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
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.
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
Quote from: kauaisolarman on October 17, 2016, 01:59:37 AM
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
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
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
@CDN-VT
Quote2 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. :D 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?)
QuoteSeries 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.
Quote from: CDN-VT on October 17, 2016, 10:45:32 PM
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
Quote from: kauaisolarman on October 18, 2016, 05:33:29 AM
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
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
@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. :-[
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.
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?
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 :o
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
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
Quote from: Powerplay on October 19, 2016, 05:37:51 PM
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?
No worries.
As someone mentioned in this thread (I think) you need to beware of "nuisance tripping" if you shave it too close on breaker ratings, especially if the breaker is used as a shutoff and gets cycled.
I find it hard to give good advice when the target audience has such a varied level of knowledge in the subject.
Things I often automatically "figure in" without conscious thought can be a bad thing to ignore in use for the unaware. Giving too much info can confuse folks, as well.
I learned electronics back when the components were huge and the hot ones were called "valves" also known as vacuum tubes. Today, they stuff a couple billion transistors (valves) on a CPU in your laptop.
Its all good. Just want to be sure I didn't miss something or develop Alsheimers.
Tom
Also, older Thermal and Thermal Magnetic breakers have developed a reputation as getting more-and-more sensitive and tripping at a lower current with more trips. Some/many were NOT "Switch_Rated" -- the ability of these breakers to be used as switches, under load.
The breakers that MN sells are all Magnetic-Hydraulic (or sometimes, Hydraulic-Magnetic).
Here is a statement from Halfcrazy Ryan, about the CBI breakers that MidNite sells, under the MNEPV part number, copied from Reply #7 in this thread:
http://kb1uas.com/mnsforum/index.php?topic=351.msg2024#msg2024
"Straight from our CBI distributor when I talked to him in Dallas at SPI. The CBI breakers are good for 10,000 or more switches under partial to no load and 5,000 or more under full load rating. We have sold millions of CBI breakers and I can count on one hand with fingers left over the failures we have had. Rony the distributor for CBI is in Pennsylvania and sells to a lot more than us and he says the same thing they just do not break.
The Over center thing you mention is just part of the spring or snap action it has to open fast to help extinguish the arc. I have a 150vdc Din rail breaker here that is on my lister and I always open it under load it runs a VOC of 195vdc and an operating voltage of 135vdc and 60 amps. I have been using this for about 6 months and have opened it a few dozen times and it works just fine.
Ryan"
Hope that stealing words of others is OK, FWIW, Vic
Quote from: Halfcrazy on October 19, 2016, 05:50:12 PM
... 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 :o
I'm not disputing this but am having difficulty figuring out how this can happen with several parallel strings of PVs if there is a blocking diode for each string and bypass diodes in the PVs of each string. How can any form of backfeeding work to burn down a house?
I understand, I think, that the MNPV6 breakers, one per string, protect the string if for some reason ( that I can't imaging) produces abnormally high current that exceeds the breaker switch rating. That scenario never concerned me as I use the MNPV6 breakers simply to isolate a string when I want to work on its wiring.
dgd
QuoteI'm not disputing this but am having difficulty figuring out how this can happen with several parallel strings of PVs if there is a blocking diode for each string and bypass diodes in the PVs of each string. How can any form of backfeeding work to burn down a house?
I just went with the premise for the sake of argument, and, like most of us electrical types, I'm a belt AND suspenders type of guy. ;) Maybe some math can tell whether there would or could ever be a serious issue despite one's super bad luck scenario of rocks - meteors - hail, etc.
Mine goes like this - -imagine one of the strings to a given combiner is shorted and open to back feeding all the current available to a given combiner box. My biggest combiner at the moment has 2000W of PV total. But one of the strings is shorted out and not producing anything - fair? Okay 2000 - 400 = 1600W. Now the PV is only about 80% efficient on a good day. (Assuming I wasn't home during the hail storm and didn't open the breakers before the sun came out. :D Okay 1280W. The voltage at the combiner 76V so 1280 / 76 = 16.8A - so that's borderline possible since the max fuse is 15A. Have to weigh the likely hood of occurrence for this I think. My other combiner only has 1500W available. Btw, it looks like a 20A breaker wouldn't even get involved in this.
I just noticed the VoC on those strings would be 90V so the the 15A breaker likely also wouldn't come into play at 1280 / 90 = 14.2A. So...
Quote from: Powerplay on October 20, 2016, 04:11:33 PM
I just noticed the VoC on those strings would be 90V so the the 15A breaker likely also wouldn't come into play at 1280 / 90 = 14.2A. So...
Also on that NOTE !!
The folks who use The Classic 200 & 250 units Your Breakers are NOT the same as what the Classic 150 use due to voltages . They also have & sell the same 10 amp breaker in 250Vdc and it the same size & money ..
SO do the MATH & order the correct size & voltages .
Inspectors with real know-how will look & judge .
VT