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I’m planning to install solar panels on the roof of one building (the charge controller and batteries are in this building), to power a DC circuit in another building. No AC equipment needs to be powered. The buildings are around 5m apart.

Is it better to link the buildings together with AC or DC? DC would be easier as no inverter/transformer is needed, but are there any disadvantages of running a DC cable outdoors between the buildings for such a distance?

Edit: I forgot to mention the load. It’ll be max 30A 12VDC.

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    \$\begingroup\$ I suspect that the cost and inefficiency of converting form DC to AC and back to DC exceed the cost of 8 AWG between buildings. \$\endgroup\$ Jun 12 at 23:56
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    \$\begingroup\$ What your acceptance criteria? Cost ? easy of wiring? Efficiency? Reliability? (Other UX?) This is important in your question to add this. Just like software specs….;) \$\endgroup\$ Jun 13 at 0:31
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    \$\begingroup\$ Sometimes simple is more important than efficient. I would probably go for the simple 12V DC cabling. Make sure you put fuses or DC-rated breakers near the battery to guard against over-current. If the load can accept 48V instead of 12V, then it might be better to use 48 V batteries like Janka said. But if you are going to need a DC-DC converter at the load, I would probably just stick with 12V and use fatter cables. \$\endgroup\$
    – mkeith
    Jun 13 at 1:59
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    \$\begingroup\$ I forgot to mention the load. It’ll be maybe up to 30A 12VDC. You should figure out your load definitely, not maybe - don't buy wire or hardware until you know for certain what your load will be. \$\endgroup\$
    – J...
    Jun 14 at 11:02
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    \$\begingroup\$ Do not use copper wire for a circuit this large. Use aluminum. Use normal Code AC mains wiring methods for attaching to it. If you're in North America, Square D "QO" equipment is DC-rated for up to 48V (up to 120VDC if you special-order certain breakers). \$\endgroup\$ Jun 14 at 22:10
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The voltage drop is going to outrule any 12V circuit.

Rearrange the solar panel, batteries and charger as an 48V circuit. That makes a voltage drop of 1-2 volts less critical. It also brings the current down to 7.5A, which is far easier to handle.

On the receiver end, use a 48V/12V DC-DC converter.

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    \$\begingroup\$ It sounds like it is a 360 W load. So DC-DC might be kind of a PITA. But it is always nice to use higher voltages when possible to keep the cable diameter down. \$\endgroup\$
    – mkeith
    Jun 13 at 2:00
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    \$\begingroup\$ Such DC-DC converters cost less than $100. \$\endgroup\$
    – Janka
    Jun 13 at 9:27
  • \$\begingroup\$ I often see this advocated (mine is 48 VDC), but I rarely see anyone actually calculating the resistive loss for the 12 V case and the 48 V case. I know 48 V is better (I'm an EE), but suspect the difference in the case of the OP is minor. \$\endgroup\$
    – donjuedo
    Jun 14 at 16:53
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    \$\begingroup\$ With 8AWG, you are losing about 0.5V at the wire plus 0.1V–0.5V at each connector. That's why I think 30A isn't doing any good. If your connectors aren't tight, the whole thing will melt and burn. \$\endgroup\$
    – Janka
    Jun 14 at 17:27
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Voltage drop and cable gauge are both bad, but what is worse is the safety profile of such a system.

Bad fire safety is an indirect consequence of the long wires and the significant voltage drop. Your buildings are 5m apart, but buildings themselves are not sizeless. I bet on 20m of wires between the batteries and the appliances at the "remote" building. See the other answers for the voltage loss.

Say, you have a short somewhere at the remote part. Near the battery, you have a fuse. It will blow. Problem solved. Or is it?

Fuses blow really quick when a great deal of overcurrent passes through them. Like, say, 20x. They are notoriously bad at disconnecting mild overloads, like 5x their rated current. And with like 2V normal voltage loss from 12V battery, a good short will make ~6x the current (actually, somewhat less for a number of reasons).

So you size your wires not for the load considerations, but for fuse efficiency. Either you use impractically thick cables, that may get more expensive than the batteries, or you do something else.

My advice:

Go 24V.

You still have a great deal of automotive gear to choose from (lamps, inverters, phone/tablet/laptop chargers, fridges, etc...) as a lot of trucks and busses have 24V systems. Most automotive electrical parts either have both 12V and 24V version or are simply rated 10-32V

LED strips are available for 24V as well. You can even get the better variety of "constant current led strip" for 24V that are somewhat resistant to the charge/discharge voltage fluctuations.

Wires at 24V will get 1/4 of the size/mass/price for the same fuse efficiency. Most appliances and solar controllers are somewhat more efficient as well.

Going for higher voltages is also possible, but you will get no off-the-mill cheap parts and 48V is already somewhat dangerous.

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I suggest you to don't convert it to AC at all.

if you use 10mm2 copper wire at 5 meters, you have 4.43% voltage drop at 12v-DC - 30 Amps. As @janka mentioned well, it's better to rearrange your panels array to increase the voltage of system and decrease its current (Amps.) Because then, you needs much thinner wire and lower power loss & voltage drop.

transfer line.enter image description here

If you convert it to AC you need an inverter in 1st building and in second building you need also AC to DC converter, (because your equipments need DC, not AC) and you have to pay more unnecessary costs for an inverter and converter, also these two equipments (DC to AC & AC to DC) have some power loss in their circuits, if they have 2% power loss in each of them, you have 4% power loss in about 5 meters

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  • \$\begingroup\$ We are talking at least 10m, more probably 15m or 20m here (down from the roof on one side, and then some slack and going somewhere within the destination building, and you typically cannot go like the bee flies). \$\endgroup\$ Jun 14 at 4:19
  • \$\begingroup\$ Agreed, but 6AWG or 16mm2 would be my preference. 8awg/10mm2 is just borderline, and it's not much more expensive. \$\endgroup\$
    – J...
    Jun 14 at 10:56
  • \$\begingroup\$ See the part where it says "copper"? There's a choice there. Choose the other one, aluminum. Your wire cost will fall so fast it's ridiculous. \$\endgroup\$ Jun 14 at 22:50
  • \$\begingroup\$ Yes , he/she can use Al. Wire to decrease the cost \$\endgroup\$ Jun 18 at 2:38
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An advantage of AC over DC is that it is much easier for a switch, fuse, or circuit breaker to interrupt the flow of AC current. Some devices have ratings that allow use of either AC or DC, but the maximum safe voltage with DC may be about an order of magnitude smaller than the maximum safe voltage with AC.

That having been said, if you do use DC, ensuring that neither wire has a positive potential relative to ground will greatly avoid corrosion in the wires. If one has a DC voltage between two wires that are floating relative to ground and any current can leak through the insulation, the current flow from the positive wire to the negative wire will end up causing metal to move from the positive wire to the negative wire. If, however, both wires are biased to a negative potential relative to a grounding stake, then the net effect will be that metal will be carried from the grounding stake onto the wires. If the grounding stake starts out much thicker than the wires, having the grounding stake shrink by a millimeter of diameter would likely be far less of a problem than having a wire shrink by such an amount.

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  • \$\begingroup\$ That's really interesting and good stuff about biasing the DC system negative relative to earth, to arrest galvanic corrosion. Betcha trolley systems wish they had known that in 1880! \$\endgroup\$ Jun 14 at 23:37
  • \$\begingroup\$ @Harper-ReinstateMonica: I don't know if people's understanding of the appropriate principles goes back quite that far, but it was probably relevant in telegraphy, and certainly in "plain old telephone system" (POTS) circuits which are all biased negatively with respect to ground. \$\endgroup\$
    – supercat
    Jun 15 at 5:59
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Frame challenge: use more batteries and a split charger

When you've got a high load like that, you want your battery as close as possible to the load for all the reasons you already know. I'm assuming there's a reason you want batteries in the first building, to supply equipment in there, so we've got two buildings you want to keep supplied from the same solar panel.

RVs and boats always have two separate batteries, one for the engine and one to drive domestic loads (lights, etc.) in the living area. (Or at least two; some have more, of course.) Solar panels are common on RVs these days, and many use a "split charger" which can charge both batteries independently from the one solar panel, pushing more or less charge to each battery as needed. This is a very standard piece of equipment you can buy from any good RV supplier. Solar panel kits generally come with the appropriate battery charging electronics, and some kits have split charging built in. If you tell the supplier that you want to do split charging, they can advise you on the best kit to buy for that.

You can then use a much thinner cable to the second building, because you only need to push as much current down that wire as the charger can supply. The charger should have four wires to the battery, two for charging and two for "voltage sense" so that it can automatically compensate for voltage drops in the wire.

Et voila, the best of both worlds. And for a bonus, if you run down the battery in one building then you can go next door and carry on working.

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30A at 12V over about 5m?

Find some long jump cables and the voltage drop will be about 1V, maybe less.

That's how I power a 35A trolling motor in my boat, and it works well. The cable was cheap, about 25mm^2, and appears to be Al plated to look like Cu!

If you're using solar cells, consider they can supply higher voltages; even 20V will help, stepped down to "12V" (14V to charge a lead acid battery) by a charge controller. So route the solar cells via the jump cable to a charge controller near the load.

Anything else adds sources of complexity and inefficiency. But once cable runs exceed about 10m I'd consider other alternatives.

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    \$\begingroup\$ Yes - copper-clad aluminium cabling is a thing. Aluminium is much lighter and cheaper than copper, but has worse electrical properties and mechanically it can sag faster than copper. The Copper plated on the outside helps produce a hybrid wire that is cheaper than copper but better than aluminium. \$\endgroup\$
    – Criggie
    Jun 14 at 0:38
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    \$\begingroup\$ (..and no shortage of moaning from audio and computer networking enthusiasts) - re CCA \$\endgroup\$
    – Caius Jard
    Jun 14 at 11:13
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Stick to your original plan. Use 12V.

We're going to make two "huge leaps": first, into the AC mains wiring parts bin, and second, into aluminum wire because there is nothing wrong with aluminum in heavy feeder.

I don't agree with the other advice. They are making compromises because they are worried about voltage drop with such a low voltage over that distance if you choose wires conventionally. They think voltage conversion is worthwhile because it'll be cheaper than copper. I'd agree, so we use aluminum instead.

Use enormous wire that is aluminum. It's cheap!

Around here, 10AWG copper wire (30A) is about the same price as 2 AWG aluminum wire (90A for feeder, 100A for service). And since it's used for very common 100A service wiring, it's a commodity and occupies a price "sweet spot".

So do the same thing in your country. Go hit up the home store/house building DIY supplier or electrical supply (i.e. the guy with crates of consumer units and conduit sticks, not the guy with transistors). Ask them what the common/cheap commodity wire is used for 100A house service entrance wiring (i.e. from where the overhead line comes in, to consumer unit/service panel).

Here in the USA that fat aluminum wire is under $1 per metre per conductor. That's not so bad, what were we ever afraid of? :) You need 2 or 3 Depending on what your local authority says about safety earthing in low voltage DC circuits.

Attaching to fat aluminum

Now certainly, aluminum is a little tricky to attach to at high currents. The issue is the differing thermal expansion rate as compared to copper. However, and this has been tested exhaustively by a great national experiment in the US, aluminum safety is a settled matter. Turns out aluminum lugs are the universal donor, playing well with both Al and Cu wires. Copper lugs are the bigot, and the real root of the problem. Et voilà, equipment often used with aluminum wire has zinc plated aluminum lugs. Easily solved, once you know the score. Still, everything should be equipment listed or certified for aluminum wire.

And these are plentiful and cheap in the AC mains wiring parts bin. What we in North America call "Polaris" connectors (aluminum-shell lug connector) are the normal choice for "merely a splice". But remember, you have two other stones to kill: first you need a circuit breaker somewhere in here, and second you need proper enclosures to make these splices. So you can kill several birds with one stone by selecting certain combo products.

For instance, in the US, my go-to is "micro subpanels" made for a 2-pole circuit breaker. Because they provide fat lugs for aluminum wire, and an approved enclosure, and space for a circuit breaker! And they're cheap. And since this is DC, in the North American market I will select Schneider/Square D “QO" models, as their circuit breakers are specifically cross-listed for DC power. And easily found at the local home store. Even if you don't need the breaker, it's worth it for the approved enclosure and lugs. In North America every breaker is rated for aluminum.

Outside North America, you're into DIN rail service panels, and you can use any commodity DIN rail breaker which is DC rated. The lugs on any larger breaker ought to be aluminum rated.

Safety and reliability of AC mains rated equipment

Is beyond reproach, obviously. If you select equipment approved in your local area for AC mains work, and install it according to instructions including screw torques, you can rest assured that the work is solid. You won't have heating and mystery voltage drops across those connections. It won't burn your house down in the middle of the night.

The breakers will work (if they are DC rated), so either select Square D QO, or in the DIN rail world, just any approved DIN rail breaker rated for DC. Beware the ones that care about polarity and are destroyed by reverse polarity, no reason for that to be an issue.

If you were pulling permits for this work and being inspected, everything I advise above would pass inspection. The main issue I can see is that the equipment will need labeling to say it is part of a DC system not connected to the AC mains.

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DC will be fine. Just use some thick wire for direct connection to your equipment.

10 gauge wire (2.6mm diameter) has a resistance of 3.28 ohms per 1,000 M Your 5 M run would have a resistance of 0.0164 ohms. A current of 30 Amps would result in a voltage drop of 0.492 volts.

3 gauge wire (5mm diameter) has a resistance of 0.6168 ohms per 1,000 M Your 5 M run would have a resistance of 0.003084 ohms.

A current of 30 Amps with 5 M of 3 gauge wire would result in a voltage drop of only 0.09252 volts. This drop is negligible

You can go with even thicker wire if you want, but 3 gauge will do the job with no expensive extra equipment needed

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