breaker rating

[eastpoint turbine]

 Hi to all,  I want to know what breaker and the rating to use with the  MNPV6 combiner . I'm using 12     255 watt panels  four strings of 3 panels   current in each string should be about 30 amps
    Jason

[Vic]

Hi Jason,

It is customary to use a Combiner breaker that matches the Maximum Fuse rating for the PV that you are using.  This fuse rating is almost always on the Data Sheet for the PV,  and essentially always on the label on the back of the PVs.

In days past,  one would multiply the Isc value for the PV by 1.56 to get the minimum breaker rating.  Now,  most PV breakers from MN and OB are rated for continuous current at the rating for the breaker,  so one could use the Isc multiplied by 1.25 to find the minimum breaker value.

If you are in snow country,  guess that it might be possible that snow reflections might occasionally cause a nuisance trip of the breaker at 1.25 X Isc.

AND,  for series strings of PVs,  the PV voltage is multiplied by the number of PVs in series,  but the current ratings remain the same as for a single PV.

Often,  the 60 Cell PVs,  which are often in the 235 - 285-ish watt range,  have Max Fuse ratings of 15 A,  some are higher ...   all depends.

FWIW,   Good Luck,    Vic

[Don Miller]

If you are in snow country,  guess that it might be possible that snow reflections might occasionally cause a nuisance trip of the breaker at 1.25 X Isc.

If that is possible then I'd go with the full x 1.56 safety rating IF the system was like mine and sits unattended for periods of time.    Ptherwise it could trip and sit disconnected for weeks, or months.

[vtmaps]

I'm using 12     255 watt panels  four strings of 3 panels current in each string should be about 30 amps

Something's not right.... you have 4 strings, each producing 30 amps?!?!  No way.  --vtMaps

[Vic]

Something's not right.... you have 4 strings, each producing 30 amps?!?!  No way.  --vtMaps

I had assumed that eastpoint  had multiplied the current from each PV in a string by the number of PVs in THAT string,   to get approximately 30 A.  Had tried to address that possible issue in my above post,  regarding PV voltages adding,  (V multiplied by # of PVs) in a single string,  but the current of a single PV is maintained for the string ...  perhaps was a bit too subtle [how could that be !!?!].

FWIW,    Vic

[eastpoint turbine]

 HI vic  the more i read your reply the more i'm confused, now if each panel has isc of 8.5 then 3 would be 25.5 .   Now you say multiply that by 1.25?   that gives me 31.8
    I want to know if " MNEACQZD 10A thru 60A "   or  "  MNTS "  from midnite solar to work on my combiner box
   my next question is:> would this arrangment  overload the classic 150 ?  i've input data into the sizing tool but ? not sure.  I know the classic is good for 90 amps
   Jason

[Westbranch]

PV in series you add the voltage, so 3 x the voltage... (3 in series)

PV in parallel you add the amperage, so 4 x the AMPerage (4 strings)


BOLD is typo correction   

[dgd]

Should basic info like this not be in the FAQ area?
Every so often we see people confused about when to add together voltages or currents in strings of PVs

dgd

[Vic]

Hi Jason,

Thanks for your reply.

OK,  with a string of PV modules,   the string current remains constant.  It will not exceed the current of a single PV.  The power from the string IS multiplied by the number of identical modules in the string.  This happens because the VOLTAGE in a series string is multiplied by the number of modules.

So you will have four strings of three modules in series.  The Isc of a single module will remain the same for the string,  but the voltage of about 31 volts will be multiplied by the three PVs in the string.  So,  this will result in a string voltage of about 93 volts.  The Imp would be about 8 or so amps.  So the power is Voltage multiplied by the Imp.  That would be about 93 Volts (the approximate string Vmp) times the  approximate Imp of about 8.2 Amps.   This is about 750 watts for that string of three PVs.   (yes,  you have 255 watt PVs,  and depending on their construction,  etc,  the Vmp will not be exactly 31 volts,  but you get the idea).

The above numbers are rough guesses,  because we do not have the exact numbers for your PVs,  but you get the idea.

The Classic (like most most MPPT CCs) is not rated based on input current.  But,  rather on output current.  The MPPT function multiplies the input current by stepping down the voltage.

Hope that this makes some sense to you.   Vic

[Vic]

And,  personally,  I would probably use a 15 Amp MNEPV breaker,  which is probably the Maximum Fuse Rating for the PVs that you are using.

FWIW,   Vic

[eastpoint turbine]

 Oh my ! oh my! what was i thinking, how did i come up with 30 amps/string ?   " Something's not right.... you have 4 strings, each producing 30 amps?!?!  No way."    you are right  Vtmaps.    I am so thankful when others see my errors and put me right.  Looking over my posts again i don't know what i was thinking  ,glad for the correction  and the advise .  I guess this is what the forum is about
gratefull to you Vic, Westbranch & others   I will order 15 amp breakers
     Thanks
    Jason

[Vic]

Hi Jason,

Yes,  we are all learning,  and in my case,  forgetting between learning sessions.

Have fun with the new PVs.    Vic