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Hondo avatar image
Hondo asked

overcurrent protection between PV and MPPT

After spending hours going through the forum, I still have my question. For an installation on a sailboat with a fiberglass hull, why is a double pole circuit breaker required in the PV+ and PV- wires between the panel and the MPPT? The DC distribution system is grounded with the battery negative to the main grounding plate in the hull.

Not only that, but why is overcurrent protection needed at all? The panel fault current is much lower than the MPPT allowable input current, and the wires are oversized even for the MPPT input current rating.

MPPT Controllers
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snoobler avatar image
snoobler answered ·

Not sure about marine requirements, and I can't imagine why double pole breakers would be required unless the Voc is very high, but I can answer the last part of the question:

Fuses and breakers (OCP) exist mostly to protect wiring from sources. It's not relevant that the MPPT input current rating is higher.

In an array of like panels, you only need OCP when 3 or more strings are in parallel because the array current can exceed the current rating of a single panel, and the panel itself would fail. At at point, you need fuse/breaker on each string. A 2P array needs no OCP. A 2S2P array needs no OCP, but any array with 3P or greater needs OCP on each string.



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Al avatar image
Al answered ·

If it's a single panel at low voltage maybe it's hard to justify, but panels on a boat are almost always touchable, boats sit in a conductive liquid, and are often wet all over, even if it's fibreglass, and in the rare chance of a short circuit to the frames, there's still a possibility of someone touching and being a conduit or something happening (Fire) that doesn't need to with breakers. As mentioned above they're also handy for disconnecting during changes and fault finding. So it's good to install them, but maybe not necessary depending on your local regulations. ABYC/ RCD

I Installed DC MCB's on our arrays on our boat, but they are on a steel boat @ 180v with a decent current I don't want to risk messing with.

This is for home installs in the UK, but still a useful guide for best practice considerations, then add on top it's on a boat.

Guide_to_the_installation_of_PV_systems_2nd_Edition.pdf


guide-to-the-installation-of-pv-systems-2nd-editio.pdf (929.7 KiB)
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Hondo avatar image Hondo commented ·
Yeah, all of that makes sense. Thanks very much for the document.
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Hondo avatar image
Hondo answered ·

I opted against arranging any panels in parallel. I have a separate MPPT for each panel. This is due to unavoidable shading inherent on a boat. Also, the cost is almost a wash (a 150/70 is only slightly more expensive than 4 100/20 MPPTs, plus diodes, connectors, etc), and the extra blocking diodes would be an additional failure point and a 0.7 V drop. I have seen anecdotal stories about having to replace blocking diodes every couple years, and it appears that they are soldered into MC4 connectors. Soldering on a boat is not good unless done exactly right, and had a hard time sourcing them reliably. They all seem to come from China with sketchy technical data and unknown origin) And I don't want to have to solder in a remote anchorage.

So, with only a single panel per MPPT, the 10AWG wires are way oversized.

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snoobler avatar image snoobler commented ·

I don't recall ever seeing a recent PV system with added blocking diodes. Even a shaded panel will produce enough voltage to prevent backfeed. It takes very little ambient light to make cells provide voltage. They need intensity to provide amps.

I see no reason whatsoever to include OCP on your panels for protection. I see no reason for a double pole breaker in any event due to your relatively low Voc. As @Sarah mentioned breakers on each string is a convenient way to disconnect, and those are cheap.


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Sarah avatar image
Sarah answered ·

it may not be a rule but I find the breakers very convenient and safe if you need to ‘turn off’ the panels (either no load on the panels or no voltage at the mppt). I have had to do this a few times recently when replacing mppts, replacing connectors and general trouble shooting and maintenance. My Voc is in the 100-120 v range.

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snoobler avatar image snoobler commented ·
I agree with this as well. I would likely opt to put a breaker on there just for convenience when I want to do an easy PV disconnect.
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