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Old 15-01-2011, 00:15   #1
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Solar- MC4 or Junction Box ?

Hello out there!

I've been waiting patiently for almost two years now to start work on getting solar on our boat, but due to a pending move x-country all I could do was dream. Now I'm close enough where I can start working on the planning phase in a lot greater detail!

I'm considering 3x135W panels (or thereabouts). One thing I'm not clear on is connections. It seems most solar panels out there except one variant of the Kyocera 135 use MC4 connectors. But, it also seems that most installs I read about on sailboats use the junction boxes. I can see the cost benefit for using junction boxes since you get to use standard wiring, but I do like the ease of the MC4 connectors. I'm also a little confused on the amp/volt/wire size relationship with the MC4 connectors, since I haven't seen a lot of different size AWG for the connectors. Surely there is a safe way to connect all the panels, since it seems like grid applications with lots more panels use the MC4 connector. Perhaps they increase the voltage to keep wire size down and use an MPPT controller to adjust?

Can someone educate me on the benefits/drawbacks of MC4 vs. junction box connections?? And on a related question, am I correct in understanding that a MPPT controller adjusts both voltage and current to provide optimum to the batteries?

Many thanks!

Frank
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Old 15-01-2011, 11:12   #2
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Dang, no input out there on this?
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Old 15-01-2011, 11:53   #3
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Junction boxes are better, but safety concerns for the high voltages in domestic installations (not a factor on boats) mean that many large panel are now only available with MC4 connecters. Get a panel with junction boxes if you can.

MPPT regulators are designed to provide more output from the panels. The batteries are not treated any more optimally, but MPPT regulators have the potential to provide more output from a solar array. The manufactures claim around a 30% gain for an MPPT regulator, but I think the gains are much less-in the order of 5% is my best guess for a good MPPT regulator on a boat, in average conditions.
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Old 15-01-2011, 17:03   #4
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Junction boxes are better, but safety concerns for the high voltages in domestic installations (not a factor on boats) mean that many large panel are now only available with MC4 connecters. Get a panel with junction boxes if you can.

MPPT regulators are designed to provide more output from the panels. The batteries are not treated any more optimally, but MPPT regulators have the potential to provide more output from a solar array. The manufactures claim around a 30% gain for an MPPT regulator, but I think the gains are much less-in the order of 5% is my best guess for a good MPPT regulator on a boat, in average conditions.
Thanks for the response, noelex.

I was under the opposite impression regarding junction boxes and MC4 connectors? I thought junction boxes would be easier to parallel than MC4 connections, but I did notice that they have adapters for paralleling MC4 connections now. I'm not clear on how to parallel 3 panels, but 2, 4, 6, etc look easy enough with the adapter. I'm still not clear on why junction boxes are better for us sailors.

Regarding the MPPT controllers, I know the Outback MPPT contollors step down voltage for the appropriate charge state, but I can't tell if Morningstar MPPT controllers also do this.

If anyone out there has used MC4 connectors, did you just cut a male/female extension in half to terminate the panels at the MPPT controller, or are they able to take the MC4 connection themselves?

Thanks again-

Frank
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Old 15-01-2011, 18:05   #5
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GeoPowers,

The MC4 connectors are a no brainer. My SunPower 230 watt panels had 10 Gage wire with MC4 ends.
I purchased some extention wires with MC4 ends and wired it to the MPPT controller. Done, and it's UV protected and waterproof.

Solar Panels and Associated Systems

Solar Panels and Associated Systems

Mark
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Old 15-01-2011, 18:43   #6
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GeoPowers,

The MC4 connectors are a no brainer. My SunPower 230 watt panels had 10 Gage wire with MC4 ends.
I purchased some extention wires with MC4 ends and wired it to the MPPT controller. Done, and it's UV protected and waterproof.

Solar Panels and Associated Systems

Solar Panels and Associated Systems

Mark

Thanks, Mark. That goes along with my inclination regarding MC4 connections, but I kept reading to the contrary when Googling sailboat solar systems and I thought that I was missing something. Perhaps people just weren't used to them yet...

Nice install, BTW, and thanks for the link. I missed that one when looking for past threads for solar.

Frank
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Old 15-01-2011, 19:27   #7
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I used a fuse box for my junction - a blue sky I think. I allows me to fuse the panels and sum them easily and, I felt, safely. If I want to work on the system or do some testing I can use the breakers to shut off the panels individually.
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Old 15-01-2011, 20:58   #8
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All MPPT controllers will step down the voltage to match the batteries. However, with three large panels like you're planning -- you'd be better off keeping them parallel (not series) unless your wiring run to the controller is very long (>20 feet).

If one panel gets shaded it will not impact the others (so long as they are diode isolated) but if one panel in a series gets a shadow it will have a major impact on overall output of the series string.
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