New PV Panels …to fuse or not fuse…?

[JRH]

Having a larger array using a combiner box , each string is fused on pos & neg…. It is 4S-4P …works great…

The new Panels I’m adding are multiple 4 panel PODS…. Each pod is 2S-2P with no combiner box …..each with its own blue CC.. the Panels have 10ga wire from factory.
Each has an IMO isolater switch and a MS SPD located at the array..
The PV cable from each pod is 6 ga tinned copper and 100 ft long back to the gear location ,slightly buried &.contains the bonding wire ..

Each series string (2) in each pod is about 70;volts / & 13a..this will be parrallel with the other string in the pod of the same values..so each paralleled pair of cables going underground from each pod ,back to the gear = about 70 volts & 26a…….

I have read here many times one doesn’t need to fuse a 2 P array…

With that said I have 2 questions…

1) why not..? Is it a money thing..? Does it hurt anything if I did .? Should I ..? It would seem to be beneficial in some cases..but maybe not …!

2) if I did , I was considering using 15 - 20a inline Staubli mc4 fuses on each series string both pos+ neg…

OR WOULD positive wire fusing only be adequate…? 2 fuses

First time with this type build …always used combiner box before , input appreciated..

I really don’t care , but want the best possible outcome when done.

Thanks …….J..

 

[GXMnow]

The PV Solar panels should have a spec for "Maximum Series Fuse Current". If this fuse current rating is more than double the ISC current of the panel, then it is safe to fuse a 2P array with a single fuse or breaker at the combiner box. But on many larger panels, the ISC current might be 10 or 12 amps with a 15 or 20 amp maximum fuse rating. If these cases, you need to fuse each string before they are combined. This maximum fuse rating is set to protect the wires and blocking diodes from possible back feed current. If the entire array is just 2P, then the odds of any situation exceeding the single panel current is very slim. But it is possible a system fault could cause an excessive back feed current. Fuses are designed to protect in the worst possible case. If each 12 amp panel has #12 awg wire, it is likely safe to fuse at 20 amps. But if you parallel 2 of these panels, then the possible current is 24 amps. See the problem? If you fuse it after they are in parallel, then you need a 30 amp fuse, and the #12 20 amp wire is no longer protected from a serious fault. No wire should have a fuse greater than the wire is rated for. And this includes the wires going into the solar panel's junction box. I fused each of my 11 amp PV rows at 15 amps before they go into the combiner box. It's cheap insurance.

 

[JRH]

The PV Solar panels should have a spec for "Maximum Series Fuse Current". If this fuse current rating is more than double the ISC current of the panel, then it is safe to fuse a 2P array with a single fuse or breaker at the combiner box. But on many larger panels, the ISC current might be 10 or 12 amps with a 15 or 20 amp maximum fuse rating. If these cases, you need to fuse each string before they are combined. This maximum fuse rating is set to protect the wires and blocking diodes from possible back feed current. If the entire array is just 2P, then the odds of any situation exceeding the single panel current is very slim. But it is possible a system fault could cause an excessive back feed current. Fuses are designed to protect in the worst possible case. If each 12 amp panel has #12 awg wire, it is likely safe to fuse at 20 amps. But if you parallel 2 of these panels, then the possible current is 24 amps. See the problem? If you fuse it after they are in parallel, then you need a 30 amp fuse, and the #12 20 amp wire is no longer protected from a serious fault. No wire should have a fuse greater than the wire is rated for. And this includes the wires going into the solar panel's junction box. I fused each of my 11 amp PV rows at 15 amps before they go into the combiner box. It's cheap insurance.

Well we are thinking similar thoughts , except your using different wire gauges and #’s than I have in place and making the answer you present slightly different than what I face…

390 watt panels
ISC - 13.7
VOC - 36.4
VMP 30.4
IMP - 12.85
MAX series fuse rating - 25a

As I posted, the panels are new and use 10 ga mc4 wires. I am using 10ga pv extensions to arrive at the 2s2p config…the resulting 2 wires ( pos & neg) then connect to the junction box posts and connect to the 6ga wire running back to where the gear is mounted..100 ft away…

My intention before posting was to fuse both pos wires before combining ..with 20 amp inline Staubli fuse …
I have no preference as to what it should have installed( IF ANYTHING) , I just want it to be done as good as it can be done..cost or labor are not a limiting factor.

How does above #’s change the scenario you present .? Or does it..?

Thank you , J

 

[timselectric]

When a panel or string of panels develops a short circuit, all other panels or strings of panels in parallel, will send all of their current to that short.
The PV panel maximum fuse rating tells how much current it can safely handle (without catching fire).
If your configuration doesn't exceed this rating, no fuse is needed.
If it does, a fuse is required.
The general rule of thumb is 2p is fine and 3p or above is not.
But this isn't always true. Do the math to determine if a fuse is needed.
If so, only the positive needs to be fused.

 

[GXMnow]

Well we are thinking similar thoughts , except your using different wire gauges and #’s than I have in place and making the answer you present slightly different than what I face…

390 watt panels
ISC - 13.7
VOC - 36.4
VMP 30.4
IMP - 12.85
MAX series fuse rating - 25a

And here is the issue using your numbers.

ISC = 13.7 amps. If you run 2 of these in parallel, you have a possible current of 27.4 amps. So you can't fuse that at 25 amps as it could blow the fuse with no fault. So I would fuse each string at 20 amps and not worry about it.

The general rule of thumb is 2p is fine and 3p or above is not.

There is some truth to this as if one panel fails in a short, you only have the current from the one to drive it, so it is all current limited to the ISC current. The problem with this thinking is that it assumes there is no other possible fault condition. The odds are vey rare, but what about the charge controller also shorting battery to PV? Maybe it was a lightning hit that caused 2 things to fail at once. Full battery current could be driven out to a shorted PV panel. It needs to be fused. If it is only fused to protect the combined panel current, we are now above the max fuse current for the panel. Even with just 2 in parallel, I still suggest fusing each string to be safe. And yes, just one end is all you need to stop the current flow in the worst case here.

With just 2P, the odds are very likely, it will never be a problem. But even 0.1% chance, is a chance. With 3P or more, there is no question, fuse each string.

My small DC system is a bit odd as I have 2 different types of panels into a single charge controller. The two rows of 5 x 100 watt panels are paralleled with no fuses. But they are only about 6 amps ISC and the series fuse rating is 15 amps. So I used "Y" connectors and a single 15 amp fuse. The other row is 5 x 200 watt panels with an ISC of about 12 amps. The max fuse rating is 20 amps. I fused that row at 15 amps as well. They each feed a separate #10 AWG drop from the roof to the combiner box. Then they connect together to a 40 amp breaker feeding #6 AWG wire to the charge controller. I can safely add another 1,000 watts of panels into the combiner without a problem.

 

[JRH]

When a panel or string of panels develops a short circuit, all other panels or strings of panels in parallel, will send all of their current to that short.
The PV panel maximum fuse rating tells how much current it can safely handle (without catching fire).
If your configuration doesn't exceed this rating, no fuse is needed.
If it does, a fuse is required.
The general rule of thumb is 2p is fine and 3p or above is not.
But this isn't always true. Do the math to determine if a fuse is needed.
If so, only the positive needs to be fused.

Thank you… I appears in my case the config could slightly exceed the max Panel rating at STC, but up here I get alot of days the panels I have now far exceed the specs on the label for a little while , today about 1 pm I was running about 12% higher for about half hour with the funky clouds and cool temps...I have seen it briefly run up over 20 % this spring…
…… so I will fuse the two pos lines…

I had been running this around in my head for a while and felt I probably needed them.
The final wiring is fixing to happen on two of the pods so it was time to decide…

Appreciate it…

J.

 

[JRH]

ISC = 13.7 amps. If you run 2 of these in parallel, you have a possible current of 27.4 amps. So you can't fuse that at 25 amps as it could blow the fuse with no fault. So I would fuse each string at 20 amps and not worry about it.

That’s exactly what I was thinking… ….I would much rather have to replace a few fuses than the panels or gear…
Murphy is always lurking somewhere at some point for us all.

Thanks ..J

 

[timselectric]

It doesn't hurt to place fuses where they are not required, if it helps you sleep at night.
The least possible number of connection points, is always my goal.
To avoid adding more possible points of heating or failure.

 

[Warpspeed]

Only useful function of a fuse is to protect wiring from over current.

Solar panels are limited to the max rated short circuit current anyway, so if properly sized wiring is used to support maximum available current, fitting a fuse to that is really pretty pointless.

Where there are multiple parallel connected strings, as previously mentioned above ^^^ a shorted string or panel could in theory be driven by a destructively high reverse current from the other functioning strings.
A much better solution to that particular problem would be fitting series diodes to each string before the strings are combined.

Some people are horrified by that idea. The idea of losing one or two percent of power in a diode is just unacceptable.
But they will quite happily put up with much larger power losses than that in the wiring back to the solar controller.

 

[timselectric]

Luckily, most modern SCC's don't require multiple parallel strings, anymore.
With higher voltage MPPT's.

 

[JRH]

It doesn't hurt to place fuses where they are not required, if it helps you sleep at night.
The least possible number of connection points, is always my goal.
To avoid adding more possible points of heating or failure.

I completely agree.. overkill sometimes is my thing….always has been…I took a lot of flack on the docks from thst belief… I was nicknamed Jim Overkill along time ago.
it’s similar to thru-hull fittings in boats .. the more ya have , the odds increase for a failure or problem where one didn’t exist without it… but ya have to some or somthing…ya just have to weigh and balence for what your comfortable with.

I worked on many big boats that had no or few water intake thru-hulls ….only discharge outlets…
They used a giant hole sealed column in the hull called a sea chest that was partly below the waterline and all input water connection were made from that point below the visable water line.

..if there was a problem with flow you knew where it happened and could usually maintain/ repair it underway and standing in the dry engine room.

I feel there is a parallel to that in the world electrical world but I don’t know enough about that type gear to picture it…

 

[timselectric]

I completely agree.. overkill sometimes is my thing….always has been…I took a lot of flack on the docks from thst belief… I was nicknamed Jim Overkill along time ago.
it’s similar to thru-hull fittings in boats .. the more ya have , the odds increase for a failure or problem where one didn’t exist without it… but ya have to some or somthing…ya just have to weigh and balence for what your comfortable with.

I worked on many big boats that had no or few water intake thru-hulls ….only discharge outlets…
They used a giant hole sealed column in the hull called a sea chest that was partly below the waterline and all input water connection were made from that point below the visable water line.

..if there was a problem with flow you knew where it happened and could usually maintain/ repair it underway and standing in the dry engine room.

I feel there is a parallel to that in the world electrical world but I don’t know enough about that type gear to picture it…

Yup
I always tried to limit hull and bulkhead penitrations to the very minimum needed. When I worked in a shipyard building boats and barges.

 

[sunsurfer]

It doesn't hurt to place fuses where they are not required, if it helps you sleep at night.

I re-fuse to comply