Fuses/breakers in parallel strings of panels

vtmaps

I, and many others on this forum have often stated that an array of panels with more than two equal strings in parallel must have fuses or breakers in each string. I think that is a pretty good rule of thumb, and if you follow that rule you can't go wrong. However, it turns out that obeying that rule may lead to unnecessary fusing (=unnecessary expense) in some cases.

I learned this recently while reading through some old threads on this forum and must give credit to 'Crewzer' for explaining this and providing this link:
http://www.nmsu.edu/~tdi/pdf-resources/CC125.pdf

The exception to the rule of thumb occurs when a manufacturer specifies a fuse rating that is much larger than the Isc of a panel. The fuse rating is really the ampacity of the panel.

Example: A panel has an Isc of 5 amps and a fuse rating of 15 amps. To account for cold weather and edge of cloud effects the Imax of the panel is 1.25 x Isc. In this example Imax = 5 x 1.25 = 6.25 amps. If you have three strings of these panels in parallel, the most current that two strings can push through the faulted third string is 12.5 amps. That is less than the ampacity of the faulted string and therefore no fusing is necessary in each of the three strings.

If you are interested in reading more about this, search this forum for "cc125.pdf".

--vtMaps

niel

Re: Fuses/breakers in parallel strings of panels

i agree that there are some ifs ands and buts and sometimes it boils down to thinking, of which i've slipped in that area quite a few times too. take for instance someone with 3 pvs. one is newer at 140w 12v and the other 2 are older 12v at 65w each. it will not be possible for the 2 65w pvs to do anything to the 140w pv so no fuse is needed inline with the 140w pv. i'd go into particulars with the isc and so on, but the overview i set here is obvious for this example for the 140w pv being added to the 2 smaller 65w pvs. now the 2 smaller pvs could be damaged and so need the fusing unless if by chance the combined output of the other 2 would happen to fall under the series fuse rating for that single small pv. we all agree a single pv can be fed it's own isc rating and take it, but that 2 others feeding 2x that isc is a no no, except when the rounding up of the series fuse rating to the nearest commercial value puts the 2x area in range. this is all due to the politics of the nec setting standards. it is none the less a good idea to not allow 2 or more like pvs to gang up on another single pv.

a 5a isc pv should not have a 15a series fuse, nor should a 5a imp pv have a series fuse of 15a as that would not go higher than 10a for a commercial value, but even if it did there would be nothing wrong imo with staying with the lower current fusing if it's a case of rounding up too far. for example of 1.25 x 1.25 x isc = 10.1a or 11a even then this would have a series fuse of 15a and i believe 10a would suffice when that close, but wiles has set rules and he doesn't want to break his own rules and that's what causes the what ifs ands and butts as one can't legislate common sense.

SolaRevolution

Re: Fuses/breakers in parallel strings of panels

niel wrote: »

...a 5a isc pv should not have a 15a series fuse, nor should a 5a imp pv have a series fuse of 15a as that would not go higher than 10a....

Wait.
The purpose of PV fusing is to protect the circuit conductors and prevent them from burning if they are subjected to fault current above their ampacity rating. No? The ISC rating x 1.56 is used to determine the minimum PV output circuit ampacity, but the module's fuse rating is the based on the PV module's internal wiring ampacity.

It is very common for PV modules to have a fuse current rating of 10 or 15 amps even if the ISC is +/-4amps.
What is the purpose of decreasing the overcurrent protection fuse rating below the PV module's listed fuse rating?

Alex

vtmaps

Re: Fuses/breakers in parallel strings of panels

niel wrote: »

this is all due to the politics of the nec setting standards.

Its not an NEC thing... Overcurrent protection for modules is a UL thing. From the code corner article referenced in the OP:

The NEC outlines protection for the wire, while the UL standard addresses protecting the module.

--vtMaps

Cariboocoot

Re: Fuses/breakers in parallel strings of panels

Just to confuse you more, You will see panel manufacturers recommend a "series fuse" of 10 Amps on a panel with an Isc of 7 Amps. Why? Because the maximum current the panel is capable of producing is not the same as the maximum current the panel is capable of taking if its resistance should go low.

But if you don't have the maker's recommended number you have to figure it out for yourself. A "short circuited" panel is not likely to be zero Ohms and take full Isc from two others. Nor is there necessarily full insolation on those panels causing them to produce full current. But if you connect a fuse that's just over the Isc rating then you are probably safe from starting any panel fires should something go wrong. The difference between Imp and Isc is usually about 1 Amp, so it is possible to have two panels put out something near Imp each and have that be above Isc for the third panel. The more panels, the greater the current difference between one "bad" panel and the others acting in parallel so the greater the importance to have fusing.

One of the things I disagree with NEC regs on is the application to solar panels of fuse sizing rules that seem to be based on standard Voltage supplied circuits. No one has to agree with me on this either.

niel

Re: Fuses/breakers in parallel strings of panels

from vtmaps,
"Its not an NEC thing... Overcurrent protection for modules is a UL thing. From the code corner article referenced in the OP:"

true, but the basics of what i said still hold as they too had the pleasure of setting standards that tend to get complex with what ifs, ands, and buts.

ggunn

Re: Fuses/breakers in parallel strings of panels

vtmaps wrote: »

I, and many others on this forum have often stated that an array of panels with more than two equal strings in parallel must have fuses or breakers in each string. I think that is a pretty good rule of thumb, and if you follow that rule you can't go wrong. However, it turns out that obeying that rule may lead to unnecessary fusing (=unnecessary expense) in some cases.

While true as far as it goes, fusing is cheap and as you say, if you follow the rule, you can't go wrong. If you have a string count of anything but three, it's a non-issue, anyway.

vtmaps

Re: Fuses/breakers in parallel strings of panels

ggunn wrote: »

If you have a string count of anything but three, it's a non-issue, anyway.

The code corner article referred to in the OP gives an example:

Some modules have a low short-circuit current and a high maximum series fuse rating. A module with a 1.5-amp Isc and a 20-amp maximum series fuse can have up to 11 strings of modules in parallel without any OCPD.

--vtMaps

niel

Re: Fuses/breakers in parallel strings of panels

"A module with a 1.5-amp Isc and a 20-amp maximum series fuse can have up to 11 strings of modules in parallel without any OCPD."

funny thing is is that you will not ever see a pv with a 1.5a isc that has a series fuse rating of 20a. they are exaggerating severely for illustrative purposes. it would be that 1.56 x 1.5a isc = 2.34a and rounding up to the nearest commercial value will be either 2.5a or 3a depending on what's available. if 3a it may squeak by as being able to take on 2 other like pvs by rules.

ggunn

Re: Fuses/breakers in parallel strings of panels

vtmaps wrote: »

The code corner article referred to in the OP gives an example:

Some modules have a low short-circuit current and a high maximum series fuse rating. A module with a 1.5-amp Isc and a 20-amp maximum series fuse can have up to 11 strings of modules in parallel without any OCPD.


--vtMaps

Well, OK, but I have never encountered anything remotely like that. Is it a real world example?

Cariboocoot

Re: Fuses/breakers in parallel strings of panels

Couple of points:

The fuse ratings given by manufacturers are maximums. Nothing says you can't use a smaller fuse that falls between the Isc rating and the 2X Imp of the other two (or more) panels.

As I said before, the Isc is the most current the panel can produce which is not necessarily the same as the most current it can take.

And it's anything less than 3 parallel connections doesn't need CP, except in cases of mismatched panels where it is prudent to protect lower current panels in the array.

vtmaps

Re: Fuses/breakers in parallel strings of panels

ggunn wrote: »

Well, OK, but I have never encountered anything remotely like that. Is it a real world example?

I think its a Wiles world example. --vtMaps

ggunn

Re: Fuses/breakers in parallel strings of panels

vtmaps wrote: »

I think its a Wiles world example. --vtMaps

...............