How can I fuse PV without a combiner box?

[somawheels]

Hi
I have 6X255w 24v solar panels wired in 3 strings of 2.
Currently I only have one 30a PV breaker, which is near the charge contoller end of the PV array. Im unsure how to fuse each string individualy or how necassary this is. I was considering using 3 Inline fuses (one for eachstring), but then began to question the effectiveness of these.
The panels are connected using 4mm mc4 connectors, before being connected to some (over sized) 35mm cable which goes to the CC.

[SunEagle]

Anytime you have more than 2 parallel strings or panels you are required to have OCPD for each string. A combiner box makes it easy by having them all in one place but an inline fuse for each string will work. Not having this fuse will violate the NEC and can cause the wires between your panels and the CC to burn up if a short happens.

[somawheels]

My 4mm interconnecting wires should not burn up, correct? As each panel has an isc of 9A (3X9=27A). Having said that, any poor connections would still be a potential fire hazard though.

[littleharbor]

Not sure how compliant this would be but these are available on fleabay; MC-4 fuse holder.jpg

[ButchDeal]

if you can do two strings of three you wouldn't need a combiner box.

[Sunking]

Arrange your panels 2 x 3 and there is no requirement for fuses and combiners. Your way is just a waste of money, time, material, and labor.

[ButchDeal]

What CC do you have? it sounds like it is not MPPT.

[somawheels]

FlexMax 60 MPPT CC. Look at page 102 of the manual and you will see the efficiency curve. Sorry I can't post on this forum right now, it messes up every time i try a post. It happens very often to me on this forum.

http://www.outbackpower.com/downloads/documents/Make_the_Power/flexmax_6080/owner_manual.pdf

[ButchDeal]

you are losing very little efficiency in the CC and likely a hell of a lot more in wire lose and definitely more in cost of equipment, to run 3 strings of 2. Unless you have VERY short wire runs or very costly heavy gauge wire, you should be running 2 strings of 3.

[somawheels]

Thanks for your help, I was considering using those mc4 fuse holders littleharbor; Is the any reason I should not?
What should the fuse rating for each string be? each PV module has an ISC of 9A.

[somawheels]

What should the rating for each string's fuse be? I assumed it would be about about 1.5X the ISC, But looking at this, I am unsure why it should be so much higher.

[Sunking]

Then you selected the wrong Controller. Sure the Controller would have been more expensive. However it is way less expensive than the combiners, fuses, larger cables, and materials you wil have to spend for 3 strings. Will your way work? Yes. Was it the best solution? No.

Your system is by no means small at 1500 watts off-grid battery. That requires a 60 amp MPPT controller at 24 volt battery. Your system is medium sized.

[somawheels]

I shall call it a medium from now on. I looked at this page

https://www.windynation.com/jzv/inf/how-properly-fuse-solar-pv-system

and dont understand why the fuse is rated based on the wire size and not the PV ISC. surely I should the minimum fuse rating as I can, not the maximum.
Sorry for posting this as a comment, but every time I try to post a reply, something goes wrong; this happens to me very often on this forum.

[Sunking]

I am just trying to save you a buck and optimize the system.

[Sunking]

I am very familiar with your controller. Going from 3 strings to 2 strings you loose 1.3% efficiency, not that significant. However I dare to ask is did you run your Wire Loss calculations between the panels and controllers?????

That needs to be 2% or less. If you used what the minimum manufacture and code recommends may bite you in the butt if longer than 5-feet 1-way. Beyond that and you start taking heavy losses in voltage and power. Not unusual 10 to 15% if the runs are fairly long. So when you go from 3 to 2 strings you reduce current by 33% and thus gain 33% more efficiency or reduce losses by 1/3.

So what I am telling you is you may be better off running two strings. Yeah your controller looses 1.3%, but you may gain 5% in wire losses because you reduced the current. I dare say you only did half a job and fell short. You may not have looked at the whole picture, only a small piece of it.

Lastly, that is a known problem of Flexmax. Midnight Solar and Morningstar Controllers do not loose much efficiency as voltage goes up. But dude at worse case your controller efficiency is pretty good at 95% on the low end with high voltage, and 98% at lower voltages. That is a 3% Delta, but what really counts is the line losses and line losses are dictated by operating voltage, current, and distance on a specific gauge wire. So make sure you are looking at the whole picture. I think you might find you can't see the Forrest for the trees.

[somawheels]

I know you are trying to help, and I am grateful that you do.
Wire losses should not be much as the max current is 27A with an average of around 8A (here in UK). Most of the distance between PV and CC is coverd with 35mm cable. There is 3.5 metres total of 4mm cable interconnecting the 3 PV strings in parallel. Another thing which made me decide on 3 strings was shade: I am surrounded by trees so small amounts of shade can have a large impact on power output.
Also, looking at the flexmax efficiency vs PV voltage vs PV power graph I estimated a greater efficiency loss of around 5%, this is due to UK weather being so cloudy, thus giving me quite low PV power, and therefore putting me on the steep, left side of the graph.