I hope I can ask this question even if its totally lighting unreleated.

My family has summer house in a small island in a midle of lake. Thus there is no possibility to connect it to power companys netwerk(actually there is, but the cost would be too expensive). So we have installed couple solarpanel. In future there also will be wind generator too. There are three separated buildings: the main building, sauna and storage/shed. In the current situation each building has its own 12v lead acid battery. Solar panel is installed in the main house. When sauna or storage battery runs low, we need to carry it to main building to load it. because this brings extra efford, I have been thinking to centralize the batterys to storage building and dig cables to sauna and main building. as the same time, there is plan to swap 12V dc to 24V dc system. Now the question is, Is there any difference in using copper or aluminium cable at low voltage dc. as far as I know, copper cable has lower resistance than aluminium, but aluminium is much cheaper than copper. The second question is how thick cable should I use? I have think that maximum current for sauna could be something like 20A and distance is about 15meters(to main fuse box to sauna fuse box). what comes to main house. I have thought maybe 25A and about 30 meters. I hope my plan is not complere mad.

You need a 70% bigger wire if you use aluminum instead of copper for the same resistance. You CAN go lower, but you loose more energy as heat. To reliably carry 25A, you need at least 12awg copper. If you use aluminum, you would ideally need 8awg, so if 8awg aluminum is cheaper than 12awg copper, go for it.

And a question: What do you power from it in the Sauna and Storage house?
If majority of it is lighting, i would consider using flat wall or ceiling mount fixtures converted to LED. Flat fixtures, because on these it is not as difficult to add aluminium plate to their back serve as good LED heatsink, mount LED modules on them and still keep their original look...


And the power draw is for short term (few minutes, up to a hour) or more steady state?
Then good idea might be to keep 12V battery local in each house and redistribute only the charging current, you would not have to deal with as high power to transfer between houses...

@Medved: In the sauna, there is usually only lighting and maybe something like loading you cellphone or laptop(in the same building there is one bedroom) so 20A is overkill but for the future there is always good to have some extra capacity left. In the main house, there is tv, radio, extractor fan, cellphone and laptop loading and ofcourse lighting. Lighintg is mainly done in DC CFL's, small halogens(reading lamps for bedside) 18W fluorescent that usees DC control gear(and some rare places low votalge incandescent lamps). I have tried some DC LED lamp but was not happy with them. and what comes from the most fixtures there is... Im not sure those could be converted with led's so easely. In the summertime, there is usually not much need for lighting(as the sun does not set at all) but respectively in the winter, you need lights all the time.

If the charging current is only distributed, will it need somekind of charger close to the charged battery? I dont see big currents so big problem. Bigges pain is to try to dig that cable in rocky ground.

@Xytrell: Im not complete familiar with these AWG sizes but I googled and if what Iv foud is correct 8AWG is 8.36 mm2. smallest aluminium cable I did find is 4X16mm2 and it cost 2,45EUR per meter. in copper closest mm2 size for 12AWG would be 4X6+6mm2 because 4mm2 is very hard to find. 4X6+6 cost 3,40 per meter. All the prices exclude the taxes(so 22% more for real price). because both of these cables have 4 conductors you could double the diameter easely. Because shot distances even thick aluminium or copper cables could be used and prices would not be a problem( example. 4x16+16mm2 copper would cost  8,50EUR per meter. and at the same price you would get 4X95mm2 aluminium cable, sure enought capacited if two pairs are both connected). Aluminium cable is just not so nice to install as copper is.

If the charging current is only distributed, will it need somekind of charger close to the charged battery?

Yes, but if the mains house with the panel uses higher voltage, it might take an easy form of simple step-down DCDC converter with readily available chips (e.g. MC34063A for up to 0.8A charging current - see datasheet for buck converter).
For 12V battery you set the output voltage to 13.8V, the current limitation should be as high as possible, taking into account the max input power draw you want to allow from the main house.

Other possibility would be to install separate wind turbines (based on your description the lighting is not needed in the summer, but solar panels would not work in the winter) on each house - then you would not need to dig the cabling at all.

And don't forgot, then with long cables and low voltage, the fusing become more difficult, as the ratio between short circuity and operating current become small: Or fuses would blow on high peak demand, or there is risk, then the fault current would not trigger them. So with the cable you have to ensure, then the failure current really exceed the fuse's triggering current with enough safety margin.

The cooper 4*6mm^2 cable used as 2+2 conductors (so 2*12mm^2) would limit short circuity current to below 200A, what is really on the border to reliably trigger the 20A rated fuse.

The aluminium 4*16mm^2 wired the same would lead to >400A short circuit current, keeping higher safety margin for tripping the 20A fuse in case of a fault. At the same time it would have lower losses, so i would go for this cable.

The solar panels are being moved to near to the storage for near future. because of the hight trees near the all three buildings, the wind generator is planned at the northen end of the island(about 20 meters from storage direction away from the main house. as there are smaller trees there so the mast does not need to be so tall to reach over the trees.

If I remember right, short circuit current should be 10 times greater than fuse rated current.

So because of the low price of the aluminium cable. I think that 4X50mm2 will be used for main house. So the capacity should not run out. There is Distribution board with its own fuses in the main house so theoretically. only shortcircuit in the "main distribution cable" should trigger the 25A fuse in the Main fuse box. Ive been thinking to use this http://www.firomet.fi/kuvat/sahkotarvikkeet/kahvasulakkeet%20pohjat.jpgkind of fuses in the main fuse box because I have dozen of that kind of fuse holders laying around with no use. Its kind a hard to describe this because I dont know all the english terms for these things. Also I have been thinking about having a current meter in the main fusebox so I could monitor the load. I should just find shunt resistor and ampere meter for that purpose.

I should also find a good yard light. There are probably no electronic HID control gears for 24V DC so I think 18 or 36W FL lamp is the only good option (maybe LED but Im little sceptical for LEDs) I would just need to find a good fixture or maybe build/modify one myself. Ive been thinking something streetlight style.

Better check their resistance.
Fuses you show are mains voltage, very high break capacity ones, so need quite long fusing wire, so they might have quite high resistance as well, causing losses in ELV circuits...

how should I do that? what kind of meter do I need(is normal multimeter enought?)? BTW what is the english name for that kind of fuses? I have seen that kind of fuses to be used in -48 telecommunication powersystems too. It would just be so easy to connect thick wires for that kind of fuseholders. There are different physical sizes for that type fuses. I would guess that smaller size one has smaller resistance? Am I right?

Edit: I found some loss numbers in ABB catalog that said that 25A size 000 fuse would have 2,5W loss in maximum current and size 1 (If remember right the ones I got is this size) has losses of 3,5W. could somebody tell me what this exactly means as this is little unfamiliar thing to me.

The fuse is in fact a resistor, so if they say at full current, so 25A, the losses are 2.5W, it mean the voltage drop is 2.5W/25A = 0.1V, what mean it's resistance is about 0.1V/25A = 4mOhm. The "larger size" would have then ~5.6mOhm resistance.  This then add to the wiring resistance and influence the short circuit current as well (the whole loop resistance count)...

Hooo.... now I understand. Thank you very much both of you for helping me with this problem.