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Old 15-12-2016, 00:29   #1
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Fusing for solar panels

4 x 100 W solar panels on arch. One MPPT regulator.

Plan:
Connect all 4 x panels to a junction box, one for minus one for plus.

Connect junction box to regulator. One plus and one minus cable.

Ok to fuse just the plus cable between junction box and regulator, or must I fuse all panels individually = 4 fuses?

What if a panel breaks and get short-circuited internally (likely?)?

Cheers
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Old 15-12-2016, 00:49   #2
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Re: Fusing for solar panels

The solar panel is self current limited. It can not deliver more than what it is spec'd at. You do need battery to regulator fusing, as that wire can take excessive current from the battery if shorted.
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Old 15-12-2016, 01:07   #3
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Re: Fusing for solar panels

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The solar panel is self current limited. It can not deliver more than what it is spec'd at. You do need battery to regulator fusing, as that wire can take excessive current from the battery if shorted.
Thanks, I will fuse between Votronic MPPT regulator and house bank. What if I fuse (better safe than sorry) with one fuse between Votronic and junction box? And no fuses on the individual solar panels? Will do?

Thanks,
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Old 15-12-2016, 04:55   #4
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Re: Fusing for solar panels

I feel it is foolish not to fuse the cable between the solar panel and the the controller. There should never be power cables running inside a boat without current protection between the power source and the cable in case or a short.
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Old 15-12-2016, 05:14   #5
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Re: Fusing for solar panels

No fuse required between the solar panels and the input to the MTTP controller. There is no large source of current. Example: 100W panel - Isc = 5.7 Amps and Imp = 5.4 Amps. There is only .3 Amps difference between the maximum current possible and the maximum power point current that the MTTP controller will be trying to operate at.
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Old 15-12-2016, 09:53   #6
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Re: Fusing for solar panels

To be clear, no fusing is required for less than three strings. Three or more parallel strings can send more current back to the panel than the cabling is rated for.
To understand why fusing is not required, you can short a dollar painal all day long, no harm will come to it.
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Old 15-12-2016, 12:39   #7
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Re: Fusing for solar panels

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To be clear, no fusing is required for less than three strings. Three or more parallel strings can send more current back to the panel than the cabling is rated for.
Doesn't that depend on the panels? And the cabling? And I would hope that the cabling would be appropriately sized for the four (or more) panels, otherwise that would be a really stupid installation.

Fuses aren't generally required in the panel / controller wiring, but switches or circuit breakers may be convenient for working on the system. The controller / battery circuit should definitely be fused or circuit-breakered.
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Old 15-12-2016, 12:49   #8
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Re: Fusing for solar panels

Pretty much what everybody said. You fuse to avoid burning up a wire.

Since your batteries could put hundreds/thousands of amps into a short on the way to the controller, you need a fuse there.

Since your solar panel wiring should be sized for Isc at minimum, there is no way to burn up the wire if it shorts. Heck, nothing even gets hot. You MAY need fuses on the wires to the panels if the current from a shorted set of panel leads cannot handle the maximum power the other panels are putting into a paralleled connection.
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Old 15-12-2016, 18:18   #9
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Re: Fusing for solar panels

I have known folk put multiple fuses in locations where they are not needed. OK, if they want to spend the money, I let them - BUT...

One element of electrical design that is often overlooked is what is referred to as "discrimination". i.e, if we have an energy source (battery) then the fuses that radiate out from there should diminish in size. We may have a 100A, then 50A, then 20A - all OK. If we had a 100A, then 80A, then 100A, then 50A etc, we have a longwinded and painful task of finding the one that blows.
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Old 15-12-2016, 22:45   #10
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Re: Fusing for solar panels

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Originally Posted by sailorboy1 View Post
I feel it is foolish not to fuse the cable between the solar panel and the the controller. There should never be power cables running inside a boat without current protection between the power source and the cable in case or a short.
Over current protection (fuse or breaker) is generally designed to protect the wire.

In the case of a proper solar charging system installation, the energy source (panels) can't produce enough current in a short circuit condition to burn the wire, therefore, no current limiting is required.

As far as installing switches in panel wiring, well, one can install as many future potential failure nodes as they desire. If panel isolation is needed for testing etc. one can just temporarily disconnect an MC4 connector.
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Old 15-12-2016, 23:13   #11
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Re: Fusing for solar panels

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Originally Posted by sailorboy1 View Post
I feel it is foolish not to fuse the cable between the solar panel and the the controller. There should never be power cables running inside a boat without current protection between the power source and the cable in case or a short.
Over current protection (fuse or breaker) is generally designed to protect the wire.

In the case of a proper solar charging system installation, the energy source (panels) can't produce enough current in a short circuit condition to burn the wire, therefore, no current limiting is required.
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Old 16-12-2016, 00:13   #12
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Re: Fusing for solar panels

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Originally Posted by ramblinrod View Post
Over current protection (fuse or breaker) is generally designed to protect the wire.

In the case of a proper solar charging system installation, the energy source (panels) can't produce enough current in a short circuit condition to burn the wire, therefore, no current limiting is required.
True for one string but not true for more than two. Unlikely on a boat.
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Old 16-12-2016, 06:19   #13
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Re: Fusing for solar panels

So what kind of fuse suits best to an inline use between controller and batteries?
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Old 16-12-2016, 06:48   #14
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Re: Fusing for solar panels

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Originally Posted by ramblinrod View Post
Over current protection (fuse or breaker) is generally designed to protect the wire.
It is in aircraft, but I don't think it is in boats
For example, the Sea Era head I just installed, called for 8 Ga wire due to length of run, but I think a 20 amp breaker. Now an 8 Ga wire can hold way more than 20 amps.
I think its common for boats to way oversize wire to prevent voltage drops.
I think boats are often fused to protect the user more than the wire
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Old 16-12-2016, 07:01   #15
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Re: Fusing for solar panels

I use breakers like these: APS 20A Car Audio Inline Circuit Breaker Fuse for 12V Protection SKCB-02-20A | eBay

Fair winds,

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