PaulRainbow
Well-Known Member
There is no danger to the cable and i never said there was. Now go away and try to learn how it works.
Fuse between solar panels and MPPT controller?
- Thread starter scr0che
- Start date
There is no danger to the cable and i never said there was. Now go away and try to learn how it works.
lustyd
Well-Known Member
Now I’m concerned some lunatic has your password because someone posted from your account about cables being underrated.
PaulRainbow
Well-Known Member
lustyd
Well-Known Member
True dat. Either there's no danger, or we need a minimum 0.5mm cable for the current.Don't worry, there is only one lunatic in this thread
PaulRainbow
Well-Known Member
Just because we allow for current requirements does not mean the wiring will catch fire if the cable is undersized, in the case of a solar installation. If the cables between the panels and the controller were rated lower than the PV output we just wouldn't be able to harvest all of the power that would otherwise be available.
Assuming no fuses:
If you wired your starter motor with underrated cable the first time you turn the key the cable will be overloaded because the motor is pulling an excess load through the cable. It will get hot, melt, catch fire etc.
If you fitted a battery charger and wired it up with undersized cable you would not get the rated output of the charger. The cables would not suffer any ill effects because there is no load, they would pass as much current as they could and be subject to voltage loss. We still fuse those cables, to protect them from shorting as they are connected to the batteries
If you fit a solar installation and use undersized cables to the controller they cannot carry the current and will suffer voltage drop, they will not melt or catch fire. If they short, they still won't melt or catch fire so no need for a fuse here.
For a solar installation current is really only considered for calculating voltage drop, the more current the greater the voltage drop, so you install thicker cable to be able to carry the available current with an acceptable voltage drop. So, in the case of the OP, we need to install 6mm cable if running the cables below deck before paralleling or 10mm if making the parallel connections above deck, to keep the voltage drop to acceptable levels.
If he uses thinner wire he won't get the full yield from his panels, the boat won't catch fire.
I'm done, good night.
lustyd
Well-Known Member
William_H
Well-Known Member
Just as few parting shots. Heavier cable is always better from the point of view of long life in a corrosive atmosphere. Bigger cable more robust. However voltage drop in the cables for a solar panel is not as bad as you might think. Say 100w 5amp panel. 5Amps through 1 ohm of cable is 5 volts dropped. ( 25% to 40% power loss )
No it does not work that way. The 5 volts dropped will reduce the current so volt drop. Better to compare 1 ohm cable to the internal resistance of the panel which is 20 ohms. So this cable resistance might be regarded as losing 5% of power. Not desirable but then not disastrous either.
Obviously if you wire panels in series the current remains the same as one panel but volt drop becomes only half as significant. ol'will (please don't be rude in shooting me down)
No it does not work that way. The 5 volts dropped will reduce the current so volt drop. Better to compare 1 ohm cable to the internal resistance of the panel which is 20 ohms. So this cable resistance might be regarded as losing 5% of power. Not desirable but then not disastrous either.
Obviously if you wire panels in series the current remains the same as one panel but volt drop becomes only half as significant. ol'will (please don't be rude in shooting me down)
rogerthebodger
Well-Known Member
There was a link posted early where the link was looking at a fault condition where one panel in a bank went short circuit and the other feed all it current into that short circuited panel.
I know panels can go open circuit with damaged to the solar cell series connection links. If one cell goes short circuit ther are all the others in circuit so you may loose one or 2 cells short circuit reducing the output voltage will simply reduce.
If a solar cell goes open circuit you will get no voltage and no current flow in either direction.
The only way that a solar panel can go totally short circuit is for the output connections to go short circuit or all cells to go short circuit all together.
What's the possibility of a complete short circuit happening in a correctly installed setup.
As I said early the internal resistance of a panel would prevent the overloading the cables from a solar setup.
This is similar to a crowbar circuit in a power supply that would reduce the output voltage thus current when the power supply tried to supply current over the rated current supply.
I know panels can go open circuit with damaged to the solar cell series connection links. If one cell goes short circuit ther are all the others in circuit so you may loose one or 2 cells short circuit reducing the output voltage will simply reduce.
If a solar cell goes open circuit you will get no voltage and no current flow in either direction.
The only way that a solar panel can go totally short circuit is for the output connections to go short circuit or all cells to go short circuit all together.
What's the possibility of a complete short circuit happening in a correctly installed setup.
As I said early the internal resistance of a panel would prevent the overloading the cables from a solar setup.
This is similar to a crowbar circuit in a power supply that would reduce the output voltage thus current when the power supply tried to supply current over the rated current supply.
lustyd
Well-Known Member
Short circuits aren't the only potential problem though.
In your scenario above if all of the power of the good panels is running through one panel, the power has to traverse cable to get there. If that cable is not rated to carry all of that current there will be a problem. My panels came with 6mm cable and MC4 connectors. If I connect up 50 of them using the handy connectors I'll be over the max current of that 6mm cable.
In your scenario above if all of the power of the good panels is running through one panel, the power has to traverse cable to get there. If that cable is not rated to carry all of that current there will be a problem. My panels came with 6mm cable and MC4 connectors. If I connect up 50 of them using the handy connectors I'll be over the max current of that 6mm cable.
PaulRainbow
Well-Known Member
So, where are you going to fit your fuse and what will it be rated at ?
rogerthebodger
Well-Known Member
So what are the other potential problems
Again I ask "what is the likely of a panel going completely short circuit in normal use"
PaulRainbow
Well-Known Member
If power was going from one panel to another it would be going through the same cables as it normally went.
How does the current from one panel get paste the blocking diodes in the faulty panel ?
I ask again, where will you fit your fuse and what will it be rated at ?
noelex
Well-Known Member
Solar panels come with bypass diodes, not blocking diodes.
The link I posted earlier shows how to determine if fuses between the solar controller and panel are required and if required the link shows the correct position . The correct value should be listed on the specifications for the solar panel “maximum series panel fuse”.
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PaulRainbow
Well-Known Member
I was specifically asking a particular poster where he would fit the fuse in this particular installation.
This is now post #94, post #18 is still correct.
lustyd
Well-Known Member
It's you that continually talks about short circuits. The liklihood of any fault is very low, yet here we are fitting fuse panels and RCDs all over the place. As I said, fundamentally if too much current is passed over a wire it will get hot and potentially cause issues, so we fit fuses. I'm not saying anything unusual there, but for some reason you're all treating me like a heretic because a well designed system which is never changed and never has issues won't need a fuse. What I'm saying is that well designed systems have a working life and things can and do change. I'm also saying that an extremely cheap, easy and common solution will change the "probably won't" to "can't" happen.
lustyd
Well-Known Member
Pretty much anywhere on the circuit to the panels, although by convention probably on the positive near the controller, rated at 80% of the cable rating, same as in any other circuit.
rogerthebodger
Well-Known Member
You can have both bypass and /or blocking diodes.
Blocking diodes are to prevent the back flow of current from the battery to the panel at night mainly when no regulator is included but are generally fitted even when regulators are included
Blocking Diode and Bypass Diode for solar panels
PaulRainbow
Well-Known Member
For the third time, in the OPs installation, where will you fit your fuse and what will it be rated at ?
lustyd
Well-Known Member
Calm down Paul, you're like a rabbid dog on this thread. I answered this above. I've not been talking about the OPs setup, I was discussing the wider question of whether a fuse should be fitted to an electrical circuit. The answer is an obvious yes unless you can guarantee that for the life of the cables there will not be too much current. You can't guarantee that as I've explained, so a cheap, simple, and convenient fuse makes sense. As I said, it's less than a pound and most of us have a box full on board anyway.
PaulRainbow
Well-Known Member
Then it's a total waste of time now, or in the future. It has nothing to protect now and if someone adds more panels it will be in the wrong place and be incorrectly rated.
When installing almost any electrical system, you design it for now, if someone changes things it may mean uprating or changing other components. You have no way of knowing what panels someone may add in the future, so you cannot instal fuses in advance to protect against such additions.
If you were installing new battery and distribution cables, it might be different. I commonly fit overrated cables to distribution centres, because you don't know what might be added in the future and having cabling to allow for those additions saves having to replace cables just to add a few new things. But the master fuse can be pre-sized to allow for this too, by sizing it for the cable. The same logic does not apply with solar arrays.
