Question regarding inline MC4 Fuses

[Solar User]

Hello to all,

I am a relative beginner to the solar world and I am in the process of setting up my first basic station. I am working on a diagram to help with what I need but I am a little confused about inline MC4 fuses and their purpose. I assumed that you would put a fuse inline right after the panel that would be the rating of the wire that you had running from the panel to the charge controller but when I look at the ads for the inline fuses, they state that they are used to protect the panels. As an example, if I have a 100 watt panel connecting to a run of 10AWG wire (30 amp rating), wouldn't I want to put a 30 amp inline MC4 fuse where the panel hooks to the run of 10 AWG wire (in order to not exceed the 30 amp rating of the wire)? Is there a way for power to run to the solar panel from the charge controller instead of from the solar panel? Thanks for any clarification on this.

By the way, in case it helps, I am trying to install a solar charge controller (thinking Victron 150/45) inline to 12.8V Power Queen Batteries. So far, I am thinking that I will be running 10AWG PV wire from a solar panel (or series of solar panels) to a 40A switch (https://www.amazon.com/dp/B09BQP924...788459f3&language=en_US&ref_=as_li_ss_tl&th=1) and then more 10AWG PV wire to the Victron. 10AWG from the Victron to the battery or batteries in series (probably with a 30amp fuse inline between the controller and battery on the positive side). Please let me know if anybody sees any issues with this setup.

 

[hwy17]

You don't need a fuse for 1 panel or 1 string of panels. You don't even need a fuse for 2 strings of panels. When you parallel 3 or more, you have to fuse.

You should fuse the battery circuits.

 

[Solar User]

You don't need a fuse for 1 panel or 1 string of panels. You don't even need a fuse for 2 strings of panels. When you parallel 3 or more, you have to fuse.

You should fuse the battery circuits.

Thanks for the information. It is most helpful as I was wondering what I was missing. Others were mentioning the need for this inline fuse but I think they were using different solar charge controllers.

 

[Norwasian]

You don't need a fuse for 1 panel or 1 string of panels. You don't even need a fuse for 2 strings of panels. When you parallel 3 or more, you have to fuse.

You should fuse the battery circuits.

I understand what you are saying to apply to the protection of the solar panels, not the wire--and I agree with that part of the theory. However, the need to protect the wire would be in the case of something unexpected; whether an equipment failure could cause such a surge, I do not know, but perhaps someone working nearby the wires, or a rat, etc., could cause a short between strings or something.

I don't suppose an MC4 fuse would offer any protection against lightning, but for other mishaps, would it not still be a good idea to protect the wires themselves with a fuse? It seems that fuses are intended to protect against the unexpected--and it does not seem as if it would be harmful to use them.

I'm open to learning. It's all good in theory, but how things work in practice is more meaningful.

 

[seneysolar]

You don't need a fuse for 1 panel or 1 string of panels. You don't even need a fuse for 2 strings of panels. When you parallel 3 or more, you have to fuse.

Huh?
What is the logic here?

A dc wire carrying up to 150vdc, even with 1 string of panels is a potential hazzard under normal conditions. Why do the panels spec a max series fuse rating?

 

[hwy17]

What is the logic here?

Think about it with a single series string first. A fuse is to protect against overcurrent. The string is designed to deliver it's maximum current into the charge controller. How are you going to fuse against overcurrent? If the string can deliver 12 amps of current, and you put a 10 amp fuse on it, your fuse will just blow while the array is producing. If you put a 15A fuse on it, the fuse will never blow even if you short out the PV positive and negative inside your charge controller, because the panels can only deliver 12A, the short can keep melting and sparking and the fuse will allow it.

Once you understand this conundrum, then you can move on to considering external power source shorts. An external power source could be paralleled PV, AC power from grid or inverter, or battery DC, or grid DC. I don't consider the parallel strings problem a lot because I don't plan to have any yet, but the maximum fuse rating basically says that one series string can short into another and it can take it.

Say your battery shorted into your PV wires. Would 48v even drive over 15A into your 12A output array in order to blow your 15A PV fuse? This is more beyond my knowledge, but I think because of the dynamics of PV cells it couldn't. You'd need two shorts, battery onto PV wires, and PV + short to - at the array, and then maybe the resistance of the PV wires would keep your battery current below the battery fuse, but above the wire rating, and then you'd want a pv wire fuse to break it.

 

[wpns]

Panels are inherently limited to Isc, I can’t imagine a scenario where a fuse would be useful.

I do have 600V fuses on a commercially installed array at a combiner box, but since it was installed in the days before RSD I just use them as disconnects.

Code may require them (check with your AHJ) but from an engineering perspective they are just another potential failure point.

 

[dstra1]

Isn't the purpose of the inline fuses for parallel setups? Simplistically, I was under the impression if you had 3 parallel strings and there was a failure of a panel in one of the strings the other two strings are going to send the voltage to the bad string where it potentially could cause a fire by overloading a panel in the 'bad' string. You would only fuse the end of each string, so in this case you just need 3. Or am I missing something?

 

[seneysolar]

Maybe its just me but I fuse any PV circuit. Fuses are for all the "what if" scenarios with fault isolation being the important part.

I don't trust the charge controller being the only stop between the batteries and the panels, I fuse both sides of the CC. Just my personal preference I suppose.

 

[Mattb4]

My experience with inline MC4 fuses are unless you buy Staubli brand you just have a point of bad connection. They seem to do more harm than good. My advice if you have multiple PV strings is to wire them through a combiner box.

 

[seneysolar]

Agreed.
Let me clarify by saying i use Midnite pv breakers for all pv circuits through a baby box.
Not a fan of the inline fuse holders.

 

[hwy17]

Isn't the purpose of the inline fuses for parallel setups? Simplistically, I was under the impression if you had 3 parallel strings and there was a failure of a panel in one of the strings the other two strings are going to send the voltage to the bad string where it potentially could cause a fire by overloading a panel in the 'bad' string. You would only fuse the end of each string, so in this case you just need 3. Or am I missing something?

That's correct as far as I understand it. Here's my Schneider's installation advice for fusing:

 

[Norwasian]

That's correct as far as I understand it. Here's my Schneider's installation advice for fusing:

View attachment 200880

The way I reason it is this:

Imagine each "string" of panels as exerting a pulling force on a "rope", tug-of-war style. If all strings are working the same amount, and on the same side, they pull together, and move together--like horses harnessed together. Now, supposing someone has four strings of panels, and one string (or even just one panel of one string) gets shaded: now the shaded string is not pulling with the others...it has stopped. The other three strings now pull against that disabled string, and will overpower it, likely damaging it as the electrical force exceeds the resistance limits of the diodes' base bias (remember that solar cells are made of a type of diode).

If there is only one string, there is no potential for this kind of competition. If there are two equal strings, one pulling against the other would be like an equally matched tug-of-war, and no damage should occur. Only with three or more strings is there a potential for this type of damage. If they are single-panel strings, I have heard that three such panels in parallel would be unlikely to present a problem, i.e. any single panel would be capable of resisting the nearly double its own potential. But I would fuse them all to be safe.

 

[hwy17]

The way I reason it is this:

Imagine each "string" of panels as exerting a pulling force on a "rope", tug-of-war style. If all strings are working the same amount, and on the same side, they pull together, and move together--like horses harnessed together. Now, supposing someone has four strings of panels, and one string (or even just one panel of one string) gets shaded: now the shaded string is not pulling with the others...it has stopped. The other three strings now pull against that disabled string, and will overpower it, likely damaging it as the electrical force exceeds the resistance limits of the diodes' base bias (remember that solar cells are made of a type of diode).

If there is only one string, there is no potential for this kind of competition. If there are two equal strings, one pulling against the other would be like an equally matched tug-of-war, and no damage should occur. Only with three or more strings is there a potential for this type of damage. If they are single-panel strings, I have heard that three such panels in parallel would be unlikely to present a problem, i.e. any single panel would be capable of resisting the nearly double its own potential. But I would fuse them all to be safe.

But is it a concern in shading if the paralleled series array are all identical, so two strings won't push any more voltage into the one string than one string could, so more current should not flow.

Edit: Oh maybe current flow in that scenario is not actually voltage limited, but supply current limited. I can sort of see that.

I'm still not sure exactly what failure modes the fusing protects against. But luckily nowadays with 600v systems, I don't think I'll ever have to consider it, because two strings would already be comfortably over paneled.

I do like the idea of doubling my 3.8kW planned string into 7.6kW 2p8s one day. With the 100A 4.8kW charge controller it would make a nice consistent flat top production curve. Especially with 1 string facing south and the other facing west.

 

[Norwasian]

But is it a concern in shading if the paralleled series array are all identical, so two strings won't push any more voltage into the one string than one string could, so more current should not flow.

In my own mind, I compare to various sizes of resistors. One can have a small 32-Ohm resistor, or a rather large one...and what is the difference? the amperage it can handle. (Just google for images of 32 Ohm resistors and compare the various sizes.) The photovoltaic panels have their own internal resistance, but it can be overpowered, even as a resistor can get burned out if handling too much current. But I'm not an electrical engineer, so I won't claim to have perfect understanding of the matter.