Fuses in Solar panel system

[asdex]

Hi, I have found a bit more information and it looks like you don't need to earth the negative in an off grid system which mine is. In that case, can I just use three inline fuses for each set of panels in series? I have three sets of two panels. I don't really want to take the three strings to the junction box as I am planning to use a 3 way MC4 splitter so from the panels will only have a single positive and a single negative. My PV array current will be 26 amps @74 volts max.
Thanks,

[SunEagle]

Hi, I have found a bit more information and it looks like you don't need to earth the negative in an off grid system which mine is. In that case, can I just use three inline fuses for each set of panels in series? I have three sets of two panels. I don't really want to take the three strings to the junction box as I am planning to use a 3 way MC4 splitter so from the panels will only have a single positive and a single negative. My PV array current will be 26 amps @74 volts max.
Thanks,

Two things you need to consider. Wire size and ease of disconnecting panels under load.

First wire size. Is the single pos and neg wire going to the junction box big enough to handle the total current of all the panels as well as keeping the voltage drop at or below 2%.

Second is ease (or safety) being able to disconnect a set of panels under load. Those MC4 fittings are real easy to put together but can sometimes be tough to quickly disconnect which can cause an arc that can damage the MC4 fitting & wire or give you an electric shock. An "in line" fuse usually does not have a way to quickly open the circuit which can also result in an electric arch.

That is why having the fuse on a swing arm or circuit breaker in the combiner box will protect the wire from over-current and allow you to safely disconnect panels under load.

Here is picture of my combiner box and fuses. They have be designed to open under load as well as covering the energized parts.

I also have #10 gauge wire that is no more than 9 feet between the combiner box and charge controller which is big enough to handle the 20 amps from the panel and is about right for the 2% VD.

PV Combiner box.jpgBehind PV panels.jpg

[asdex]

Ok thanks, so it looks like you have four panels and the positive of each panel goes though a circuit breaker in the combiner box. You have no inline MC4 fuses. It looks like your four panels are connected together in parallel. Can you tell me what is the item to the right of the breakers? I see you also have the negative earthed. Does that go to an earth stake?
Thanks for your reply and photos,
Cheers,

[SunEagle]

Ok thanks, so it looks like you have four panels and the positive of each panel goes though a circuit breaker in the combiner box. You have no inline MC4 fuses. It looks like your four panels are connected together in parallel. Can you tell me what is the item to the right of the breakers? I see you also have the negative earthed. Does that go to an earth stake?
Thanks for your reply and photos,
Cheers,

The item to the right of the four fuses is a 5th fuse so I can add a 5th panel. The negative wire from the panels is also connected to the green ground wire which gets connected to an earth stake.

[asdex]

The item to the right of the four fuses is a 5th fuse so I can add a 5th panel. The negative wire from the panels is also connected to the green ground wire which gets connected to an earth stake.

Ok thanks, I guess as I have two panels in series then the three sets in parallel I will need something to protect the individual panels in series? Would an inline fuse for each set of panels in series then three circuit breakers be ok? Or if possible three inline fuses for each series panels then two circuit breakers for the total positive and negative before they run to the controller in the house?
Cheers,

[ILFE]

I guess as I have two panels in series then the three sets in parallel I will need something to protect the individual panels in series?

No.

Would an inline fuse for each set of panels in series then three circuit breakers be ok? Or if possible three inline fuses for each series panels then two circuit breakers for the total positive and negative before they run to the controller in the house?

You are overthinking this. Just have one breaker for each set (pair) of modules (panels) in series, at the combiner box. So, three sets (pairs) of modules (panels) in series, each with two wires going to the combiner box. Attach the negative wires to the negative bus bar and each of the positive wires to their respective breakers.

mnpv3open.gif