Solar Panel DC Cable Length - How Long?

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Re: Solar Panel DC Cable Length - How Long?

Postby GregoInc » Mon Oct 01, 2018 3:22 pm

australsolarier... have to agree, running the DC to the shed would be my preference. I used the cable calculator on this site and it scared the crap out of me when it gave me the cable size I would require between the solar panels and the equipment in the shed.

Perhaps you can give me some guidance on how the load is calculated, and how to work out the size of the cable? The solar panels will be Trina Honey M Plus 300 watt, and I will be starting with 4 panels. I calculated the amps by dividing the watts by the volts.... 4 x 300 = 1200 watts ÷ 48 volts = 25 amps. Does that sound right?

Then I put the amps into the energy matters calculator and it came back with a crazy cable size :shock:

Maybe I am doing something wrong?
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Re: Solar Panel DC Cable Length - How Long?

Postby Privatteer » Tue Oct 02, 2018 10:06 pm

GregoInc wrote:My setup is currently configured as a 48 volt system, with 24 volt panels.


300w panel = Approximately 11Amps per panel @ at an assumed 27V. Spec sheet will give exact amount.
You then have 2 sets of 2 in series as your running 48V. Thus multiply current by 2.
If you had 6 panels you would be multiplying current by 3.

While your result was close not quite the right method.
Low voltage DC cable losses can get quite substantial over long distance which is why the cable size is getting so large. A battery inverter that allowed a higher DC voltage on solar side would be more suitable.

While some of the Victron gear does support AC coupling with compatible invertors there is some factors to cover as a start.
https://www.victronenergy.com/live/ac_coupling:start
Stage 1:
Fronius Primo 5 + 6.6kW Canadian Solar
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Re: Solar Panel DC Cable Length - How Long?

Postby australsolarier » Wed Oct 03, 2018 6:51 am

keep in mind, that amperage of 25A is probably not there at all. it is the max. the mppt voltage of two panels is much higher than 48v. more like 60v. (65 volts in your case actually) besides you can configure the panels for 3 in series. the max current of your trina panels is 9.2A. so your max current 2 strings probably be around 15A at the most. besides you can configure the panels for 3 in series. your trina panels seem to be Vmax 39.7V. you might just slipp in for ELV. so you could configure 3 in series, which would make most sense for you. you just have to get a solar charger like for 150v. not 80v or so. commonly available.
putting that gridfeed inverter on the wall, you could not do any work yourself. besides you need to have way more than 4 panels to get into the mppt working voltage.
i have a few 25m runs for dc solar, at 6mm2. i don't think there is really a big difference.

like i said, keep in mind, that max amperage is probably only there for like 5%.
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Re: Solar Panel DC Cable Length - How Long?

Postby GregoInc » Wed Oct 03, 2018 8:01 am

Hi australsolarier, great advice, thankyou.

From the advice I've received it's looking like I will go with strings of 3 panels, and most likely will start with a single string of 3 but will install railing mounts and cabling to accomodate probably a top capacity of 3 of 3 strings (9 panels total). I have a Victron SmartSolar 150/45 so it should be fine. Chime in if that sounds crazy :)

I am looking at using 6mm2 from each string down to a combiner point nearby and then will use 10mm2 from there to the MultiGrid in the home DC. That should give me enough capacity to add the 9 panels in total. How does that sound?

I've had suggested I should invest in in-line diodes for each string, do you have a view on that?

My biggest job right now is to design the tile roof mounting hardware layout, so I can at least mount the panels.
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Re: Solar Panel DC Cable Length - How Long?

Postby australsolarier » Wed Oct 03, 2018 8:35 am

i have up to nine strings on one solar charger and they do not have inline diodes. newer solar panels should have bypassing diodes built in. i don't think inline diodes are needed, but they certainly won't hurt the system except lower the voltage a bit (0.2V or so)

as for the combiner box, get a dc breaker for each string, then one breaker for all combined. so, get a box the size you need, in your case 3 strings + 1 total = 4. so you need minimum a 8 pole box, but recommend a larger one as the wiring is rather on the thick side and you can use the spare room for the wire runs inside and for later expansion. the box has to be water proof if it is outside.

don't forget to run a 4mm2 earth wire when you lay the wires. goes from the solar charger to the rails for the panels. there is some more info on the specifications on the internet too.

then do another dc breaker in the shed before the solar charger, so you can isolate the pv when you are fiddling around with your solar gear.

keep in mind, after a while you probably want to add more things, panels, solar chargers etc. that is how i started out. the stuff just grew and grew. then decided to build a large second system. the first 12v system is still there. as a backup and could run the fridge/freezer in an emergency.
10mm2 should be enough, but should be double insulated. use a 35mm2 conduit. cheap to buy in bunnings.
invest in a box of bootlace ferrules and pliers. it makes for neat endings of the wire runs and good contact in the breakers. it doesn't cost much.
i have a 16mm2 run from the combiner/dc breaker box to the solar charger. 8 strings connected. something like 80A max.
make sure you get NON POLARIZED DC breakers. you cannot connect them the wrong way. ( noark, easy to get on ebay, but somewhat costly, there are langir and bluesun non polarized breakers on aliexpress for about half the price. probably the same manufactures.
as for the battery connection:
get a hydraulic clamping device for the lugs for the battery terminals and the inverter. something like 40odd dollars. lots of fun crimping the lugs. you need to order the lugs for wire size AND the bolt size.

another piece of advice, there are thousands of videos on youtube how people do there solar things. american systems are very different to the rest of the world, keep this in mind. also there are a lot of tutorials how NOT to do things. so it might be worthwhile reading the comment sections.
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Re: Solar Panel DC Cable Length - How Long?

Postby GregoInc » Thu Oct 04, 2018 1:48 pm

australsolarier wrote:i have up to nine strings on one solar charger and they do not have inline diodes. newer solar panels should have bypassing diodes built in. i don't think inline diodes are needed, but they certainly won't hurt the system except lower the voltage a bit (0.2V or so)


Thanks australsolarier, that seems to be the general thinking I am getting, and what you say makes sense.

australsolarier wrote:As for the combiner box, get a dc breaker for each string, then one breaker for all combined. so, get a box the size you need, in your case 3 strings + 1 total = 4. so you need minimum a 8 pole box, but recommend a larger one as the wiring is rather on the thick side and you can use the spare room for the wire runs inside and for later expansion. the box has to be water proof if it is outside.


Yep, will find a reasonable sized PVC enclosure and plan to attach to the brick wall of the house. The isolator switches will be on the roof and the combiner enclosure will be down at ground level and will contain circuit breakers etc.

australsolarier wrote:don't forget to run a 4mm2 earth wire when you lay the wires. goes from the solar charger to the rails for the panels. there is some more info on the specifications on the internet too.


Yes, have a roll of solar earth cable, so will probably do overkill :lol:

australsolarier wrote:then do another dc breaker in the shed before the solar charger, so you can isolate the pv when you are fiddling around with your solar gear.


Yep, have a suitable DC circuit breaker, but not sure if my size guesstimate will be right, my amperage calculations suck.

australsolarier wrote:10mm2 should be enough, but should be double insulated. use a 35mm2 conduit. cheap to buy in bunnings.
invest in a box of bootlace ferrules and pliers. it makes for neat endings of the wire runs and good contact in the breakers. it doesn't cost much.
i have a 16mm2 run from the combiner/dc breaker box to the solar charger. 8 strings connected. something like 80A max.


Yes, will grab a decent amount of conduit, with the sweeping radius bends. I may even run 2 x 10mm2 cables, just to be sure and for future capacity. I have a NoArk breaker, but as per above not sure if it's the appropriate size. Re the ferrules, any brand or place to buy you'd recommend?

australsolarier wrote:get a hydraulic clamping device for the lugs for the battery terminals and the inverter. something like 40odd dollars. lots of fun crimping the lugs. you need to order the lugs for wire size AND the bolt size.


Yep, have the hydraulic crimper and lugs for the cable sizes.

australsolarier wrote:another piece of advice, there are thousands of videos on youtube how people do there solar things. american systems are very different to the rest of the world, keep this in mind. also there are a lot of tutorials how NOT to do things. so it might be worthwhile reading the comment sections.


Thanks again for the advice mate, very much appreciate you taking the time to reply. I know it takes your time, so thanks again.
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