This weekend, weather permitting I'm hoping to install 5 100 watt solar panels on the roof of my fifth wheel. The controller I'll be installing is a Bogart SC-2030 working in conjunction with a trimetric 2030. I have 10 AWG wiring I'll be running to a junction box on the roof for a parallel connection, then drop down to the battery compartment using either 6 AWG wiring on 4, depending on the route I'm able to take and how long that ends up being. My initial thoughts were to fuse each panel using a small automotive fuse block and 7.5amp fuses in the junction box, but now that I'm thinking about it, is there any reason to do this? Looking over the panel specs, the short circuit output for the panels isn't much more than the normal nominal output which would more than likely result in any fuse I used that would handle the normal max output not blowing even if my wiring shorted straight to ground. Are there any gains to be had that I'm not thinking about? I'll put a switch in line between the panels and charge controller so that I can isolate it if desired, and I'll fuse the output side of the controller as I can see where there could be issues there a fuse would provide protection for.I'm open to thoughts, ideas, or lessons learned by others the hard way.

Searching_Ut wrote: My initial thoughts were to fuse each panel using a small automotive fuse block and 7.5amp fuses in the junction box, but now that I'm thinking about it, is there any reason to do this?Not that I've ever considered.

I dont think its neededi have almost 700w my fusing is at the input to the controller,NOT on each panel

MrWizard wrote:I dont think its neededi have almost 700w my fusing is at the input to the controller,NOT on each panelI guess my primary question is why fuse the input side of the controller at all. If the fuse is sized large enough to handle the peak power output of the panels, it will more than likely be large enough it won't blow even with a dead short. There isn't really any "Current pool" waiting in solar panels to dump in the case of a short like there is in a battery, or other type power supply. Being a PWM type controller, when the batteries demand it, the controller pretty much passes full panel output straight to the batteries anyway with the possible current flow going up less than an additional amp per panel under dead short when you take the battery resistance out of the circuit. On the output side of the controller, you could get a large current flow from the battery back to controller if something shorted in the controller, so it makes sense to me to fuse there, I just can't envision any situation on the solar panel side of the circuit where a fuse would protect anything, or add any safety to the circuit.

Searching_Ut wrote:I guess my primary question is why fuse the input side of the controller at all. I don't know such a reason. For a convenience, you may have just a switch on the input side, in case if you need to work on the controller. On the controller output you need a fuse or breaker.

From the Rogue MPT-3048 Manual wrote:Pursuant to National Electric Code requirements (and others which may have jurisdiction in your locale), disconnects and/or overcurrent protection devices are required for both the input and output of the MPT-3048. It is recommended that the PV overcurrent protection device be rated for 30 amps to comply with NEC requirements.So, as I wanted a switch anyway on the solar inputs to the controller, I used a 30A breaker. It's right next to the breaker controlling output going to the batteries.No idea why NEC wants it.

Fusing Solar Installation | Good Sam Community

red31
Explorer
Apr 28, 2016
Does array max Isc exceed continuous current rating of conductor selected?

http://www1.cooperbussmann.com/pdf/9df1f7ec-8c62-4210-8cf8-9504927394f0.pdf
page 5
2oldman
Explorer II
Apr 28, 2016
Perhaps this is another benefit of series panels. If one shorts.. what happens? Seems it's now just a conductor.
Searching_Ut
Explorer
Apr 28, 2016
KD4UPL wrote:
...The fuse is to protect the panels wired in parallel.....

Your input is very much appreciated, and has made me decide to stick to the original plan with a fuse block in the combiner box. I'd already purchased all the bits and pieces, but then started questioning myself as to why add the additional hardware instead of keeping it as simple as possible.
KD4UPL
Explorer
Apr 28, 2016
The parallel string fusing required by the NEC in the combiner is not to protect the wires. Many people think this, realize it's ridiculous, and eliminate the fuse.
The fuse is to protect the panels wired in parallel. If one of them shorts internally the others will try to feed their full Isc thru the shorted panel. If your panels are 5.5 A Isc and 1 of them shorts the others could try to send 22 amps thru the shorted panel. This is the reason for the fuse.
Any installation we do on a home with more than 2 parallel strings has to have fuses or it won't pass inspection.
Almot
Explorer III
Apr 27, 2016
MEXICOWANDERER wrote:
Is not the purpose of fuses between panels and controller to prevent the aggregate total of potential panel amperage from a worst case scenario of overloading a single branch circuit to ground - a direct short?

Seems logical placing the fuse close to the panels then. Not necessarily "between" the panels - only where the cable for the whole array begins.
MEXICOWANDERER
Explorer
Apr 27, 2016
Is not the purpose of fuses between panels and controller to prevent the aggregate total of potential panel amperage from a worst case scenario of overloading a single branch circuit to ground - a direct short? A thousand watt short is not pretty with 10-gauge brand cicuitry wiring.
pigman1
Explorer
Apr 27, 2016
Beside the fuse ability to protect the panel which is very minimal, having a fuse at the combiner box gives you the ability to drop off one or more individual panels to check the output of individual panels or to cut off a panel that's not performing properly,
pianotuna
Nomad III
Apr 27, 2016
I would use bigger wire from the roof to the controller. #6.

I would have a disconnect near the controller input side.

I would fuse the controller to battery side as close to the battery bank as possible.
red31
Explorer
Apr 27, 2016
Searching_Ut wrote:
is there any reason to do this?

if one panel's wire shorts, all the current from the 5 panels can feed the short if unfused.

all 5 may not exceed the limit of 10g panel wire or a 30A fuse.
brulaz
Explorer
Apr 27, 2016
From the Rogue MPT-3048 Manual wrote:
Pursuant to National Electric Code requirements (and others which may have jurisdiction in your locale), disconnects and/or overcurrent protection devices are required for both the input and output of the MPT-3048. It is recommended that the PV overcurrent protection device be rated for 30 amps to comply with NEC requirements.

So, as I wanted a switch anyway on the solar inputs to the controller, I used a 30A breaker. It's right next to the breaker controlling output going to the batteries.

No idea why NEC wants it.

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