Combiner box...

Here is a typical combiner box and how it would be wired. Does viewing this image help?

midnite_combiner_box.jpg


EDIT: Alternatively, if it were possible to wire your array into a single series string, rather than parallel (as you have depicted in your drawing), you would only have one input to the charge controller and no need for a combiner box.

Tomato Tomato.

There are still 3 breakers touching each other at one end like in the drawn (poorly) bitmap. Those breakers are not unidirectional - so what is stopping the current from two 15 amp breakers form tripping the third? Is the electrical current incapable of traveling backwards if there is a forward current? (which wouldn't make sense in that combiner box as it would seem like the two 15a breakers on the right would put a > 15 amp current on the breaker on the left)

Any more thoughts?

First, I am beginning to wonder if we are on the same page here. I may be off just a bit, but will do my best to stay with you.

Second, the DC current that passes through breakers in the combiner will only go one way, from the PV array to the controller. To my knowledge, no power will ever be sent back from the batteries, through the breakers and into to the modules. (If it were, it would drain the batteries at night time when no light were shining on the array. This is due to the protection diodes in the modules. I would hazard a guess that modern controllers have similar protection to prevent this from happening as well??)

Third, unless the panels are going to be installed in a parallel string, there is - realistically, no reason to even install a breaker between the PV array and the controller.

The only real purpose a breaker will serve, is as a switch to disconnect the PV array from the controller in order to work on modules in a series string. (If you have a two conductor wire that will carry more current than a single module can produce, the circuit protection will never be needed and the wire can never be damaged by the current coming from the PV array.)

When wiring two (or more) strings together in a combiner, will be the only reason / time to have breaker protection.

Breakers should be bidirectional (certainly the ones pictured), there are no diodes in them to only cause the current to flow one way - in fact most of them are just a simple thermal trip.

I understand that no current will flow from the batteries back to the Panels, but that is because there are diodes in the charge controller and on the back of the PV panels - not because of the breakers.

And yes, my 4 panels will be installed in parallel and not in a series - 4 panels in parallel.


Ok, as I am writing this it finally hit me why this works...

Its simply just the definition between AC and DC current. DC only flows one way (from the power source) where as AC alternates forwards and backwards : /

I do quite a bit of AC wiring and I knew there was no way this would work as is - but I forgot that DC (unidirectional) doesn't quite work the same.

A good AC/DC practice model I suppose.

Yes, that is what I said, too.

1. No modern panels have blocking diodes to prevent reverse flow. They have bypass diodes to protect shaded panel sections.
2. The breaker is there to deal with a double failure: simultaneously 3. The breaker is also there in multiple (3 and up) parallel strings to prevent two or more good strings from producing damaging current back into a failed string.

So, the controller is the only thing stopping the reverse flow of current back to the panels, draining the batteries at night? Wouldn't it be a good decision, then, to have heavy diodes between the panels and controller, to help prevent this from occurring - in case of hardware failure?

Those heavy diodes are not needed and would cause an unnecessary voltage drop.

As far as the fuse or breaker protecting against large current flow from the battery (in the event of a controller fault AND a shorted panel), there is already a fuse or breaker between the controller and the battery.

If you use polarized breakers, the only breaker with the correct polarity to interrupt the double-fault current flow from the battery is the breaker between the controller and the battery.

--mapmaker

Keep in mind that DC will flow either way depending on the potentials. DC flows from the highest potential (voltage) to the lowest potential. One way to look at it is DC negative is the "less positive" or the lower potential terminal. In DC the "power source" is the terminal with the highest potential. For example, if a battery charger (power source) positive lead is +14 volts and the positive battery terminal is +12 volts the current will flow from the battery charger lead to the "less positive" positive battery terminal when connected. However, if the battery charger fails and shorts the voltage of the lead becomes less than +12 volts, then the same battery terminal that was "less positive" now becomes the power source and current will flow the other way toward the "less positive" charger. Just another way to look at it.

Indeed the breakers are there incase a panel shorts, preventing the backflow across the short, Small din rail breakers like these are currently polarized in that they need to be setup so the + is toward the greatest potential. I believe the NEC is working to change this, but there currently are few if any un polarized breakers in small din rail mounts.

OK you do not quite understand what the main purpose of any Over Current Protection Detection is for. It has only one purpose, to interrupt and disconnect excessive current flow. Nothing more, nothing less. Breakers are sized by the normal operating current inspected, and there is a minimum size wire requirement based on the breaker current rating.

But here is what you are missing. The breakers are not there to keep current from flowing, and they do not care what direction the current is flowing. Their only job is to protect the wire, and nothing more. Why are they there you ask? Really simple when you go putting 3 or more parallel string, this is why the code requires combiners with fuses for 3 or more parallel string, if one string were to short out, the current flowing through it are its own and all the current from all other parallel stings. For example if you have 4 parallel strings with each string of panels with a 15 amps current, on estring shorts out and now has 60 AMPS flowing on wiring designed to only have 15 amps.

What happens if you have a breaker in the string when that happens? It operates and your system returns to normal operation missing one string.

I think in most cases the breakers are to protect the panel. The wire on most currently produced panels is usually capable of handling much greater current than the series string rating of the panels.

That you need string OCP (breakers or fuses) on 3 or more strings is certainly true for many/most currently produced panels. But the code, as I understand it, only requires OCP on a particular string if the sum of the currents from the other strings could exceed its series string fuse rating.

You may need OCP on only two parallel panels if, for example, one panel with a string rating of 5 amps is parallel to another panel with an Isc of greater than 5 amps.

Likewise if you have panels with a string rating of 10 amps and an Isc of 2 amps, you could have 5 in parallel without OCP.

--mapmaker

Actually Most/all? din rail DC Breakers are polarized (magnetic) and need to be setup with the + toward the greatest potential. I believe the NEC is moving to make these no longer acceptable, likely, like the dead front combiners boxes, when they become available... https://www.youtube.com/watch?featur...&v=Mtq3cs9GuccI'll agree with this if you include the solar panel as part of the 'wiring' I think it's confusing to people to say the breaker is there to protect the wiring, I've had arrays set up with 10 gauge wire where the combined strings didn't total 30amps the capacity of 10 gauge, but require a 15 amp breaker to not over load the 'wire' in the panel it's self.

Here is the code specific language from 690.9

PV source-circuit overcurrent devices are required
to be rated so that the source-circuit conductors are protected
in accordance with Article 240 and so that the overcurrent
device ratings do not exceed the maximum
overcurrent device rating marked on the modules. Possible
backfeed currents from the other PV source circuits, other
supply sources through the inverter, and storage-battery circuits,
if any, have to be considered.


Blocking diodes (possibly required by the module manufacturer
for specific applications) can lose their blocking
ability because of overtemperature conditions or internal
breakdown. Therefore, overcurrent protection has to be considered
with a condition of shorted blocking diodes if they
are used in the circuit.

Here is 690.8 D

(D) Sizing of Module Interconnection Conductors.


Where a single overcurrent device is used to protect a set of
two or more parallel-connected module circuits, the ampacity
of each of the module interconnection conductors shall
not be less than the sum of the rating of the single fuse plus
125 percent of the short-circuit current from the other
parallel- connected modules.
[/QUOTE]

What it boils down to is this statement in 690.9

do not exceed the maximum overcurrent device rating marked on the modules.

So if you have 10 amp listed modules, each sting cannot be larger than 10 amps; no if ands or buts