rated output of the combined solar ,sometimes nothing like it !!!.
The point I'm getting at is ,you are well within your particular requirements of cabling , (See if you can use TINNED conductors everywhere,I would if I started over again.and buy yourself a hydraulic crimper(50bucks) they and the crimp lugs are a real necessary (not the cheap thin walled Chinese lugs , get the Aussie made real ones). and use heat shrink ,it lessens the chance of corrosion.
Just my 50 years experience of climbing ladders and fiddling with cables...
BruceBushboy
Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578
Posted: 01:39am 12 Mar 2021
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Thanks all for the feedback, Very helpful and enlightening.
The project has grown overnight to upsizing to 10 Panels now so definitely going to be 10mm wire. Still much smaller than the 26mm the calculators were advising.
Bruce you touched on my next question, do I have to use solar cable which as I was thinking, is tinned ( and finer stranded?) for DC as being different to 10MM for AC or will the Ac do? ( which I think it will not.)
From what I'm seeing, Might also have to join the 10mm as I can only find it as 30M rolls and I'll definitely need some extra. does anyone know the proper name for the connectors for joining 2 ends of cable so I can look them up?
I'll look for a crimper as doing that gauge is going to have to be done right.
I'm always amazed at how inefficient wire is for carrying power!
Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406
Posted: 04:58am 12 Mar 2021
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The only reason cables are stranded is for flexibility. Above a certain size, not even Superman could bend it if it was a single solid core.
Building wire and similar power cables are designed to be enclosed in a wall cavity, enclosed in conduit, or similar, and never moved again once installed.
Something like welding cable, or the leads on your multimeter, are moved every time they are used, and have dozens or hundreds of very fine hair like strands. And are more expensive...
Solar cable probably falls somewhere in the middle.
The big issue with outdoor or underground cable is corrosion. If water can find its way into one end of a stranded cable, the strands can wick the moisture tens of metres. Your nice stranded copper cable quickly turns to green powder and goes open circuit.
The finer the strands the more fragile the cable, and the more readily it sucks up moisture. Tinning the copper wire helps a very great deal with this problem, but its not a completely fool proof solution, just one more step in the right direction.
So for outdoor solar, probably stranded cable, but big strong strands will be more robust, and tinned wire is a much better choice than raw copper. Terminate the ends in a weatherproof box with cable glands so the end stays dry. A pin hole at the lowest point is a good precaution.
Its one of Murphy's laws, that anything totally waterproof lets water in, but prevents water from ever escaping once its in.Cheers, Tony.
Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578
Posted: 10:09am 12 Mar 2021
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Very helpful thanks Tony.
Was going to use the AC link boxes to bring all the paired panel cables together then run the main cable from there. That will probably go under a panel and I was going to mount the link boxes on some plastic board then fill the whole damn things with silicone and put the lids on letting the silicone ooze out and hopefully sealing them like electronics are encased in that potting Media.
Long as I seal them, seems the AC type cable will be OK then. That will be a saving and easier to get.
Thanks again for the info.
brucedownunder2 Guru Joined: 14/09/2005 Location: AustraliaPosts: 1548
Posted: 10:38pm 12 Mar 2021
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Yes Dave, Tony is thinking along the same lines as myself. You digest the posts a couple times over a coffee ,or what ever is your choice of poison,lol.
The jointing of the cables , this may not be necessary if you do the joints in a junction box or breaker box, just use the plastic joiners (I prefer the double screwed ones that they require for earth wiring. sort of get more "tail" of bare wire twisted together, and two screws to really do the job ).
But ,yes, you can buy the little joiners designed for crimping and then heat shrink . I think I got 4,6, and 16 mmsquare ones --very handy for cut mistakes and sometimes joining two cables into one, but make sure they are a very tight fit, before crimping. and the heat shrink.
also, I use "Bootlace ferrules " on all my cable ends. you can buy 25 or 50 in a packet for a dollar or so and different sizes will do all your jobs.
you'll find the 10mm cable very expensive,IF you can find it tinned ??-best of luck ,might have to get a load from the bank ?.
Solar cable is nearly always tinned by regulation.
I would suggest getting a couple of 100m rolls of building wire from a wholesaler. I use these guys: https://www.tradezone.com.au/product/10mm-building-wire-cable-red-100-metre-drum-31073.html
I don't know price they'll give you if you don't have an electrical licence, but for me their prices are about as good I can get at any local wholesaler, and for most stuff they only charge $9 freight, but you may have to pick it up from your local freight depot.
Get a roll of red and a roll of black. You can get it by the meter as well, but its a bit cheaper if you buy a whole roll. Its up to you if you think its worth the upfront cost of the whole rolls. Depends if you think you'll make use of it or not. Personally I've never regretted having extra cable on hand. It always comes in handy for some project or another. Doesn't go off if sits in the shed for a few years.
Bear in mind that technically it is supposed to be in conduit as its only single insulated. But even double insulated cable is supposed to have mechanical protection of some kind if you run it anywhere where it could possibly be damaged.
For the low voltage you're talking about I wouldn't get too stressed about it, and I think you're smart enough to make sure nothing stupid happens to the cable.
Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406
Posted: 10:06pm 13 Mar 2021
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Conduit is dirt cheap, and always a good idea for any wiring open to full sun and rain.Cheers, Tony.
brucedownunder2 Guru Joined: 14/09/2005 Location: AustraliaPosts: 1548
Posted: 11:06pm 13 Mar 2021
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Yes,and I use Aussie Duct ,in the 50mm square range and the smaller ranges if you are contemplating running a few wires .
Very handy, as you just un-clip the lid and add or subtract cables so very easy, especially if you are working off a ladder. I live out in the bush ,where critters luv to build their home in ducting,Etc, so squirt a very small (a drop) , amount of that fitting oam cavity sealer in the ends, stops the nasties .
BruceBushboy
Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 814
Posted: 07:03pm 08 Apr 2021
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240v 2.4mm 'mains building cable' or 'TPE' in conduit is perfect for series arrays (use the UV stabilised mains conduit)- its good for up to 25A in shorter runs (use the standard voltage drop tables for the lengths you are talking about)- it does require the use of conduit (either fixed or flex) and is easy to fit and reasonably cheap (100m rolls about $150)... and the insulation is quite adequate for the job
I'll be using it in my system (hell there's going to be a lot of cables coming from the roof lol)- I can put 1.5kw of a 3S, 2P array of 37vPOC 250w panels, which means a grand total of 12 cables coming from the roof to the 3kw controllers, with 2 cables to each controller for the 18kw of panels (This is a common usage with many professional installers btw)
Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406
Posted: 10:46pm 08 Apr 2021
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The standard building cable has a third earth wire and a fairly thick white outer covering, none of which is really needed for solar. But agree the wire itself can be ideal.
You can buy rolls of just the red and the black 2.5mm sq (single insulated). Its the exact same stuff, much cheaper, and its a lot easier to feed through conduit if you have a whole bunch of wires to deal with.
If you have a single series high voltage array of maybe about 10 amps capacity, the 4mm sq standard "solar" cable may be heavier than you really need, unless the distance is quite far.Cheers, Tony.
Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578
Posted: 08:30am 09 Apr 2021
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I did some re arranging and cleaning up the solar setup today actually. I had some 2.5 Mm 3 core building cable up there which I put up about 2 years ago for a short term test. Nothing so permanent as temporary as they say.
The insulation was in perfect condition., No powerdering, cracking, still flexible.... I was surprised. I'd still of course put it in conduit if I was using it on a proper setup but for 2 years and no breakdown I could see, pretty Impressive.
I have been able to buy the solar cable cheap or cheaper than the 3 core cable. I think building cable looks a bit dodgy and it does not suit the MC4 Connectors well but it certainly does the job. Most mains solar is under 10A on the DC side so plenty of capacity in 2.5 Cable.
With high current and longer DC runs, I have known people whom shall remain nameless to protect myself, to join all 3 Conductors together and use an entire 3 Core cable for each Pole. Other questionable practices have been to use 3 phase cable to bring multiple strings back to the inverter in one cable. Hey, I....errrr, THEY got it for nothing so may as well put it to use.
I have also heard conflicting info that the earth in building cable does not carry the same current as the active and Neutral but then read where it has to be able to carry the full current of the other sides so I'm not sure what the situation really is.
I have also noticed some significant differences in building cable. Some is just easy to work with, strips well, the conductors seem flexible and resilient and others are a Pain in the arse, The conductors break and fall apart the second you try and twist them together. I'd like to say this has been with cheap foreign crap but unfortunately I have seen it repeatedly with an Australian made cable which is disappointing.
Murphy's friend Guru Joined: 04/10/2019 Location: AustraliaPosts: 648
Posted: 10:35am 09 Apr 2021
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You are right, that earth wire is smaller (at least it looks so) but then it does not carry *any* current until there is a fault. In a fault situation it only needs to carry the fault current for the time until the fuse blows so it can be over rated for that task.
Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406
Posted: 10:49pm 09 Apr 2021
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That is my understanding as well. It needs to be made a certain minimum size for robustness, but beyond that, it need be no larger than to carry a momentary high fault current.Cheers, Tony.
Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578
Posted: 03:34am 10 Apr 2021
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Sounds reasonable. I agree that it always looks smaller to me as well. Maybe I was reading something from other countries. Sometimes hard to know where info originated from.
Just had a look on my electrical wholesalers site. 3 core 2.5 is $146 100M roll, red and black twin 2.5 is $100 so a decent saving there especially if the 3rd core is going to be used for DC and not needed.
Otoh, 4mm Solar cable is available for $100-125 on Gumtree and other sites.
The earth conductor in 1.0, 1.5 and 2.5 mm cables is the same as the active and neutral but when you go to larger sizes the earth is smaller, for example 4mm and 6mm twin and earth use 2.5mm for the earth conductor, and 10mm cable uses 4mm for the earth. As mentioned, the earth conductors don't carry a load, but they must be able to carry a fault current long enough to trip the protection device.
Quote from AS3000
The cross-sectional area of a protective earthing conductor shall ensure a. adequate current-carrying capacity for prospective fault currents for a time at least equal to the operating time of the associated overcurrent protective device b. appropriate earth fault-loop impedance see Clause 5.7 c. adequate mechanical strength and resistance to external influences and d. for parts of the protective earthing conductor that do not consist of cables, or parts of cables, that there is an allowance for the subsequent deterioration in conductivity that may reasonably be expected.
Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me.
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