Off Grid System Design Review w/ questions

DKMA · May 2013

OK guys, need a sanity check. I am putting together a small off grid system and have asked and received good advice in other thread. Wanted to post a picture of what I have, what I think I need, and what I plan to add later. This is the design i am working to. Everything in grey, I have, everything in yellow, I need to purchase. Does this look correct? Any suggestions on what breakers/boxes to purchase? I looked at the MS boxes on NAW&S but am not sure what I need or What size breakers I need. Plan to add 3 additional panels later in series to the 3 I have now. (Provided I can find 3 that are a good match) As always thanks for your help.

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Cariboocoot · May 2013

Re: Off Grid System Design Review w/ questions

I'm going to guess the wiring isn't exactly as planned.

One thing I notice right away: if you are going to add more panels in the future they need to go directly to the combiner box, not "piggy backed" on to other panels. Again I hope this is an error in depicting the design rather than the design itself. As such the combiner box should be one that can handle six panels/strings. The fuses/breakers should be no greater than the maximum series fuse rating of the panels.

The array disconnect needs to be able to handle the combined current of all panels, as does the wiring from the combiner to charge controller. The distance will have a bearing on that size too, as you don't want too much V-drop creeping in.

The disconnect between the batteries and charge controller needs to be large enough to handle the maximum expected current (40 Amps probably given the panels although it would not hurt to go for the controller's max out). Same with the wiring.

Same with the connection to the inverter. On 24 Volts a 2kW inverter can pull 84 Amps, so this circuit should be designed around 100 Amps minimum. The manual for the inverter should provide specific information on this. 6 AWG is not going to be heavy enough here.

Photowhit · May 2013

Re: Off Grid System Design Review w/ questions

Those panels are going to be very hard to matchup, VMP of 41.3 is much higher than 'normal' panels lately. If you've got these and more are available, you might stretch your budget now. I think I understand what your saying about adding 3 more panels in series, then your fuse/breaker doesn't change in your combiner box as the amperage will not change, that said charge controllers work best/most efficient at about 2x the output.

Your fusing/breakers can be based on your wire diameter. Most of your equipment will give you some guide lines on wire sizes and fusing.

DKMA · May 2013

Re: Off Grid System Design Review w/ questions

Thanks for the reply’s guys. OK I will upgrade the wiring to the controllers max out. Same for the inverter. Appreciate the tip as I do not want to change the wiring later when I add more PV.

I know what you mean about matching the VMP. I bought the three panels I have on-line for .90 a Watt without knowing ANYTHING! One of those shoot.. fire…aim moments. Like other’s I have seen on this forum, I put the horse first and then tried to tie a cart to him.
After finding this forum and reading a lot, and asking a few other questions, I am trying to STOP making costly mistakes. That is why I made my little drawing and asked for input.
I have casually started looking around on line for a panel that would “match” the three I have, and have not found any yet. My thoughts are that if I can find a close VMP or Imp match then I will purchase three more and either wire them in series or parallel depending on what I can find. That was one of my motivating factors in selecting the Midnight classic 150, because I think it can handle the addition either way.
Not the best way to approach things I know, but that is where I am at now. So back to my original question.. Can you guys recommend a box/breaker combination that will provide the disconnects in my drawing? I looked at the Midnight boxes on our hosts site and am not sure what I need. Thinking I need two different boxes. Is this correct?

Thanks,
DKMA

Cariboocoot · May 2013

Re: Off Grid System Design Review w/ questions

You would need a combiner box for the panels. A separate disconnect for the whole array is optional (unless required by jurisdiction).
http://www.solar-electric.com/pvarco.html
Exactly which you pick will depend on the final design and desired location of the combiner and/or disconnect (some are weather tight, some are not).

If you wanted to add some lower Voltage panels in series with the ones you've already got you could increase the array size but maintain three strings. This will work on an MPPT controller, providing the Imp rating of both panels is close.

Photowhit · May 2013

Re: Off Grid System Design Review w/ questions

Cariboocoot wrote: »

...If you wanted to add some lower Voltage panels in series with the ones you've already got you could increase the array size but maintain three strings. This will work on an MPPT controller, providing the Imp rating of both panels is close.

To be clear, 'Coot is suggesting adding 1 panel each (or more but the same/each) to each high voltage panel in series. So you would need 3 panels for the addition, near the 5.9 IMP of the current panels.

Off Grid System Design Review w/ questions

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