Breakers size, orientation and wiring?

[sigp2101]

All I could read up to now points me that breakers should be installed between:
PV array and Controller,
Controller and Battery Bank,
Battery Bank and Inverter.

Questions I am struggling with are as follows.
Size in Amps and Voltage for each breaking point,
+ and - orientation,
cable hook ups, breakers will not accept required wire size.

Here is my off grid system description:
3 X 235W 36V 8A PV in parallel,
Midnite Classic 150 controller,
12V 6 X 6V Battery Bank. Inverter is yet to be determined but it will be no bigger then 1000W.
Already have Junction box with 3X15A breaker on each wire going in to serve as disco.

So, will all this info be sufficient enough to provide some suggestions? Appreciated.

[Westbranch]

have you seen this diagram, it needs another panel, ++ but you get the idea..

http://www.midnitesolar.com/pdfs/Kid_12V_2p_Par_REV_B.pdf

You NEED to tell us more before we can get started..

[sigp2101]

have you seen this diagram, it needs another panel, ++ but you get the idea..

http://www.midnitesolar.com/pdfs/Kid_12V_2p_Par_REV_B.pdf

You NEED to tell us more before we can get started..

Interesting, what I need to know is how they come up with those values 15A and 30A for the breakers.
Typically those panels would be around 6A each. When you bring them down to 12V from 36V as shown they will be almost tripling amperage.

What additional information community needs to know? I will be glad to answer.

Thank you!

[ClassicCrazy]

You size the PV input breaker for the size wire you are using . Same for the breaker size for output to the battery.

Of course you are limited to the amps that the controller is capable of  so  for example if you had a controller that only could do 15 amps you could use 12 gauge wire and a 15 amp breaker. But in the real low voltage world you have to consider voltage drop so you use larger gauge wire - so lets say you used 10 gauge wire you would have to use 30 amp breaker ( sized for the wire ) even though it was only 15 amp load on it.  Those are just generalizations but hopefully you get the idea.

My first setup with 24v battery system had 63 amp input breaker because I had 6 gauge wire in. I had to up the output to the batteries up to 100 amps since the Classic is capable of charging 90 some amps at 24v .  I had a 63 amp breaker on battery output at first but it tripped a few times so I upped it to 100 amp.

So you need to know how far you will run your wiring so you can determine voltage drop in order to decide what size wire you need. And use the Midnite string calculator or other calculators online that will give you all the voltages for your particular system.

Larry

[zoneblue]

Here is my off grid system description:
3 X 235W 36V 8A PV in parallel,
Midnite Classic 150 controller,
12V 6 X 6V Battery Bank. Inverter is yet to be determined but it will be no bigger then 1000W.
Already have Junction box with 3X15A breaker on each wire going in to serve as disco.

You have the right idea. If you think about the battery positive being a bus, and EVERYTHING that is connected to it having a breaker, youll do fine. (Dont forget to ground-bond the DC negative though).

PV: this breaker is more of a switch as the PV is current limited. Make it more than the combined Isc by at least 25%, and assuming your wire guage is big enough to limit voltage drop to 5%, youll be good to go there.

CC: this one is important, take your peak charge rate (45A), go to the next size up, or one more if its close (60A). If its a CBi orient it so that the positive(line) is to the battery. Then size that cable so that a voltage drop of under 0.2V occurs. This is much more important than teh PV drop, as it dictates charge setpoints. Double check the ampacity to be sure.

Inverter: You want the voltage drop low there too, to stop the inverter dropping out on surge. 0.2V is cool there too. The breaker is sized to peak discharge (100A), next size up. So 125A. Thats an issue because that puts you into a Carling, which are big, cost more and require a step up from Big Baby to MNDC.

Are you sure you would rather goto a 24V system? Then you can use a 60A for the inverter breaker.

You also need 3 breakers to combine the three PV modules. You can use a 3 slot combiner and some Cbis there for that, set at the nameplate fuse rating.

For the drops hit the calculator:http://www.calculator.net/voltage-drop-calculator.html

[russ_drinkwater]

Zoneblue, why do you need breakers to combine the 3 PV modules?

[Westbranch]

Russ it is protection for any of the 3 panels should there be a back-feed issue, 2 could back feed a third and there is a possibility of a fire.... needed when > 2 panels are combined...  the amperage for the CB is shown on the back of your panels, or should be...

[russ_drinkwater]

One amperage is 8 and the other in the specs is 8.5 amp.
I only have 2 panels combined (series) which gives me 58-59 volts for a 24 volt system.
But there are 4 lots/strings (parrallel) of 2 if that's what you are talking about.
No mention of circuit breaker on the back of the panels unless I am missing something.
And that of course happens to me!

[Westbranch]

here is the spec sheet for one of my old PVs, attached, see page 2 for breaker size, called FUSE rating


you use ISC * 1.25 *1.25 and go to not highest CB rating

[russ_drinkwater]

Yep, had another look at the specs on the back of a panel.
There is nothing even remotely similar to fuse rating listed.
I will have to do a newt search for further specs.

[Resthome]

Yep, had another look at the specs on the back of a panel.
There is nothing even remotely similar to fuse rating listed.
I will have to do a newt search for further specs.

They show a Maximum Reverse Current of 15A on their Data Sheets

[russ_drinkwater]

So i should be right with only 2 panels?

[Westbranch]

2,3,4 or more in a string (series) all have the same amperage as one panel,  only the voltage increases, it is in parallel that you need the circuit protection from any of the other strings backfeeding / (Maximum) Reverse Current