panel wiring question

[goldhunter_2]

I used SMA design software , the power losses even with 14awg is only like 0.58% with 6awg its only 0.02% But the strings required more panels then I expected

Positive ground grid tie inverter, if I understand correctly the positive is both grounded and carries the current, the negative wire is basically just a neutral wire is that correct?

If I run three strings of 12 280 watt 44.4v 7.63 amp panels with individual string 6awg positive wires can I run one one single common negative for all three strings back to the fuse combiner box , and a separate common earth ground for all the panels frames or I that going to mess up my voltage/amps

[inetdog]

Positive ground grid tie inverter, if I understand correctly the positive is both grounded and carries the current, the negative wire is basically just a neutral wire is that correct?

Regardless of which wire is grounded, or if neither is grounded, the + and - wires carry exactly the same current (unless there is a fault.)
And if both wires are grounded, lots of current may flow but you will get zero power.

In terms of voltage drop, you have to consider the drop in both wires.

[goldhunter_2]

I did notice the SMA software did actual figure two wires for those power loses mentioned, I probably only need 25'-30' feet but figured 50' in case there was something I had t o route the wire around and the SMA software figured 100' (50+50) so it included two equal length wires


"Regardless of which wire is grounded, or if neither is grounded, the + and - wires carry exactly the same current (unless there is a fault.) "

ok so no bus bar combining the three negative strings to single wire sounds like I need to run separate neg/positive for each string back to the fuse combiner box

Thanks for the input

[inetdog]

I
ok so no bus bar combining the three negative strings to single wire sounds like I need to run separate neg/positive for each string back to the fuse combiner box

Thanks for the input

No problem combining the negatives into a single wire, but it will have to be larger to handle 3 times the current.

[goldhunter_2]

I will just run two wires if I gotta run another line anyway ,was trying to be lazy/cheap


Another question for you

WHY do people say you can only use certain solar panel brands with a positive ground system ? , I asked the panel manufacturer (perlight) ad they said that has to do with the inverter not there panels .............. is there any reason I can not use a regular perlight PLM-P280-72 panel on a negative ground inverter ?

[inetdog]

WHY do people say you can only use certain solar panel brands with a positive ground system ?

A very good question. My Scientific Wild Ass Guess (SWAG) is that it has to do with charge movement inside the material of the actual cell.

The specific models that I have seen described this way have all been from Grape Solar. The factor that apparently makes a difference is whether the cell is positive or negative with respect to the grounded backing of the panel, since the cell itself has no reference to ground to affect its behavior.

The symptom when one of those panels is connected in a negative ground situation is that it will be fine initially, but that over a few hours or days the output will start to go down, eventually dropping well below half of what you started with. If you then reconnect it with positive ground it will recover, without any apparent damage.

So it looks like in the course of the photons from the incoming light knocking electrons loose inside the active material of the cell, some of them are ending up going to the wrong place. If the backing plate behind the cell is positive relative to the cell, the "loose" electrons will be repelled from that direction and will instead eventually pile up somewhere and create an overall negative charge in way that prevents the "normal" photoelectrons from crossing the gap to create an output voltage.
If the backing plate is instead positively charged compared to the cell voltage (which is what will happen with positive ground) the loose electrons will be attracted toward it and will not end up accumulating somewhere else in a way that will decrease output.

Reversing the polarity to the preferred direction or just letting enough time go by will allow this region of charge to bleed off and the cells will work again.

So the root cause is something odd about the way the manufacturer produces the cells themselves, combined with a grounded metallic back plate behind them but insulated from them.

There you have it. Any other guesses (or even actual knowledge) is welcome.

PS: It may be easier to talk about the positively charged "holes" moving around and being attracted to the negative back plate, etc. and that type of language is common when discussing semiconductors. But I think it would be confusing rather than useful in this context.

[goldhunter_2]

SWAG's are always good

[Naptown]

I did notice the SMA software did actual figure two wires for those power loses mentioned, I probably only need 25'-30' feet but figured 50' in case there was something I had t o route the wire around and the SMA software figured 100' (50+50) so it included two equal length wires


"Regardless of which wire is grounded, or if neither is grounded, the + and - wires carry exactly the same current (unless there is a fault.) "

ok so no bus bar combining the three negative strings to single wire sounds like I need to run separate neg/positive for each string back to the fuse combiner box

Thanks for the input

Generally the combiner sits at the array or near the arrray. And does exactly what it is called it will combine 3 strings into one circuit or pair of wires.
In your case you would run the three strings to the combiner connect all - wires to a common bus and the + wires to fuse holders with fuses which then will connect to a combiner comb which is essentialy a common bus for the three strings then the 2 larger wires are run back to the inverter.
some inverters contain a combiner within so you could just run the 3 strings to there and combine within the inverter

[goldhunter_2]

`I was looking at the midnight solar 3 or 6 breaker combiner so I could use it as both a combiner/fuse/dc disconnect all in one type of thing . Actual where I plan to put a mast head through the roof is about 5-10' off set form where the main panel and inverter are below thus the short wire spans. I had planed to mount it on the wall next to the main and the inverter. The inverter will combine two line but I may have made a mistake buying it since it can not be changed to negative ground , I can't used the negative ground panels I had gotten a quote for and finding positive ground panels is not working very well. I talked with sunpower (unfriendly people IMHO) they said talk to the dealer I talked to the semi-local dealer was very friendly but said no sunpower would not let him sell me just panels, but he did say he would do the finial inspection when I get ready so I have that documentation for the power company requirements.

I guess I got to excited and in a rush to start my solar system now I am going to have to slow down and see what I can find in positive ground panels and how they affect the budget or I mite have to consider selling this SMA 6000U and buy a different inverter so that I can use negative ground . Another newbie mistake is I didn't realize how big a SMA 6000 watt inverter physical was , there is just no way to fit two of this where I plan to mount them I may have been better space wise to just buy a 10-11kw inverter and be done with it.

[Naptown]

What model SMA did you buy
Some if not all can be changed from + to - ground and visa versa

[goldhunter_2]

it's a sma 6000U , I already talked to SMA this one can not be changed