Solar Panel Fuses on 400w RV system

On the blueprint page for the "400w w/ alternator charging" about 3/4 to the end, Will recommends installing 20-30 amp fuses on each panel before the branch connector:

"Because this blueprint unfortunately requires parallel connection of the solar panels (which I dislike, but the charge controller voltage is quite limited), then you need to add a inline mc4 fuse to your individual solar panels. This is not the case in series strings. But because this blueprint requires so many solar panels in parallel, adding a inline mc4 fuse is very smart to do."

Can someone explain this a bit more? The blueprint recommends an MPPT DC-DC charger (12v, 50a). If the charger is rated for 50a and I put 30a fuses on each panel, won't that allow 116a [(30-1)x4] to come through before a fuse is blown? At that point, won't the charger either break/overheat or render the fuses null with it's own safety features because there's 116a coming to a 50a charger?

Will also says to get 20-30a fuses, but when I click on the link to amazon, I have to chose 20 or 30...so which is it? Does it matter?

Sorry if this has been addressed already, I'm new and still wrapping my head around this stuff. Thanks for patience and to Will for making this stuff accessible.

If the total pv module current does not exceed the max circuit fuse rating of any module or conductor rating, you dont need a fuse in the pv source circuit. For most residential panels thks is 2 strings and less than 20A. Always need to check specs.

There is need for it where combining strings or modules in parallel produces higher current than a component in that line can tolerate during a fault.

Hey Kernel, thanks for the response.
So if I'm planning a system on an RV based on the above blueprint (which requires wiring panels in parallel), I should get the fuses. 20a or 30a?

pkamps said:

On the blueprint page for the "400w w/ alternator charging" about 3/4 to the end, Will recommends installing 20-30 amp fuses on each panel before the branch connector:

"Because this blueprint unfortunately requires parallel connection of the solar panels (which I dislike, but the charge controller voltage is quite limited), then you need to add a inline mc4 fuse to your individual solar panels. This is not the case in series strings. But because this blueprint requires so many solar panels in parallel, adding a inline mc4 fuse is very smart to do."

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I may get kicked out of this forum for saying this, but "you need to add a inline mc4 fuse to your individual solar panels" is bad advice that gets repeated without being examined. I recently addressed this very question in a different thread.

My response: No, there's no need for fuse on the solar side of wiring for low voltage panels. A set of panels can't magically produce more Amps than they are designed for, unless the Sun goes supernova. You're already protected on the battery side of the controller with a breaker/fuse properly rated. Simply add a automotive type switch on each solar +ve so that you can disconnect each panel, if needed. For residential, grid-tie type of installations, local electrical codes may or may not require fuses on the solar side; and whatever they dictate stands, not what I (or the rest of the forum) recommend.

@fat_old_sun, your quip about the sun going supernova was what I thought when I read the recommendation to use fuses.
To build on this:
If I have 4x 12v 100w panels wired in series to a 12v/ 50a controller, I shouldn't need the fuses because each panel puts out ~6 amps during peak sun hours. (6 amps per panel) x (4 panels) = ~24 amps. Controller is rated for 50 amps. So unless the sun's output roughly doubles (supernova), I don't need fuses coming from the panels.

Does this jive?

pkamps said:

@fat_old_sun,

Does this jive?

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Yes. A downstream fuse of 25 to 30 Amps on SCC to battery + segment is more than adequate. A simple switch on the solar +ve wires makes the task of isolating the panels easier in case you want to troubleshoot some issue.

While I'm bucking conventional "wisdom", let me share something else that should make your system reliable. Do not combine the panels on the rooftop using MC4 Y connectors (or any other device). Cutoff the MC4 terminals from the panels (you *will* void the warranty) and hard crimp pairs of wires to each panel. Bring the 4 pairs to the spot where you have the SCC. If you're running a PWM type controller, you can get away with using thinner gauge (AWG 12, 14, or even 16, depending on the run length) since you're not boosting current by combining the panels on the rooftop. Voltage loss is not an issue, since the controller is going to reject it anyway. With this approach, you're essentially accepting the 'weakness" of a PWM controller and exploiting it for your benefit. If you're using a MPPT, you have to run thicker cable; in this case series connection on the rooftop is recommended.

Label each pair " driver front,", "driver rear" etc. Add a switch to each +ve. Combine the +ves & -ves using separate busbars and then take a single pair of thicker gauge to the SCC. The advantage of this approach? - you'll never have to climb on to the roof to examine the MC4s.

My advice for small RV based solar is to put reliability/redundancy before efficiency. Go with two inexpensive PWM SCC instead of one more expensive MPPT controller. When you're boondocking 20 miles from the nearest road and have to rely on yourself, it makes a big difference.

The fusing on the panel side isn't to protect the controller, it's to protect the wiring of the panels. Code says any time there are 3 or more parallel panels or parallel strings of panels, each panel or string must be individually protected by a fuse. This is so that if one panel or string of panels shorts, the remaining panels or strings don't dump their current into the shorted panel and melt its wires.

Granted, this code is per ABYC and IEEE, so doesn't strictly apply to mobile systems, but the principle -and the science- remains the same.

Edit: Please keep in mind that fusing is nearly always there to protect the wiring of the circuit, not the device attached to it. That seems to be being overlooked in this thread.

Justin Laureltec said:

The fusing on the panel side isn't to protect the controller, it's to protect the wiring...<snip>

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Per those codes, if we have multiple parallel strings, are we protecting with a CC or midget fuse? Hoping midget because I already bought.

FAQ AutomationDirect.com "CC fuses are current limiting fuses that have one end "pinched down". ... Midget fuses are defined as supplemental fuses and are not rated for current limiting"

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My build example; Four 100 amp solar panels in parallel. Each panel short circuit amperes is 5.75A. Wires to combiner are 10 awg, good to 60 amps. No problem there. But if one solar panel has damage and shorts, the other three panels will send 17.25 amperes into that short and effectively shut down my charging circuit. By having a 15 ampere fuse on each panel, the defective panel's fuse will open and the three panels will continue to charge my battery. The problem with mc-4 fuse holders? They are a bear to replace or even check the fuse. They suck. I prefer a fuse block on the positive side. I use this with the load terminals for the panels and the battery lug for the combined cable to the CC. https://www.bluesea.com/products/5045/ST_Blade_Compact_Fuse_Blocks_-_4_Circuits That's the last of the free advice today. I'm feeling drained.

Justin Laureltec said:

The fusing on the panel side isn't to protect the controller, it's to protect the wiring of the panels. Code says any time there are 3 or more parallel panels or parallel strings of panels, each panel or string must be individually protected by a fuse. This is so that if one panel or string of panels shorts, the remaining panels or strings don't dump their current into the shorted panel and melt its wires.

Granted, this code is per ABYC and IEEE, so doesn't strictly apply to mobile systems, but the principle -and the science- remains the same.

Edit: Please keep in mind that fusing is nearly always there to protect the wiring of the circuit, not the device attached to it. That seems to be being overlooked in this thread.

Click to expand...

I have been up and running since November 7 2020 but just had a 30 A fuse blow between the PV and the Growatt. My PV array is 3 panels in parallel wired to a fuse box with a fuse for + and one for -. I did not identify which one blew but regardless now I am worried since I don't know if each panel was/is individually fused. I had electricians install and wire the panels as I'm 79 YO and don't care about ladders.
Do I have a real problem here requiring rewiring with separate fuses?
Thanks ahead for the help, Eric

to Justin Laureltec,
I have been up and running since November 7 2020 but just had a 30 A fuse blow between the PV and the Growatt. My PV array is 3 panels in parallel wired to a fuse box with a fuse for + and one for -. I did not identify which one blew but regardless now I am worried since I don't know if each panel was/is individually fused. I had electricians install and wire the panels as I'm 79 YO and don't care about ladders.
Do I have a real problem here requiring rewiring with separate fuses?
Thanks ahead for the help, Eric

I haven't seen Justin on the forum for a while. He may not respond.

What are the specifications of your solar panels? Do you have only three panels?

Thanks, yes 3 panels. They are 24V 320W each. Under Nominal Module Operation each one puts out 30.9 V and 7.73 A.

Open circuit V is 37.7 and Short circuit current is 8.18 A. These numbers are higher for Standard Test conditions which is performed with
irradiance of 1000W/m squared. I mention this because I checked the sun output from weather data and yesterday at 1:00pm
it was a 1,025.20 W spike!

After replacing the fuses everything seems normal but a blown fuse is of course NOT.
Thanks,
Eric

Worst case 8.18 amps x 3 panels = 24.54 amps. The usual 25% bump gets you to 30.675 amps, which is slightly higher than the 30 amp fuse you're using.

I would be wary of going to a 35 amp fuse. Before doing so, check connections, especially at the solar charge controller. It's interesting that you had a spike over the standard irradiance.