Solar Panel DC Cable Length - How Long?

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Solar Panel DC Cable Length - How Long?

Postby GregoInc » Sun Sep 30, 2018 7:25 pm

Hi Folks,

Some background on my question... I have a large bank of EV batteries that I am using to power an off grid power supply, via a Victron MultiGrid. The EV batteries and Victron equipment are stored in a small building separate from the main house. The roof of the small building is limited, so I am considering installing solar panels on the main house east facing tile roof.

I am thinking about running a DC cable from the solar panel isolator switch on the main house roof to the small building, but I am not sure how long a DC cable can be run before it becomes a problem? Or if it is even allowed to run DC cables within a domestic house? If I were to guess I would say the cable distance between the solar panels on the main house and the small building would be around 40 meters.

So, do I run a 40 meter DC cable, or do I do a short DC cable run to an inverter (mounted on the house wall), with the inverter converting the DC to 240 AC and then run an AC cable to the small building and connect it to the Victron MultiGrid to charge the batteries?

Nothing has been installed on the main house roof, which is why I am asking the question. Appreciate any guidance.
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Re: Solar Panel DC Cable Length - How Long?

Postby australsolarier » Mon Oct 01, 2018 8:22 am

there are tables on the internet (less than 5% loss) that tell you how thick a cable has to be at a certain length at certain currents. like with AC wiring there are rules for laying DC cables in a house, for example have to be in conduit, and labelled.
the longer the DC run the higher you want the DC voltage, limited by your mppt solar charger's specification. those parameters should be in the specs of your solar charger.
elv, extra low voltage, DC under 120v, generally does not need a licensed electrician to install, so you will have to consider this aspect as well if you want to do the work yourself.
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Re: Solar Panel DC Cable Length - How Long?

Postby GregoInc » Mon Oct 01, 2018 10:03 am

Thanks for the feedback. I actually wouldn't mind a contractor doing the install for me, as I have zero experience with installing tile roof solar panel mounting systems or solar panel cabling. And am also not sure if I would need a permit from the council?

My setup is currently configured as a 48 volt system, with 24 volt panels. So I figured if I were to run the DC cable it would be carrying 48 volts. But as I wrote originally, nothing is installed on the main house so very much open to options.

I also figured I could run multiple DC cables, to try and minimise the loss. I have a 100 meter roll of certified solar cable, so figured two runs would be doable.

But there is also sense in my eyes with mounting an inverter on the wall of the main house, and then running a single AC cable the 40 meters.
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Re: Solar Panel DC Cable Length - How Long?

Postby australsolarier » Mon Oct 01, 2018 11:50 am

are you suggesting hanging the victron inverter/charger up on the wall?
and hanging your ev batteries on the wall underneath?
i take it the victron also charges the batteries? you still need a dc cable for the batteries and this time a really thick and expensive one.
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Re: Solar Panel DC Cable Length - How Long?

Postby GregoInc » Mon Oct 01, 2018 12:39 pm

Apology for any confusion. Right now I have a Victron SmartSolar that converts from the 48 volt solar panel feed in and outputs 48 volts to the charge the Lion batteries. The Victron MultiGrid takes the 48 volt from the batteries and produces 240 volt AC.

The Victron MultiGrid has the option to take 240 volt AC input and charge the batteries, so for the panels on the main house I purchase a DC to AC inverter (mounted on the house wall), run an AC cable to the small building and feed the AC into the MultiGrid, which in turn would charge the batteries.

Hope this helps.
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Re: Solar Panel DC Cable Length - How Long?

Postby australsolarier » Mon Oct 01, 2018 12:48 pm

i think you are planning something that is impossible unless i am misunderstanding you.
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Re: Solar Panel DC Cable Length - How Long?

Postby GregoInc » Mon Oct 01, 2018 1:55 pm

Any standard solar setup includes solar panels connected to an inverter... yes. So if I were to install a 'standard' setup on my house roof, I would end up with a set of solar panels feeding into an inverter (mounted on the house wall in close proximity to the solar panels), and the AC output from that inverter would normally go into the house electrical system, with excess feeding into the grid. All good so far.

So from the above example... Instead of feeding the AC from the inverter into the house electrical system, what if I were to run an electrical cable from the inverter directly to the MultiGrid (which can support multiple AC sources). The MultiGrid would then take that AC power generated from the roof solar and use it to charge the batteries connected to the MultiGrid.

This Victron diagram may help :D
https://www.victronenergy.com/blog/wp-content/uploads/sites/10/2016/11/hub4-assistant-without-meter_22569079.png

I have the PV on DC already setup, and I am looking at installing the PV on AC output part. But I wont be grid connecting it as in the diagram... it will be offgrid.
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Re: Solar Panel DC Cable Length - How Long?

Postby Gordon-Loomberah » Mon Oct 01, 2018 2:56 pm

What happens when the batteries are fully charged and you dont have much load in the house? Most grid-tie inverters generally only operate at max power, so you will need some sort of useful or dummy load to use up excess power.
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Re: Solar Panel DC Cable Length - How Long?

Postby australsolarier » Mon Oct 01, 2018 3:10 pm

gregorinc might have a victron inverter that is shifting the frequency and therefore able to turn the grid feed inverter off automatically. (but he is going to need a licensed electrician to do that kind of work and i do any bet he will have an extremely hard time to find a licensed solar bloke that knows what frequency shift is and does.
we really do not know enough of what he wants or what he already has.
personally i would draw the solar dc to the shed, have solar chargers. gregorinc can then legally do all the work right up to the victron inverter and only needs the electrician to hook up the victron to the house and the grid.
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Re: Solar Panel DC Cable Length - How Long?

Postby GregoInc » Mon Oct 01, 2018 3:13 pm

The load on the batteries is currently constant because I have various servers etc running, so going low on load probably wont be an issue. In fact right now it's the opposite, not enough battery/solar capacity to run what I want to run. Hopefully with more solar and battery capacity I can run more :)
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