Sizing breakers

[Jphoenix13]

Decided I want to use four 340 watt panels on my RV, 2 strings of 2 parallel and the sizing tool says a classic 150 will be within limits, all green and OK for a 12VDC battery bank.

Will use a MNPV3 box, but having difficulty figuring out breaker sizes. I plan two breakers for the two strings coming in, rated PV array current for each string is 17.94 amps. Not sure how to determine breaker size for each string?

Is there a document,ent, video or other tutorial that describes how to size the Pv breakers as well as the breaker coming out of the cc and to the batteries?

Thanks,

Jim

[ClassicCrazy]

You usually size the breakers at the combiner box  to the wire you are using. So if 12 gauge you can go with 20 amp breakers. If it is 10 gauge you can still use 20 amp breakers if you want - or go up to 30 amp . It is okay to go smaller breaker on high gauge amp capacity wire - but you can't go the other way around.

I guess it will depend some on the wire loss calculations to determine what wire gauge  you need to use going to  your combiner box.
You say you will have 18 amps current on each string - are you sure ? If two of your panels are in series you would not add the amps - only add the voltage.  If you panels are 9 amps - two in series would stay 9 amps so that is what each string would need a breaker for 9 amps ( so use at least 15 amp breaker )   . When you combine the two strings the total current from the combiner to  the controller would be 18 amps so that is what the PV breaker at the charge controller would need to be. But often people use the maximum size wire from combiner to controller for lower voltage drop and maximum current possibility - so like 6 gauge and then use a 60 amp breaker for PV input at the charge controller .

For 12v you would probably want to use at least  100 amp breaker from charge controller to batteries.

I believe I have that all correct !

Larry

[Kent0]

Breakers for the individual strings of modules should be no larger than what is specified by the module manufacturer. That might be 15 amps, or possibly 20 amps, for these modules. From your description, it sounds like you are planning four strings. That requires four breakers and a combiner that is larger than the MNPV3. If wiring the modules two in series and then wiring those in parallel, you'll have two circuits, and the MNPV3 will work, but I think that each circuit is will only be about 9 amps.

[welmore]

1.5 times the current, short circuit (Isc).

I know, it sounds strange. Normally you size the protection to open before full current, but solar is different.

Use the Isc from one panel.  Do not add Isc for series strings.

[Vic]

1.5 times the current, short circuit (Isc)   ...

Just to try to clarify;

There are several ways to size  PV breakers,
one is,  when using DC breakers  that are NOT designed to run at the rated breaker current continuously to multiply the String Isc by 1.56 (1.25 X 1.25= 1.56),   and round up to the next higher available current rated breaker.

Or,  for DC breakers that are rated to operate continuously at the Handle Rating,  to multiply String Isc by 1.25,   and round up as necessary.  MidNite DC breakers ARE designed to operate continuously at the Handle Rating,  so using 1.25 X PV Isc  is best.

Or as a maximum breaker current rating,  use the Maximum Fuse (or breaker) size stated in the PV module spec sheet,  or on the sticker on the back of the PVs.

FWIW,   Vic