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Can You Wire Circuit Breakers in Reverse?

Ralo

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I've been researching how to AC Couple my Sunny Boy inverters to my Sunny Island inverters in an Off Grid configuration. I've read in the SMA manual that they are supposted to connect to the protected loads panel where the Sunny Islands are also connected. However, it instructs you to connect each SB to it's own circuit breaker. My SI's are not connected to circuit breakers, however, the outlets are. I understand that layout, but how do I put a circuit breaker between the SB's and the loads panel, connect them to the output of the circuit breakers? Is that what is happening in this video where the guy is running 3 SB's through 3 circuit breakers? That looks like the only way to do it? Can you wire circuit breakers in reverse?
 
Yes, In most solar installations the AC power from the Inverter is delivered to the main panel (or subpanel) via a standard breaker. Remember we are talking about AC not DC so technically the breaker is NOT being used in a reverse flow manner.
 
Yes, In most solar installations the AC power from the Inverter is delivered to the main panel (or subpanel) via a standard breaker. Remember we are talking about AC not DC so technically the breaker is NOT being used in a reverse flow manner.
@BentleyJ, that's a good point. AC is always going back and forth, right?
 
This may not be specific to the question about SMA inverters but my Outback Skybox ports are bidirectional. My AC Input on Skybox comes from my main panel and the AC output goes through a breaker on my critical loads panel. In an off grid situation I presume it would only have an AC out going to the loads panel.

From an AC coupling perspective my Enphase inverters are AC Coupled to my Skybox through that critical loads panel. SMA may have more specifics but it was helpful to me to understand in AC Coupling the Skybox was the device forming the AC signal and the Enphase micros were the devices following that AC signal.
 
Interesting, My system is similar. I'm using a Conext XW+ in the same fashion as your Outback Skybox and I also have Enphase microinverters that are AC coupled. In my case, there is no critical load panel, the whole house is backed up at the main panel.
Regards,
 
The breaker is mainly to give a method to AC isolate the device. There is really no need for a breaker on an inverter if the wiring supports more current than the inverter can supply.
 
SI is supposed to be protected against excess backfeed current in the event of a fault. Maybe that could happen if you had 24kW of SB on your 12kW of SI (although SB would shut off quickly if AC voltage collapse.) Or if you stupidly connected grid or generator to the loads panel?

Mine is 4x SI wired 2s2p, so I landed those on 2x 70A 2-pole QO270 breakers. Mismatch in resistance cause problems with grid feeding through. Could also happen with a large generator. No problem for your 2x SI wired 2s.

Backfed breakers ought to be secured against popping out and having electrically hot pins exposed. SB shuts off when grid/island voltage lost. SI keeps producing. My QO breakers have exposed contacts. I think yours are Homeline, so recessed.

You could put a 70A main breaker in your panel if desired.

After I had problems with QO270 (imbalanced current through SI from grid) I switched to main lugs with no breaker. Then I put Schneider DIN rail 63A 2-pole breakers in a box.

Connect SI at the main breaker or main lug end. Put breakers for Sunny Boy at opposite end. Put breakers feeding any critical loads and breaker feeding load-shed relay to house somewhere in between. This is so you don't have sum of SI and SB current added together and flowing through busbar, might exceed its ampacity.
 
The breaker is mainly to give a method to AC isolate the device. There is really no need for a breaker on an inverter if the wiring supports more current than the inverter can supply.
This is not right. Circuit breakers are to protect the wire and should be sized correctly. Breakers/fuses are required on all wires.

The breaker should be located on the end from where the power is coming from....In solar many times the current will flow in both directions and thus need a breaker at both ends. If the wire is short most people would look the other way and just put one breaker.

Many DC breakers require the flow of current to be only in one direction but many solar storage systems have flow in both directions so a fuse would be a better choice for DC.
 
This is not right. Circuit breakers are to protect the wire and should be sized correctly. Breakers/fuses are required on all wires.


If you have wiring capable of carrying more current than the inverter can produce, what will a breaker achieve?

You are parroting information you have been told without any understanding of what you are saying.

Much the same applies to single panel PV applications, if you use 15a wiring on an 8a panel - what fuse would you use, and what do you think it would achieve?
 
If you have wiring capable of carrying more current than the inverter can produce, what will a breaker achieve?

You are parroting information you have been told without any understanding of what you are saying.

Much the same applies to single panel PV applications, if you use 15a wiring on an 8a panel - what fuse would you use, and what do you think it would achieve?
Sorry toms, not sure your experience but by code you size a breaker to wire size or smaller. All wire to a component requires a breaker or fuse per NEC. The breaker is to protect the wire only incase it is cut or damaged or overloaded so it doesn't cause a fire.

All components in use are required to be UL listed and the UL listing takes care of the device design, if the unit needs a certain breaker size to make it fail in a safe way then it should be built in.

But in your original post you stated an inverter doesn't need a breaker if the wire size is larger then the load needed by the inverter and that is not right.

I am assuming this is in the US home which is bad on my part for not checking.
 
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U.S. NEC, current limited circuit, PV being the source, there are situations where no fuse is required:

"In this scenario, each string of modules can produce a maximum circuit current of 1.25 x I sc. As long as the connecting cables are rated at 1.56 x I sc or higher, the combined circuit fault current is not large enough to cause damage to the cabling or modules, thus fusing is not required."

 
If you have wiring capable of carrying more current than the inverter can produce, what will a breaker achieve?

You are parroting information you have been told without any understanding of what you are saying.

Much the same applies to single panel PV applications, if you use 15a wiring on an 8a panel - what fuse would you use, and what do you think it would achieve?
If it’s PV where the worst case is Isc of 8A, I wouldn’t use a fuse in that situation.
 
I appreciate all the replies. I have to read them multiple times to get my head rapped around them. Still working at it. ?
 
I appreciate all the replies. I have to read them multiple times to get my head rapped around them. Still working at it. ?

SMA have very good documentation and circuit diagrams, you won’t go wrong if you exactly follow their instructions.
 
Just realised i have a polarised breaker connected backwards and it still works during overcurrent/heat events.

Actually not surprising. What circuit is it in?
Some applications, voltage across breaker when it trips isn't very high.

We don't want breakers tripping. What happened?
 
Actually not surprising. What circuit is it in?
Some applications, voltage across breaker when it trips isn't very high.

We don't want breakers tripping. What happened?

It's on the inverter positive cable to the battery.

Running a high power AC motor from my inverter with too small of cables to the battery (which get hot after a few minutes) ... doubling up my cables and appropriating the breaker size are on my list of things to do. The breaker I have is really too small and was intended for a different project originally.
 
OK, so slight over-current of the breaker, and at the moment it tripped capacitors would of held up voltage briefly. So in a 48V system, maybe about 12V across the breaker while arc was extinguished.

More difficult to interrupt arc would be full battery voltage feeding a short to negative rail, and short-circuit current from battery.
In Midnight's tale of burning a breaker, they connected a string of car batteries to put about 165V across a 125V 63A breaker and resistive load (99A), and closed it, then opened. Connected backwards, the breaker burned. Forwards (with a new breaker), it interrupted successfully.

 
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