I'm fusing my solar panels individually in the combiner box before they are combined. Initially, I purchased the 15a MC-4 fuse holder and the connectors. This came out to about 6-1/2" long. Too long for my combiner box with all the other hardware in there. So, can I use any automotive inline type fuse and fuse holder so long as the the fuse size is right and the wire size(10ga) is the same? The reason I ask is the conductor/fuse is real small on an automotive 15a and I always concern my self with not carrying as much juice cause of the small fuse link compared to the 10ga wire? Meaning, is there a "special" solar fuse that carries more juice?Thanks,

fuses are rated for interupt current, interupt time, and arc interuption voltage. that is how much current before it trips, how long an overcurrent takes before it blows, and how high the voltage can be and still interupt the current and not sustain an arc thus not interupting the current flow.AFAIK for solar panels, since most are used in houses and many are interconnected to the house power feed and power line there are spec's for the fuses to be used.Now, when used with a trailer, likely another story. no line connection, no feed to the power grid.Now I am by no means an expert on this application, but if it was me using a 15A auto blade type fuse and fuseholder I would feel comfortable.BTW 14Ga wire is rated for 15A, 10Ga wire is rated for 30A in romex, higher in individual strands. The 10Ga gives lower voltage drop.

You have more than two panels in parallel? What are the panel ratings? link?If you are using MC4 why even have a combiner box? MC4 combiner is fine and you can get #8 MC4 down to the controller if you need it.Otherwise yes any 12v fuse is fine.

Fuse needs to be rated to handle the full voltage of the panel in full sunlight. And for safety I like to double that. So a 24 volt panel would need 48 volt rated fuses (Or at least 36) and DC rated.The only other thing would be the "inline resistance) of the fuse (The resistance pin to pin not blown) likely too low for your meter to measure but I'm not sure how the "Solar rated" fuses compare to standard automotive/house fuses in this aspect.And as someone else mentioned... The fuse's "Speed" Fast. slow. standard. ultra fast) That is how quickly it blows if overloaded.

time2roll wrote:You have more than two panels in parallel? What are the panel ratings? link?If you are using MC4 why even have a combiner box? MC4 combiner is fine and you can get #8 MC4 down to the controller if you need it.Otherwise yes any 12v fuse is fine.I have a couple reasons for a combiner box. First, it came with a Zamp combiner box that was rather butchered up when the dealer installed the existing Solar. There was MC-4 cable wrapped round and round in circles on the roof and in the attic. When I yank the the butchered Zamp box, something needs to go back in it's place and since I'm adding a third panel, that something might as well be a combiner box to tidy up the roof wiring. And I'm not really using any MC-4 Connections. Was going to use an MC-4 Fuse holder but they don't fit in the combiner box and I'm trying to keep all connections in the box.

Not sure I understand why one would fuse a solar panel. What rating fuse would you use? ...the solar panel is simply incapable of sourcing higher current than its maximum output.....a fuse could never blow. The other end is not a current source assuming it is a charge controller, so nothing that could provide current to blow a fuse there, either..... am I missing something?

Is a fuse a fuse? | Good Sam Community

CA_Traveler
Explorer III
Nov 11, 2019
Solar cells fail and short, doesn't take a tree branch or wires shorting.
time2roll
Nomad
Nov 11, 2019
BFL13 wrote:
Need a scenario. Say a tree branch falls on one of your RV roof panels and smashes it all to heck. Perhaps somehow a cell gets shorted.

Cells are all in series with each other, so hard to figure how a short can occur with pos and neg touching. But even if a cell is shorted, what will happen? Shorting the whole panel is harmless. That is how you measure Isc and Voc.
If the panel shorts the other two panels feed current through. If the panel fuse rating is 15 amps and you push 20 amps through the short (10 amps x2 panels) they you might get some heat.
BFL13
Explorer II
Nov 11, 2019
Need a scenario. Say a tree branch falls on one of your RV roof panels and smashes it all to heck. Perhaps somehow a cell gets shorted.

Cells are all in series with each other, so hard to figure how a short can occur with pos and neg touching. But even if a cell is shorted, what will happen? Shorting the whole panel is harmless. That is how you measure Isc and Voc.
time2roll
Nomad
Nov 11, 2019
Ed_Gee wrote:
Has anyone heard of a solar panel short circuiting in its internal power connections? I have not.... just seems like a lit of work for nothing.
Not on an RV. Tesla did recently battle Walmart due to some Solar City installations that caught fire. Lawsuit was resolved and withdrawn.

Doubt the risk is significant at these low voltages. Residential/commercial systems often run 400 to 800+ volts. Big difference.
Ed_Gee
Explorer II
Nov 10, 2019
Has anyone heard of a solar panel short circuiting in its internal power connections? I have not.... just seems like a lit of work for nothing.
TechWriter
Explorer
Nov 10, 2019
Vintage465 wrote:
I'm fusing my solar panels individually in the combiner box before they are combined . . .

OP, I suggest checking out the MidNite Solar Forum. These are the guys who manufacture, test, and install solar equipment . . . actual experts.

The advice you get on this forum may be well-meaning, but it can be mixed, and even outright dangerous . . . for example, one of the current posters suggested that using a piece of wire was just as good as a fuse.
time2roll
Nomad
Nov 10, 2019
Yes with three panels the array (-1) may exceed the individual panel rating.
MC4 is designed to use outside of a box. If you have them and the panels still have the connectors you can certainly use them snapped on the end of the panel connector.

3 to 1 MC4 connector.

Of course to really clean things up you may need to cut the cables and attach your own MC4 at the proper length. At that point you only need a basic box to seal the cable entry on the roof.

Can also get MC4 with #8 wire if you prefer for the run down to the controller. Just cut the cable in half with the MC4 attached to the 3 to 1 adapter and the cut end on the controller.

Just an additional option to consider.
pianotuna
Nomad III
Nov 10, 2019
Hi,

There is no need to fuse the panels at the combiner box. In fact it is undesirable to do so.

A switch after the combiner box close to the charge controller is a good plan.

If using MPPT controller, wire for highest input voltage and use a DC rated switch or circuit breaker.

Vintage465 wrote:
I'm fusing my solar panels individually in the combiner box before they are combined. Initially, I purchased the 15a MC-4 fuse holder and the connectors. This came out to about 6-1/2" long. Too long for my combiner box with all the other hardware in there. So, can I use any automotive inline type fuse and fuse holder so long as the the fuse size is right and the wire size(10ga) is the same? The reason I ask is the conductor/fuse is real small on an automotive 15a and I always concern my self with not carrying as much juice cause of the small fuse link compared to the 10ga wire? Meaning, is there a "special" solar fuse that carries more juice?

Thanks,
BFL13
Explorer II
Nov 10, 2019
It is all here as to when you need fuses or not depending on your solar set-up. Scroll down to the discussion page 6 and on.

http://www.cooperindustries.com/content/dam/public/bussmann/Electrical/Resources/technical-literature/bus-ele-an-10191-pv-app-guide.pdf
CA_Traveler
Explorer III
Nov 10, 2019
Vintage465 wrote:
Some people recommend fusing them. Some don't. Renogy recommends(or shall I say I saw in of their solar "101" info write-ups) fusing each panel. Samlex doesn't mention fusing at the panels. I don't have any issue from a technical perspective about fusing, whether or not it should be. Only from a "does it still charge as well" perspective.
Good plan and no effect on charging.

Many don't understand the possible hazard of a parallel panel catching on fire.

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