cable sizing / panel distance

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Re: cable sizing / panel distance

Postby dmatter261 » Fri Jul 04, 2014 4:16 am

Hi Dennis,

The cable drop calculation needs to be done for a complete loop,i.e. double(connecting both your pos and neg).

Reg your ques -
1. Earthing can be done two/three ways, either all the earthing wires which are connected to the panel frames are connected directly to a DC earthing pit( made specifically ) or connected to the existing AC earthing pit. Are you using a combiner box?
In this case you need to remember and make sure you ground your inverter body as well and connect it to the earth pit again, separately.
Otherwise you can bring all the earth wires to your inverter and finally connect the wires to the earth pit.
In the second method, you just have a single pt of connecn and some seem to ppl prefer this for this reason.
For the earthing wire sizing and other queries I suggest you refer American NEC standards( 250.122 and 690 if I'm not wrong).
I live in India and refer lot to NEC literature avail easily online and also since design standards for solar PV don't really exist in India.

2.Commercial Combiner boxes avail for use in solar PV systems generally have a fuse for each string(some might use a DC Circuit breaker, however this might be more expensive).
In the same box, a surge protection device and DC disconnect switch is provided. Input to this switch is o/p of each of fuses. The switch o/p connects to the inverter. SPD might/might nt b reqd. If your roof has any spikes or sharp objects ,acting as lighting arrestors, SPD might nt be reqd.
( wrt to point 1. You can run your earth wires till this combiner box and body earthing wire of combiner box and finally obtain two final earth wires to be connected to inverter or directly grnded as explained earlier)

Now while selecting the fuses in your combiner boxes make sure that you check the rating and not exceed the max fuse rating mentioned on the panels( shud in the range of 10 or 15 A).

Reg the poles, since yours is a fully offgrid system and inverter might be quite old, I'm assuming your inverter hs a transformer and its a bonded system.( where the negative of your solar panels are connected to earth).
So for this case you need to connect the fuses and DC disconnect switch to the positive o/p of panels only.

Since you have only 4 strings and its a v small system , you can do away with fuses altogether.
For one of the systems I installed, with 3 strings we didn't use fuses for each string and only used a DC disconnect switch.
I guess I'll write about this after your reply in another post.

3. Don't think can help you with this ques ;)

Ok I think I typed a lot! :)
Hope this helps.

Akella
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Re: cable sizing / panel distance

Postby dinkum » Sun Jul 06, 2014 8:22 am

hi kurt and akella,,,sorry i took so long...[interuptions ]
offgridQLD wrote: 6mm2 from the shed wall junction up to each 3s string, 7 stings in total.
are you using 60 cell panels ? . i will run the under ground cables myself [as per installer instructions] , and build tracker , install panels,to save some money,, then get installer to hookup...
offgridQLD wrote: I'm sure it helps you ending up with the best outcome if your somewhat informed.
.i agree,,my first and only installer nearly burnt my solar shed down, due to inadequate fuse/wiring...[not saying it can't happen to me ]..sparky is asking $2.50 m for cable,,is this a good price ?...
dmatter261 wrote: a surge protection device
i can't recall reading about these in any post's,,are they a good idea ?..i think i like the idea of fuses on panel strings at the array,,[needed or not].. ] ...thanks for all info....dennis..
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Re: cable sizing / panel distance

Postby offgridQLD » Sun Jul 06, 2014 8:40 am

I am using 72 cell panels. So each string is 110v 5.45A - 600w max. Seven strings like this.

$2.50m for 6mm2 .I can't really say it would depend on the quality of the cable. It dosnt sound to bad If its ok quality cable . I can't remember what payed. I just purchased two 100m rolls.

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Re: cable sizing / panel distance

Postby dinkum » Mon Jul 07, 2014 7:56 pm

thanks kurt,
offgridQLD wrote:I am using 72 cell panels.
for some reason i had the [ocv] in mind when i asked that ?...would it be possible the [ocv] of 136V could ever could ever exceed the 150V limit of classic 150,[cloud edge effect],,,i'm also curious as to what type of cable you used,as a solar installer came and looked at my system yesterday [sun] and i mentioned the xlpe cable,which he said won't comply as the solar panel cable has to be [tinned copper] now...i can't recall reading any post's on this!!! anyone??? . thanks...dennis.
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Re: cable sizing / panel distance

Postby offgridQLD » Mon Jul 07, 2014 8:22 pm

Perhaps the voltage could go over if it was very very cold in the morning though not not in my location. The classics have a hyperVOC so you can go up to the maximum operating voltage PLUS the nominal battery voltage. So on a 48v bank 198v is your upper limit before your (fry it) beyond 150v though it will go into hyper voc and it will not fry. Though it won't put out any charge power until the voltage drops back to 150v or below.

Yes the solar cable is tinned, double insulated, Uv rated and marked with the appropriate marking on the outer sheath. There are standards it needs to meet. All my 6mm2 cable from the roof PV thats exposed to the elements (actually its in a conduit with very little exposed) all the way down to the combiner on the shed wall meets this spec .

The 50mm2 cable in the trench from the shed wall the power room running in HD conduit isn't tined its basically just like orange double insulated welding cable.

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Re: cable sizing / panel distance

Postby dinkum » Tue Jul 08, 2014 8:47 pm

thanks kurt, i have been searching for info on how much a solar panels [ocv] can rise over it's rating ,in regards to temp and solar irradiation...the hyper VOC , you mention reassures me a lot...thanks...dennis..
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Re: cable sizing / panel distance

Postby Cherokee Solar » Thu Jul 10, 2014 8:29 am

Hi dinkum,

Just spotted this thread. Apologies if I'm repeating what anyone else has said:

Voltage drop over a length of cable can be calculated using the following formula:

Voltage drop = 2 x L x I x 0.017 / A

where:
L = Cable length measured in metres
I = Maximum current measured in Amps
0.017 is the resistance factor in Ohms assuming you use copper cable. Swap 0.18 for steel and 0.028 for aluminium. Just use the 0.017 and don't worry too much about what it all means. Most cables are copper anyway
A = mm2 of the cable you intend to use.

Given that you are using an mppt controller (I think?) it shouldn't matter too much if you lose a little bit of voltage.

However, underground cables (and I have about 100m of them here off grid) run hotter than cables which can lose their heat to the air. Always over specify and ensure that you never run them at their normal maximum rated capacity. Just sayin...

I use Plasmatronics PWM controllers here (yeah, yeah, whatever :lol: ) so voltage drop is an issue over long cable runs, but in reality given the cables are usually oversized here anyway, I haven't really noticed any great losses.

I wouldn't expect 6mm2 cable to run much more than 30A. But given you use much higher voltages than here, it is probably unlikely ever to see such high currents...

Oh yeah, Cables Plus in North Melbourne is the place to go for well priced cables. For DC applications they sell a black/red welding flex cable in all sorts of sizes. It is good stuff.
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Re: cable sizing / panel distance

Postby davidg » Thu Jul 10, 2014 5:51 pm

Cherokee Solar wrote:However, underground cables (and I have about 100m of them here off grid) run hotter than cables which can lose their heat to the air. Always over specify and ensure that you never run them at their normal maximum rated capacity.

AS3000 and AS3008 for underground cables keeping it simple specify for a depth of 500mm to de-rate (subtract) 2% extra to compensate for the reduced cooling for being underground in conduit.
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Re: cable sizing / panel distance

Postby dinkum » Sat Jul 12, 2014 9:15 am

thanks , c/solar and david g ..i have seen the formula ,but was unsure if [ 2xL ] meant ,it had taken twice the distance into account ,or do you still double the distance [complete loop] when doing calculation ?..
davidg wrote: de-rate (subtract) 2% extra to compensate
doe's this mean,to add 2% extra to cable size ?.. just one other ? , " an amp is an amp is an amp " seems to be a lot of differing opinions on this subject.. will a given cable size carry the same amount of DC amps as AC amps ,or does dc run hotter...in other words , is the formula the same for ac and dc.... dennis...
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Re: cable sizing / panel distance

Postby Cherokee Solar » Sat Jul 12, 2014 9:49 am

Hi dinkum,

The 2 x L - I believe - takes into account that there are two wires (positive and negative) so no need to double the cable length.

If I'm incorrect in this thinking can someone please correct me?

Generally I try to oversize long cable runs and have never really found that voltage drop is much of an issue. The longest single run here is 70m (50m of which is under ground). The cable ended up costing more than the panels.... There's a vote for mppt regulators...
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