maximum series fuse rating?

[kauaisolarman]

hello

i am a little confused about the series fuse rating for my solar panels.

the way i understand it is that if the blocking diode on a panel fails the panel can handle up to 15A without being damaged correct?

so if i am running 3 panels in series do i need 15A breakers between the series connection of each panel?

or will the 15A breaker in the combiner that connects the series string together suffice?

thanks in advance

[TomW]

In basic electric circuits 101 terms, one fuse / breaker in a series circuit will protect the entire circuit. By definition the current must flow through all components to complete the current path in a series circuit. So, one breaker per string.


Hope that helps.

[kauaisolarman]

yes that does

it also saves me a lot of money not having toput circuit breakers/fuses between each panels series together,  just the series strings that i hook in parallel needs the breakers.  and the breaker size should be at or around the maximum reverse current rating of the panels that you are using.


sorry about the newbieness of my questions still trying to learn and figure everything out.

this forum is extremely helpful

[TomW]

sorry about the newbieness of my questions still trying to learn and figure everything out.

this forum is extremely helpful

No worries.

Not one of us was hatched knowing this stuff.

Glad you found your answers.

Tom

[CDN-VT]

I posted this on another forum , So it's just a Copy & paste .

SO Rules first !!
2 stings of same value (two 250w panels in series = a string) , No need for any breakers ( not wise due to adding or checking one string against another string [Breakers only] ) , Three strings (now we have 6 panels of 2 in series & 3 parallel strings ) , then the rules kick in due to back feeding of two OR more strings into one that is the MAJOR .

Kids (nice girl & boys ) wack a ball & it hits a roof , but your panels are in the way , crack & a short between cells length so the (im picking on high voltage output !! ) gets shorted to but are fused so all power is going from 2 strings into one1 shorted string  broken & it starts to overheat on one panel .

YOUR imp is 8.xx AMPs  but your breaker is 15 amps
Series adds voltage / per panel , but AMPS are the same !!!
SO now we have 2 series strings feeding a shorted panel 16 Amps , went the max currant out of each string is 8,xx (in a perfect world) times 2 , so just under (losses) 15 amps , breaker holds & roof is on fire !!


OCB's /Breakers / Fuses/ Cut-outs are ONLY to protect the WIRE , NOT the appliance or supply ..
My combiner is 10 Amps @ 150Vdc type ,my panels are 8.41 amps
Wire size is 10AWG 60'foot run max

VT

[kauaisolarman]

so when you say 3 strings you mean 3 panels in series together or in parallel together?

sorry  again just trying to be 100% clear.

so if i want to put 3 or more panels together in series do i need to run circuit breakers between the + & - connections of all the panels that are being connected together?

or

do i only need circuit breaker when paralleling more that 2 strings together that are in series?  like if im connecting 3 sets of (4 panels together in series)?


just trying to make sure dont want fire hazards.

to be safe i will run all 10A breakers just want to make sure i'm putting them in the right places

thanks again for all the helpful info

[Powerplay]

@CDN-VT

2 stings of same value , No need for any breakers ( not wise due to adding or checking a string against another) ,

A few questions since my coffee hasn't kicked in yet.     2 strings (Is the definition of a string a set of PV-in wires that terminate at a combiner box?)  No need for breakers (not wise) (Does this mean not wise to leave out breakers on this case or not wise to put in breakers?)

Series adds voltage / per panel , but AMPS are the same !!!
SO now we have 2 series strings feeding a shorted panel 16 Amps , went the max currant out of each string is 8,xx (in a perfect world) times 2 , so just under (losses) 15 amps , breaker holds & roof is on fire !!

If the breaker holds, then the wire won't be on fire so what is?  Did the diode blow in this scenario?  Doesn't the regulator have a say in this? 

I'm having trouble following the scenario, but the 'roof is on fire' quote did catch my attention.  Maybe my breakers are too big in my own system. 

Thanks.

[Vic]

I posted this on another forum , So it's just a Copy & paste .

SO Rules first !!
2 stings of same value , No need for any breakers ( not wise due to adding or checking a string against another) , Three strings , then the rules kick in due to back feeding of two OR more strings into one that is the MAJOR .

Kids (nice girl & boys ) wack a ball & it hits a roof , but your panels are in the way , crack & a short between cells length so the (im picking on high voltage output !! ) gets shorted to but are fused so all power is going from 2 strings into one1 shorted string  broken & it starts to overheat on one panel .

YOUR imp is 8.xx AMPs  but your breaker is 15 amps
Series adds voltage / per panel , but AMPS are the same !!!
SO now we have 2 series strings feeding a shorted panel 16 Amps , went the max currant out of each string is 8,xx (in a perfect world) times 2 , so just under (losses) 15 amps , breaker holds & roof is on fire !!


OCB's /Breakers / Fuses/ Cut-outs are ONLY to protect the WIRE , NOT the appliance or supply ..
My combiner is 10 Amps @ 150Vdc type ,my panels are 8.41 amps
Wire size is 10AWG 60'foot run max
VT

Part of this discussion has occurred in the Breaker/Disconnect Topic ...   there,  posted the following ...m

"
Not quite certain just how to read some of the above ...

kauaisolarman's HIT 190 PVs have an Isc rating of 3.75 A,  and a stated Maximum Series Fuse rating of 15 A.

As I read it,  a 15 A DC breaker would be safe,  and the minimum size would be 6 A (3.75 X 1.56, and rounded up).  MNEPV breakers are available in a 6 A rating,  and that size breaker should not be subject to nuisance tripping.

SO,  IMO,  Sanyo is saying that  15 amp DC Combiner breakers are safe,  but breakers between 6 and 15 amps would be safe and should work well.

But,  that's just my take.    Vic"

FWIW,   Vic

[Vic]

so when you say 3 strings you mean 3 panels in series together or in parallel together?

sorry  again just trying to be 100% clear.

so if i want to put 3 or more panels together in series do i need to run circuit breakers between the + & - connections of all the panels that are being connected together?

or

do i only need circuit breaker when paralleling more that 2 strings together that are in series?  like if im connecting 3 sets of (4 panels together in series)?


just trying to make sure dont want fire hazards.

to be safe i will run all 10A breakers just want to make sure i'm putting them in the right places

thanks again for all the helpful info

kauaisolarman,

A "String" is normally a set of PV modules in series,  technically two or more PVs in series.   Each String must have Overcurrent Protection (fuse or breaker) if there are three,  or more Strings of PVs paralleled,  into the Combiner.
EDIT to add: The current in a series circuit is the same at any point in the circuit.  When adding PVs in series,  the current remains the same as with a single PV, but the voltage of the string is multiplied by the number of PVs in series (as Tom mentioned,  previously)<

For PV modules in Parallel,  the same rule applies  --  two PVs in parallel do not require a fuse or breaker,  but three or more PVs in parallel would require a fuse or breaker for each of the PVs that are paralleled.

Regarding weather protection (fuse or breaker) is required,  it does not matter,  the number of PVs in each string,  just the number of strings.

The reason that protection is required,  is due to the amount of current from other strings (or other single PVs in parallel)   that could flow into a faulted/shorted PV.  If there are only two strings or two paralleled PVs,  there is not enough current available to cause a problem  --  the current available is the same for the faulted PV as is available when that string/PV is working normally.

In the event of a fault,  with three or more PVs or strings in parallel,  the current available from the other PVs (current from two or more other PVs)  could cause more current to flow into the faulted PV than the cables or PV busses were designed to carry.   This  increased current could cause excessive heating and possible fire.

10 A breakers should be fine for the Sanyo HIT 190.  The Data Sheet for HIT 190s note that the Isc is 3.75 A,  so a 6 A breaker should work fine,  as would a 10 A breaker.

Breakers are inexpensive,  fit inexpensive Combiners,  and can help in testing the system,  and in troubleshooting.   Fuses work fine,  but cannot be used as a disconnect (switch),  and sometimes cost more than the MNEPV breakers that are usually used on most Combiners.

FWIW,  Vic

[CDN-VT]

Thanks Vic for the clearing up.
Yes 15amp would work , but Id rather be closer to 10 amps for his panels. I too would worry about setting up a  subject to nuisance tripping, being so close.

nuisance tripping. = finding a breaker tripped for NO real reason !!
Breakers have a life , if they are tripped all the time for testing , they will wear faster than one that is never touched. Also if they are so close to the load ratings they will also have a shorter life.



VT

[Powerplay]

@kauaisolarman;  Your topic & question is very timely for me since I'm just getting ready to configure my first 3X parallel into the combiner.  I tend to re-think all my breakers location and sizing frequently since I add to/move/reconfigure my system frequently.  In my case, my PV module manufacturer states the max series fuse at 15A.  My series Isc is a modest 5.75A.  So if my thinking hat is on correctly, the 3X could back feed 11.5A into the single series or panel that contains a short circuit by accident.  Therefore, a 15A breaker would not be required to protect from this scenario since 11.5A is below the 15A max series fuse rating of the panels.  However, It does require a 15A breaker in the combiner box to prevent back feeding from other strings of panels that are or might be terminated in the same combiner box.  Its 400W -> 5.3A X 75.6V series X 3 parallel (1200W total).  Any feed back on this thinking would be welcome.

@CDN-VT;  Those pictures of the hail damage give me the chills.   

[TomW]

I see some confusion in this thread. Maybe me.

As far as I know fuses / breakers do not "prevent back feeding"?

Seen it mentioned in this thread several times.

I just don't see how it does that?

Normally, a diode does that job.

As far as I know the fuse / breaker prevents excess current from flowing through the panels, ether forward or reverse.

Anyway, will someone above my pay grade please set the record straight here.

These little things can get repeated and become "fact" and that is just not good.

No offense to anyone I just want the advice here to be solid and useful.

Tom

Edit:

The series fuse rating is probably just the maximum current the panel is rated to handle. So just use the recommended breaker / fuse rating.

[Powerplay]

Sorry Tom,  I was just using the term back-feeding to mean excess current (fries the wires and busses) and that's sloppy.  Otherwise, did my logic sound Okay to you?

[Halfcrazy]

So part of the confusion I think is the idea that if a module has a "Series fuse rating" of say 15 amps that if it is fed 16 amps the house catches on fire. This just isn't true. The 15 amps is the rating of the over current protection device and to be honest UL would have tested them to close to double that 15 amps. The theory here is that if you have 3 strings of modules or more and one module shorts the other strings can backfeed that one string. Thus giving it to much current. If you use no series fusing and had say 6 strings of pv paralleled and one string shorts YES the house will burn down but if you had the proper fuses or breakers life is good. See picture for how NOT to parallel modules 

Also be careful undersizing the over current device as it can lead to nuisance trips.

Think about it another way 14-2 Romex style wire has to be protected at 15 amps by code but it has a short circuit current that is WAY over that, Maybe pushing 75 amps or more, A PV module is the same way, if it says 15 amps it can handle 15 amps back feeding into it all day long

[Vic]

Hi Tom,

Well,  this is probably above my pay-grade,  as well.

What I was trying to do in this Thread,  and a similar one on the Disconnect ...   Thread,  as well,  was to note:

The breakers/fuses that we are trying to discuss are there to protect a FAULTED PV  (some short,  or very low resistance path),  from other PVs from feeding excessive current into that fault.

Modern PVs do not any longer have diodes that protect PVs from reverse current  --  Blocking diodes.   Almost all current PVs DO have Bypass diodes,  which shunt current around individual strings of cells in an individual PV,  when that PV is Shaded/partially Shaded.

As is the common use of fuses/breakers,  Combiner breakers are there to protect currents above the current Rating of that cable/conductor,  or buss of individual PVs.

The traditional approach has been that the Minimum breaker/fuse should be the PV's Isc X1.25 X 1.25,  and round up to the next highest breaker size  --  this is where that 1.56 X Isc came from.

Most DC breakers that we use in Combiners,   these days,  are rated to Carry its full rating, without tripping.    So,  really only one X 1.25 would be required,  plus rounding up.

It has become customary to just use the Max Fuse rating of the PV,  which is often considerably more than the above calculated values.

On systems with many strings that feed a Combiner's DC buss,  the Fault currents (into a single PV)  could fairly high,  so this level of protection is not a trivial little detail.

The 1.56 X Isc  has been in the Code for some time,  IIRC.

Have never seen a Nuisance-Tripped breaker.   Of course in very cold (and windy) climates,  perhaps with reflections from snow or water,  the Imp and Isc could possibly be exceeded in direct,  bright sun ...

The older PVs with Blocking diodes should be protected from reverse currents,  and were primarily used to keep the battery from being discharged by the PVs at night.   But they added one diode forward drop,   which meant a bit of a loss of power when the PVs were producing power.

My read on things ...     FWIW,   Vic