Understanding Breakers Between Panels and Charge Controller

[ericbakuladavis]

I see people recommending using a circuit breaker between the solar array and the charge controller and I'm not sure why.

Is it true that solar panels can be shorted without damaging them?
If a breaker were sized above the short circuit current of the array (Isc) what scenario(s) could cause it to trip?

If all we want is a way to disconnect the panels from the controller, we could use a simple switch....

 

[Supervstech]

All wiring needs to be protected from shorts. Each string should be fused, a breaker protects the combined PV current from damaging the wiring. Individual PV can handle a short, but can the wiring handle the short of all panels in a string?
also, the PV connects to a charge controller. The breaker also protects the PV string from a failure in the controller.

 

[ericbakuladavis]

@Supervstech Thanks for your reply!

Each string should be fused, a breaker protects the combined PV current from damaging the wiring. Individual PV can handle a short, but can the wiring handle the short of all panels in a string?

For the panels I'm working with, the current at pmax (Imp) is 11.76A. The short-circuit current (Isc) is 12.45A. The panels will be wired in series. It seems that the difference between the current that we want/expect from the panels (Imp) and the maximum possible current form the panels (Isc) is very small. So if the wire were big enough to handle 11.76A but too small to handle 12.45A, then I can see how a perfectly sized breaker could save the day. Are there other scenarios (given my setup) where a breaker could protect against current from the panels? If the wires were too small, wouldn't the breaker have to also be undersized in order to do any good?

The breaker also protects the PV string from a failure in the controller.

Okay. I think this makes sense to me. Are you saying the controller could fail in such a way that it would pass current from batteries to the panels?

 

[ericbakuladavis]

I kid you not, I just stumbled across this guy talking about the PV breaker issue. He's saying exactly what I was thinking...

Here's the video, queued up to where he starts talking about PV breaker amperage:

 

[toms]

If the maximum current possible doesn’t exceed the current rating of the wire, no need for a fuse.

This is very common in RV setups

 

[12VoltInstalls]

If the wires were too small, wouldn't the breaker have to also be undersized in order to do any good?

Yes.
There is a marine spec for that, also an NEC spec. Often people say no more than 25% more than the wire amp rating. Some say 15%
Me? I usually overwire stuff at the 12/24V level: if the chart says x-gage, I use one size bigger than x-gage but I fuse/breaker the wire at the smaller x-gage rating. If I can’t go exact with the breaker/fuse I’ll go slightly over.
Many will say that’s overkill and unnecessary. I’d say they are correct. However, I’ve never had a failure, and for the pizza or two pizzas difference in price for small projects I’ve never regretted it or had to do stuff over.

I’m not telling you to overwire except maybe the batteries to inverter feed: one size bigger is potentially better performance electrically (though the distance is usually short) and if you decide to go from 1500W inverter to 2400W you don’t have to buy new cables. Also, if you have a good inverter say 1200W the ‘surge’ is probably 2400W; i size for the 2400W for example, and fuse for the 2400W