clarification on the mini dc breaker box

elesaver · August 2014

I'm mulling about a wiring plan for my solar set up which will be ground mount. I will have 6 ea 300W panels in 2 strings of 3 panels ea. My system will be 24V using 2 strings in series of 4 ea Trojan 125 batteries. I've read some of the threads on the forum and an uncertain if it is still necessary to have a PV disconnect before the PV wires enter the building? I want a disconnect for the PV and a disconnect for the battery and a disconnect for the controller. So, if there is no requirement for a disconnect for the PV outside the building, it seems the midnite solar mini dc breaker box would be a good purchase. It would have a breaker for the inverter and for the controller but it is not clear if the battery strings can be wired into a breaker here, too? If not, what would be a good battery disconnect? The dc mini breaker box would also have a mounting spot for the whizbang jr shunt. Any help and/or explanations are really appreciated. This box will also have a location to mount a midnite solar SPD. Thanks again for the time spent in helping.

westbranch · August 2014

Re: clarification on the mini dc breaker box

with 3 strings you want to have each set of panels on a breaker to prevent back-feeding in the event of a failed diode or ? where 2 panels send their A output through the third panel string, there is a fire risk here. The CBs will act as s disconnect.

zoneblue · August 2014

Re: clarification on the mini dc breaker box

elesaver wrote: »

I'm mulling about a wiring plan for my solar set up which will be ground mount. I will have 6 ea 300W panels in 2 strings of 3 panels ea. My system will be 24V using 2 strings in series of 4 ea Trojan 125 batteries. I've read some of the threads on the forum and an uncertain if it is still necessary to have a PV disconnect before the PV wires enter the building?

If you use two strings of PV you do not strictly need a combiner. HOWEVER, coming from someone who took this shortcut, i recomend that you do. From NAWs a 3 breaker combiner is about 70 bucks, plus 30 for the breakers. That gives you the ability to swtich off one string at a time for testing purposes, it gives you the ability to completely isolate dangerous pv voltages from your main disco.

If you go with 3 72 cell panels in series thats over 120Voc. Which as you know is dangerous. So proper interconnections and industry std combiners and disconnects is the way to go.

I want a disconnect for the PV and a disconnect for the battery and a disconnect for the controller. So, if there is no requirement for a disconnect for the PV outside the building,

It depends where you are and how concerned about code you are. Increasingly firefighters are pushing for rules in most areas that require a external disconnect. And that becomes reasonably involved to do properly, weatherproof, also shuts off battery etc. ON balance what most people do currently until those laws hit, is put the combiner at the array, and have a second pv breaker in the disco.

it seems the midnite solar mini dc breaker box would be a good purchase. It would have a breaker for the inverter and for the controller but it is not clear if the battery strings can be wired into a breaker here, too? If not, what would be a good battery disconnect? The dc mini breaker box would also have a mounting spot for the whizbang jr shunt. Any help and/or explanations are really appreciated. This box will also have a location to mount a midnite solar SPD. Thanks again for the time spent in helping.

We are talking MNDC here, not Baby / Big baby right? If so then yes, they are great general purpose discos. You cant go wrong with them, and the price is reasonable. Unless you have or plan to get a name brand inverter, in which case you will most likely be better off looking at the midnite epanel range for that inverter. They make a (slightly dizzying) range of disco/cabinets. DOwnlaod the manual for the MNDC and absorb it. It will help your install achieve best practice.

vtmaps · August 2014

Re: clarification on the mini dc breaker box

westbranch wrote: »

with 3 strings you want to have each set of panels on a breaker

That is true, but the OP wants to have 2 strings (3 panels in series per string).

What you are suggesting (3 strings, 2 panels per string) would be a more optimum configuration for the controller, but might entail heavier cable between combiner and controller. The OP hasn't told us how far the array is from the controller.

The OP also hasn't told us where the installation will be, or what controller he will use. Therefore, I don't know whether the Voc, corrected for temperature, will exceed his controller's limits. If (as zoneblue suggests) these panels are 72 cell panels, the cold weather Voc may be too high for the controller if the OP makes 2 strings of 3 panels per string.

--vtMaps

elesaver · August 2014

Re: clarification on the mini dc breaker box

Thanks for the input on my question but now I have even more questions. I thought I had given complete data but, once again, I was wrong. :-) The controller is a midnite solar classic 200. The array will be approximately 50'-60' from the controller. So, perhaps a better question would've been...what is the best configuration of 300W PV panels (36.73V nominal voltage; 8.18A nominal)? The inverter at this time is a 1500W Cotek with a plan to change to a split phase inverter in the future, possible changing to a 48V system. This is a totally off-grid system. Hopefully, I've included enough info this time. I really do appreciate the help.

BB. · August 2014

Re: clarification on the mini dc breaker box

A classic 200 can take higher input voltage (~130 volts or so) Vmp-array.

And running a MPPT charge controller higher than ~2x battery charging voltage causes the charge controllers to be slightly less efficient (more waste heat from controller).

So--You will have to look at your array configuration.

Higher Vmp-array means lower current from array (smaller gauge wiring for wire run from array to charge controller+battery bank). Great for longer wiring runs.

However, this means that you might "care" how you configure the array for 24 volt vs 48 volt battery bank... You need Vmp-array minimum of ~35 volts for a 24 volt battery bank and 70 volts minimum for a 48 volt battery bank. And wiring for an over 70 volt Vmp-array for your 24 volt battery bank--While perfectly OK--May be a percentage point or two more losses in the MPPT charge controller.

You will want to make sure that you have good air circulation around the Charge Controller so that it does not get too hot (especially in summer). 1% more loss does not sound like much--But for a 96% efficient charge controller, 1% more is 25% more waste heat at the controller--Getting good air circulation around the controller is important (don't install in a small room/closet, don't put controller near the ceiling, under a shelf, etc.).

If you will be increasing the array size when you eventually go to a 48 volt battery bank... You will need to make sure your present wiring will support the larger array--That the array is easy to reconfigure to different Vmp-array (if you do that).

And, mixing/matching solar panels can be a pain. You may have problems finding Vmp/Imp compatible panels 4+ years down the road.

Sometimes, it ended up being necessary to have the "old array" on one charge controller, and the "new array" on a second MPPT charge controller because the old panels were unavailable or just too expensive to purchase vs the newer/cheaper/larger panels "available today".

And you need two sets of Vpanel cables from the array (pull a second set through the existing conduit if you have room or buried a second empty pope... Or dig a new trench for the new array, etc.).

-Bill

vtmaps · August 2014

Re: clarification on the mini dc breaker box

vtmaps wrote: »

What you are suggesting (3 strings, 2 panels per string) would be a more optimum configuration for the controller, but might entail heavier cable between combiner and controller. The OP hasn't told us how far the array is from the controller.

elesaver wrote: »

The controller is a midnite solar classic 200. The array will be approximately 50'-60' from the controller. So, perhaps a better question would've been...what is the best configuration of 300W PV panels (36.73V nominal voltage; 8.18A nominal)?

String length of two panels will be your optimum for a 24 volt system. With #8 cable you will have a 2.57% voltage drop over the 120 ft round trip distance. With #6 cable the voltage drop will be 1.62%.

At 70 volts input, the classic 200 is rated to handle 2238 watts on a 24 volt system. The classic 150 could handle 2700 watts in the same circumstance. The classic 150 would have been a better choice for your current system... more conservative design.

If you upgrade to 48 volts, you will want to put three panels in series. Then, depending on your climate (I still don't know where you are located) you might drive the classic 150 into hyperVOC, but no harm will be done and no power lost.

--vtMaps

elesaver · August 2014

Re: clarification on the mini dc breaker box

OK, BB, you're getting my attention on this. What comes in the future is what comes, I guess, so let me slip back to the now and now and get some things figured out. I want to configure these panels in the most efficient way possible. (I may not ever be able to make too many changes in the future, so set up for the now and now seems logical to me. Far be it from me to be logical!) It had been suggested to connect 3 of these panels in series, then connect the two strings in parallel to run to a disconnect and then to controller. With the suggestions in this thread, this may not be the best way to do it if I am understanding what has been said. I am now searching for info on connecting the panels. Any help with explanations on that? I would really appreciate it.

BB. · August 2014

Re: clarification on the mini dc breaker box

As vtMaps said... The most efficient for the charge controller is probably around 60 volt Vmp (two panels in series) for a 24 volt battery bank.

Have you looked at the Midnite sizing tool yet?

MidNite Solar Classic Sizing Tool

If you are going to trench--I like getting bigger diameter conduit/pipe so you have lots of room for more cable... and I like to bury at least once extra conduit so you add cable/data/AC power/etc. later if needed.

-Bill

elesaver · August 2014

Re: clarification on the mini dc breaker box

Thanks, Bill. Yes, I had looked at the sizing tool and it indicates that either 3 panels series with 2 strings or 2 panels series 3 strings will work with MNCC 200. Thanks for all the help and suggestions.

inetdog · August 2014

Re: clarification on the mini dc breaker box

elesaver wrote: »

Thanks, Bill. Yes, I had looked at the sizing tool and it indicates that either 3 panels series with 2 strings or 2 panels series 3 strings will work with MNCC 200. Thanks for all the help and suggestions.

The difference in wiring complexity between two and three strings is pretty major, in that with two strings you are not required to fuse each string. (Although most would want to put a breaker of fuse in just to make it easier to isolate strings for testing.)

vtmaps · August 2014

Re: clarification on the mini dc breaker box

elesaver wrote: »

Thanks, Bill. Yes, I had looked at the sizing tool and it indicates that either 3 panels series with 2 strings or 2 panels series 3 strings will work with MNCC 200. Thanks for all the help and suggestions.

Both configurations will work. One configuration (two panels in series) is optimal. --vtMaps

zoneblue · August 2014

Re: clarification on the mini dc breaker box

Vt i assumed that all 300W panels are 72 cell. (At least for mono) i thought it was something to do with the 150mm wafer size barrier.
36Vmp, 45voc ballpark.

OP, my setup is exactly the same 6 x300W, 50ft, and i went with 16mm2 cable, 3 strings of two. Classic 150. You can see the logic i used for this layout in my system notes.

Also be wary of the sizer tool, because it only tells you about hard limits. Theres nothing about efficiency, and also it gives the impression that oversizeing (watts not volts) your array is not allowed "Excessive" when it is fine in most circumstances to slightly overpanel.

In hindsight i would have loaded the 150 up with two more panels. And i'll be retrofitting the combiner soon anyway.

clarification on the mini dc breaker box

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