Bus bar instead of combiner box

[severin20]

Hey Solar Fam,

I'm trying to figure out the cheapest way to connect the most panels to the cheapest charge controllers without it being a safety nightmare.

If I get an 80amp 24v charge controller and use panels ISC 9amps VOC 40v, can I...

Put the panels in strings of 4 parallel using y branch connectors. Then using the relatively cheap 8awg wire run the wires to a positive and negative bus bar. Put a 40amp fuse on the positive wire before the bus bar.

Then do the same thing for a second string of 4 panels.

Now I've got a bus bar with 80amps and I could run a very short thick wire from the bus bars to my charge controller.

Is there a reason why I shouldn't do this for safety?

I'm trying to grasp all my options for hooking up a large number of panels to these cheap pwm charge controllers I see on aliexpress.

I'd love to hear other cheap diy ideas also.

Thanks

 

[mrdavvv]

You can do your own combiner box for cheap with din terminal blocks and cross conectors between them, start small and increase connectors according to your needs, or buy the specific DIN combiner blocks. You should also fuse the negative wire i believe. With this setup you can also add fuses with breakers, and also some SPD's in the future or other devices. If you want to go for cheap, there is a lot of chinese units that are decent, like Suntree / Tomzn:

 

[perpetual]

I bought these din rail terminal blocks for my combiner box

https://www.aliexpress.com/item/32961099960.html

I also bought the fixed bridge (allows you to connect wires to both sides of terminal block if needed) they connect in the middle of the terminal blocks as an alternative to plug in bridges

https://www.aliexpress.com/item/32974793584.html

 

[Hedges]

I'm trying to grasp all my options for hooking up a large number of panels to these cheap pwm charge controllers I see on aliexpress.

Lots of panels and watts? Get a higher voltage MPPT charge controller.

If you do have many panels (or strings) in parallel, they should be fused to protect against excessive backfeed current if there is a fault.

 

[Joe BoyKey]

Build your own - Pelican 1200 box with din rail breakers, surge arrester and bus bar - kinda, Joe.

 

[Tecnodave]

Ok so here is what is the bad........

you can connect 2 solar panels in parallel or even 2 series strings in parallel, but when you connect more than 3 panels in parallel you need fuses or breakers on each string......WHY..you ask...if one panel in a parallel setup with 3 in parallel shorts the good panels will short all their power through the shorted panel....NOT GOOD....houses have burned due to one panel in an array causing all the power from that array to flow through that panel. One panel in parallel does not have enough power to short out the protection diodes built into the panels. BUT two or more have enough power to blow out the diode and set the faulty panel ablaze. Electricity is lazy.....very lazy...it takes the shortest path, doesn’t matter that your batteries need a charge the short circuit is the shortest path so ALL of the array power will be shorted through that shorted panel. Wanna see firefighters flinch, do you think that they want to risk their lives putting water on shorting panels with live electricity.....Nada....They will stand back and let it burn. Now you are in deep dodo.....you have a pile of ashes and how you installed it was not in compliance with the National Electric Code and not approved by the building inspector , you have handed your insurance company an automatic out, it’s coded into your insurance policy that your home, structure , etc. must be in compliance with the NEC (national electric code) and have a valid building permit. Not a good trade off. Larger systems almost always have series strings of panels with a combiner box and fuses or breakers to insure that a faulted panel will not burn down your house. That is the reason for more costly MPPT controllers. PWM controllers belong in small 2 or 3 panel setups, they do not belong in 12 panel arrays. The cost of the MPPT controller is offset by the reduced wiring, fewer breakers, etc. and by the fact that PWM controllers are only 60% Effiency due to the voltage mismatch between the panels and the battery...Good MPPT controllers are 97-99 % efficient.

Note on the breakers: AC breakers do not protect you on a DC circuit, they weld closed and burst into flames when exposed to DC. The only household breaker that is rated for DC use is the Square D. QO line (up to 48 v.d.c.) (not the more common homeline breaker from Square D) No other common breaker is rated for DC. Solar supply houses often stock DC breakers from MidNite Solar, both the MNDC line and the MNPV line are suitable. The MNPV line is specifically for PV use, it is a din rail mounted breaker and as such mounts directly in combiners. MNPV breakers are rated 150 volts dc, there are also 300 volt units available, basically a 2 pole with the poles in series. Fuses are a viable but due to the greater effect on the human body by dc it is required to use “touch safe” fuse holders. A good place to search is Northern Arizona Wind&Sun.....NAWS.......solar-Electric.com


NOW....the disclaimers....

MidNite Solar......I’m a long time customer of MidNite Solar and am biased towards their gear
“ “ .......I”m a beta tester for MidNite Solar on their Kid product line
NAWS..................long time customer, my first line for solar products, my go to dealer..
Square D , a division of Schneider Electric......I’m a stocking dealer of Schneider and Square D products
Chinese solar.....never did like their krap, never will, it’s cheep krap....

I am not a shill for these companies, I like their products, I don’t hesitate to recommend them, I get nothing back from them for my recommendations.

hope this helps you to make a safe system..

David

 

[Tecnodave]

JoeBoyKey,

Do you realize that Some. DC breakers are polarity sensitive, If they have polarity markings they are polarity sensetive
I see red wires in the negative and black wires in the positive,

BUT, I do not know what the power flow direction is in your setup....
the + terminal is the “more positive(source) of power”

for an inverter the positive goes towards the battery
for a charge controller the positive goes towards the battery which seems counterintuitive but in fault mode where the controller is shorted the battery is “more positive” than the controller....
for the panels the panels are always “more positive“ than the controller and the positive goes towards the panels

I recognize the left most breakers as CBI...Circuit Breaker International....also reflagged as MidNite Solar MNEPV.
The others, I haven’t seen, don’t recognize, I assume you are in Europe....?

Just flagging what I see but I don’t have the whole picture......I don’t know how you are wired up

You just might gain some knowledge from my post above.....about paralleling unfused panels...
Each of my arrays have a MidNite Combiner with MNEPV Breakers for each string. 12 panels, MNPV6 combiner feeding a MNDC15 panel

My breaker boxes are MNDC15 , MNPV6 combiners and MN baby boxes with MNEDC and MNPV breakers

Can you tell I’m a MidNite kind of feller

David

 

[Hedges]

you can connect 2 solar panels in parallel or even 2 series strings in parallel, but when you connect more than 3 panels in parallel you need fuses or breakers on each string......WHY..you ask...if one panel in a parallel setup with 3 in parallel shorts the good panels will short all their power through the shorted panel....NOT GOOD....houses have burned due to one panel in an array causing all the power from that array to flow through that panel. One panel in parallel does not have enough power to short out the protection diodes built into the panels. BUT two or more have enough power to blow out the diode and set the faulty panel ablaze.

for the panels the panels are always “more positive“ than the controller and the positive goes towards the panels

I think both orientations are wrong, when using a polarized breaker for PV strings.

If I have half a dozen strings connected in parallel through polarized breakers (with "+" of breakers going to positive of panels),
What happens if one string shorts and the other five dump their current in the backwards direction through its breaker?

 

[Tecnodave]

The breaker on the shorted panel will trip, the others will not as they are not overcurrent

The whole thing is an issue...the new codes do specify “non-polarized” breakers and as such MidNite and Circuit Breaker International have redesigned the MNPV breaker to be “non polarized”
The MNDC breaker is manufactured by Carlingswitch and is non polarized.

My info came from boB at MidNite Solar who has posted quite a few times about this on MidNite forum

david

 

[Hedges]

The breaker on the shorted panel will trip, the others will not as they are not overcurrent

The whole thing is an issue...the new codes do specify “non-polarized” breakers and as such MidNite and Circuit Breaker International have redesigned the MNPV breaker to be “non polarized”
The MNDC breaker is manufactured by Carlingswitch and is non polarized.

My info came from boB at MidNite Solar who has posted quite a few times about this on MidNite forum

david

Correct, the breaker on the shorted panel will trip. But it is carrying current in the backwards direction, current from 5 panels (about 2.5 times the breaker's rating.)
What happens when a polarized breaker opens while carrying current in reverse direction?


Sounds like NEC figured out what I figured out.
We probably didn't hear about catastrophic field failures, because only a few bypass diodes in a string are going to get shorted, not all of them.

 

[Tecnodave]

Bypass diodes only bypass shaded portions of the panel, blocking diodes prevent reverse flow through the panels

 

[Hedges]

Bypass diodes only bypass shaded portions of the panel, blocking diodes prevent reverse flow through the panels

Yes, and some panels have bypass diodes that can't handle full current, overheat and fail.
If all bypass diodes failed shorted, the breaker would have to interrupt current from all the other strings, opening to about Voc. A polarized breaker would not be able to do that in the reverse direction.

With several strings in parallel, shade on a panel could overheat a bypass diode. But only a small percentage of panels/diodes in that string could suffer such a failure before voltage of that string drops too low to pass much current at Vmp of the other strings. So I think only a few diodes fail for this reason, not enough to cause a bad short and high fault current.

If a wire was damaged and shorted, that could result in severe overcurrent assuming the inverter doesn't detect and shut off for ground fault.

Blocking diodes would be one way to avoid failure of those polarized breakers.

 

[Tecnodave]

I agree,

I’ve had it happen to me one time with some Sharp prototype NE-165 panels I had in an array of 2S6P with a MidNite MNPV6 combiners and MNPV-15 breakers, I was limiting power at the time and had only 3 strings on. All the bypass diodes shorted on one 1 string, I was there and the breaker did not trip, I was very surprised by that, the Classic output dropped, so I shut down. at the array both junction boxes were swelled , panels were noticeably warmer, so I covered up and removed that string . Later inspection in the junction boxes discovered all diodes with magic smoke released. Removing those diodes restored operation in those panel. These were panels in the testing phase and not yet released. I never heard back from Sharp about it. These were back door freebees, it’s handy to live near to where they make them.

Probably pretty rare but that breaker did not trip, I knew within seconds when the Classic went to zero power, less than 3-4 minutes to array and trip all breakers, I have wondered what would have happened if I were not right there when it happened
That’s only 1time in many years, many types, etc. that I’ve had a panel failure like that. I started doing small scale solar in the late ‘60 doing radio communications sites on remote mountain tops. Kinda pioneering back then, there was no one to ask. Our fail mode was tons of ice falling from the towers, etc

 

[Hedges]

You only had 2s3p connected, so with one string shorted, current was 2x Isc. That is only about the fuse rating, so fuse probably doesn't blow. It also doesn't exceed panel rating for max fuse by very much. Fuses would take a longer time to blow at moderate overload, not guaranteed they will at 110% of current. If all 6 strings were connected then 300% would have blown it.

I have some Sharp 165W poly panels. Isc = 5.46A, max fuse 10A.

Surprised to hear all diodes failed; my theory said only some should. Was there shading? Maybe they couldn't hold reverse voltage.
If you still have them, should replace the diodes. Won't matter without shade, but does matter with partial shade on a string.

 

[Tecnodave]

@Hedges,

I missed answering you on this issue. 2 of the panels had their wires cut and the connectors installed wrong polarity by the former user. I missed that while i was setting up and testing the panels. The two that were wired backwards were on the same string and reverse polarized and the breakers did not pop. None of the breakers popped. I was using a MNPV-6 with MNEPV-15 polarized breakers. Classic had a zero output and when i went to check the panels they were hot, opening up the junction boxes both diodes were smoked......Those panels are still sitting in my spares pile. I agree with you on putting blocking diodes in series with my strings, but i have not done so yet. The panels when tested had zero voltage output when exposed to sun but when the burned diodes were cut out the panels were at full voltage.

 

[Robin Gudgel]

The present day MNEPV series of breakers is polarized. You connect the positive side of the breaker to the PV+ circuit as described above. But, in the event that a panel does short, the current from all the other panels will backfeed the broken panel and the current will be flowing in the opposite polarity. All 600,000 of our combiners work this way. We have tested the MNEPV breakers in reverse polarity and have never had one fail until we put 200+ amps through a 15 amp breaker. So it isn't a real life problem and up until now there are no non polarized breakers for these types of combiners. CBI is now coming out with a couple new series of breakers. One is a 150VDC UL listed non polar breaker. This would be perfect! The other series is a single pole 300VDC UL listed breaker, but it is polarized. The good thing is that we can now have an affordable small (2 spaces wide) 600VDC breaker for combiners. Have we tested the polarized breakers backwards yet? No we haven't. We have only a hand ful of samples of these breakers. They are supposed to be in stock by the end of the year though. I will attach a poster of all the breakers we will be offering soon. I suspect there are still a couple of missing breakers on this poster, but it is pretty complete. All these new breakers can make your head swim. IT is now getting really complicated. By the way CBI will be discontinuing the old QY (MNEPV) series breaker in favor of these new ones. The new ones will cost a bit more too, so if you are expecting to be needing this series of breaker in the next few months, get some now. MidNite makes more residential PV combiner boxes than anyone else. They are cheap and they work. I didn't quite understand combiner boxes when I designed my first one back in 1993 or so. Hmmmmm, I still have that in service. It uses wire nuts! I probably should replace it.

 

[ko6kL]

I bought these din rail terminal blocks for my combiner box

https://www.aliexpress.com/item/32961099960.html

I also bought the fixed bridge (allows you to connect wires to both sides of terminal block if needed) they connect in the middle of the terminal blocks as an alternative to plug in bridges

https://www.aliexpress.com/item/32974793584.html

most of the din rail stuff is flimsy looking, and hard to work with.... i have one of these I'm going to try for a charging battery buss and also to combine the negative leads in the combiner box... it's a heavy metal and looks ideal ( the 125 model ) !

need some low cost 6 amp dc breakers for the 47v panels for din rail it anyone has ones they like???

ko6kL

https://www.amazon.com/dp/B08JJCGKGR

 

[featherlite]

ko6KL,
Thanks for the lead!

 

[mrdavvv]

Have you ever used modular terminal blocks?, they are very common on industrial panels, you can join them together with busbars / bridges, altough i have never used them in high power DC:

https://www.te.com/commerce/DocumentDelivery/DDEController?Action=showdoc&DocId=Data+Sheet%7F1-1773980-7%7F2001%7Fpdf%7FEnglish%7FENG_DS_1-1773980-7_2001.pdf%7F1SNK505310R0000

 

[ko6kL]

we use the DIN's at work , its fine for low current ( still a pain to put a wire into someplace you cant see most of the time ) crimp fare'al would help ( more tools ) but not so good for higher power levels.