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Making branch connections - MC4 vs direct crimps

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Leon

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I need to connect 12 250W solar panels into 6 parallel pairs. Each pair will be connected to the charging system using a pair of 6mm2 cables. I am not sure of the best way to make these parallel connections. The panels have 4mm2 leads, currently with MC4 connectors.

Should I:
A) Connect the panels with MC4 parallel branch connectors, then add the relevant MC4 connector to the 6mm2 cable.

B) Cut the MC4s off the panel leads and use some sort of butt crimp.

Some other details:
  • I need the paired panel setup as this is an Electrdacus SBMS0 based system.
  • I have a typical hydraulic crimper, but would have to buy one for MC4 (I think?)
  • This is going on an easily-accessible veranda roof, in a permanent installation.
  • Since this is low voltage, I want to minimise the cable lengths as much as practical.
I am leaning toward a permanent crimp - type yet to be determined. My thinking is that this greatly reduces potential points of failure via corrosion or high resistance.

I would need to find a butt crimp that can join cables of slightly dissimilar sizes and work out a reliable way to weatherproof the connection.

I would really appreciate any advice you can offer.
 
Get a splice crimp the correct size for two wires, fold the single wire in two. Cover with adhesive filled heatshrink. I avoid MC4 whenever possible - they are a cheap and nasty solution for easy install.
 
A combiner box is ideal. You need fusing between the series strings. If you don't use a combiner box, you can still easily fuse them with MC4 fuses. I made a homemade combiner box of sorts. Inside the box, I put some terminal strips, then brought in some PV wires (through a water tight gland) with MC4s hanging on the outside for the input. The combined output is into a conduit wire run that comes into the box.
 
Leon said:
I need to connect 12 250W solar panels into 6 parallel pairs. Each pair will be connected to the charging system using a pair of 6mm2 cables. I am not sure of the best way to make these parallel connections. The panels have 4mm2 leads, currently with MC4 connectors.

Should I:
A) Connect the panels with MC4 parallel branch connectors, then add the relevant MC4 connector to the 6mm2 cable.

B) Cut the MC4s off the panel leads and use some sort of butt crimp.

Some other details:
  • I need the paired panel setup as this is an Electrdacus SBMS0 based system.
  • I have a typical hydraulic crimper, but would have to buy one for MC4 (I think?)
  • This is going on an easily-accessible veranda roof, in a permanent installation.
  • Since this is low voltage, I want to minimise the cable lengths as much as practical.
I am leaning toward a permanent crimp - type yet to be determined. My thinking is that this greatly reduces potential points of failure via corrosion or high resistance.

I would need to find a butt crimp that can join cables of slightly dissimilar sizes and work out a reliable way to weatherproof the connection.

I would really appreciate any advice you can offer.
Click to expand...


Are you sure you want to connect all the panels together? I think you're referring to an array 2s6p. But it appears SBMS0 (or DSSR20) ratings are 49V and I think 20A? I think the idea is one or two parallel PV panels/strings into DSSR20, so you wouldn't parallel all.

With only two PV strings in parallel, no need for fuses, could just use "Y" connectors.
If crimp, usually we connect inside a box to make waterproof. There are some filled connectors for irrigation system, but not sure any good for your application.

What are the ratings of your panels? Most 250W panel are "24V", which have higher Voc so only one would be within 49V spec of the electronics; two in series would be too high a voltage.

I prefer connecting panels in a series string, 12 or 6s2p, and using an MPPT controller.
The Electrodacus SBMS0 diagram looks like panels are just connected to battery through FETs, a PWM regulator.
PWM may be good for some applications (especially small systems) but can only match or slightly beat MPPT power capture under narrow conditions. MPPT is able to adjust electrical parameters to work better over a range of temperature, illumination, shading, capturing more power.
 
MC4 connectors may be inexpensive, but solar panels come with them and those are used for roof installs. With their IP 67 rating, I trust those to weather over the years better than a butt splice with a couple of pieces of heat shrink with the glue inside, even if wrapped in tape afterwards.

Even though its permanent installation, these MC4s make troubleshooting or upgrades easier down the line.

When professionals install solar panels on roofs, I don’t think they are cutting off MC4 connectors and making butt slices to make the wiring last longer.
 
Hedges said:

Are you sure you want to connect all the panels together? I think you're referring to an array 2s6p. But it appears SBMS0 (or DSSR20) ratings are 49V and I think 20A? I think the idea is one or two parallel PV panels/strings into DSSR20, so you wouldn't parallel all.

With only two PV strings in parallel, no need for fuses, could just use "Y" connectors.
If crimp, usually we connect inside a box to make waterproof. There are some filled connectors for irrigation system, but not sure any good for your application.

What are the ratings of your panels? Most 250W panel are "24V", which have higher Voc so only one would be within 49V spec of the electronics; two in series would be too high a voltage.

I prefer connecting panels in a series string, 12 or 6s2p, and using an MPPT controller.
The Electrodacus SBMS0 diagram looks like panels are just connected to battery through FETs, a PWM regulator.
PWM may be good for some applications (especially small systems) but can only match or slightly beat MPPT power capture under narrow conditions. MPPT is able to adjust electrical parameters to work better over a range of temperature, illumination, shading, capturing more power.
Click to expand...
You are correct about the Electrodacus system. There are no series connections in the system at all. Each pair of panels is paralleled, then a pair of cables is run to the battery, through the solid state relay (the DSSR20) which controls charging. There will be 6 pairs in total, each with their own relay. A typical MPPT system with series string of panels has advantages over the Electrodacus system, notably simplified and less costly wiring, but was not suitable for other reasons in my application.

The panels I am using have VOC of 27.9V and ISC of 8.78A. Each of the relays can handle 20A, hence the 2P setup for each.
 
DThames said:
A combiner box is ideal. You need fusing between the series strings. If you don't use a combiner box, you can still easily fuse them with MC4 fuses. I made a homemade combiner box of sorts. Inside the box, I put some terminal strips, then brought in some PV wires (through a water tight gland) with MC4s hanging on the outside for the input. The combined output is into a conduit wire run that comes into the box.
Click to expand...
There are no series strings. A typical combiner box is not needed, just a waterproof way to connect 2 4mm2 cables to one 6mm2 one.
 
chrisski said:
MC4 connectors may be inexpensive, but solar panels come with them and those are used for roof installs. With their IP 67 rating, I trust those to weather over the years better than a butt splice with a couple of pieces of heat shrink with the glue inside, even if wrapped in tape afterwards.

Even though its permanent installation, these MC4s make troubleshooting or upgrades easier down the line.

When professionals install solar panels on roofs, I don’t think they are cutting off MC4 connectors and making butt slices to make the wiring last longer.
Click to expand...
Yeah. It's hard to argue with the success of the millions of MC4s on roofs everywhere. It just makes a lot more sense when panels are simply strung together in series. Troubleshooting is a real consideration.
 
chrisski said:
MC4 connectors may be inexpensive, but solar panels come with them and those are used for roof installs. With their IP 67 rating, I trust those to weather over the years better than a butt splice with a couple of pieces of heat shrink with the glue inside, even if wrapped in tape afterwards.

Even though its permanent installation, these MC4s make troubleshooting or upgrades easier down the line.

When professionals install solar panels on roofs, I don’t think they are cutting off MC4 connectors and making butt slices to make the wiring last longer.
Click to expand...

I’ve seen plenty of MC4 connections fail. Keep in mind a MC4 is a crimp and an interference connection. By crimping only you remove one point of failure.

UV rated, quality adhesive lined heatshrink and a well formed crimp will be more reliable.

Professionals use what is fast and easy. MC4 are made to just meet the standards required - they are literally the minimum acceptable connection.
 
What I'm now wondering about is the heat shrink seal. The side with a single cable is easy, but what about the other side of the crimp, where two wires come out side by side? Will the adhesive have enough squish to fill the gap between the two?
 
Leon said:
Will the adhesive have enough squish to fill the gap between the two
Click to expand...
Not reliably enough to keep you from waking with a cold sweat in the night worrying about it, maybe 99.44% failure heat shrinking pairs of wires imho

This is not meant for this application (or marine) but I’ve used it for ??20?? years below the waterline or for various things like panel adhesive and installing fiberglass columns at storefronts:

Squirt it liberally in the paired juncture, wrap with ‘nice’ 3M electrical tape.

Or use the nice tape over liquid electrical tape (there’s a mil spec product somewhere out there that I can’t find lately) and then paint the tape with it.
 
Leon said:
What I'm now wondering about is the heat shrink seal. The side with a single cable is easy, but what about the other side of the crimp, where two wires come out side by side? Will the adhesive have enough squish to fill the gap between the two?
Click to expand...

I have used this method for sealing splice joins in a harsh mining environment - never seen one fail.

If you take a look at the join after you complete it, and look at the adhesive surrounding the two wires, you will have a clear idea if you are confident or not.

I would always choose this method over a connector.
 
toms said:
harsh mining environment - never seen one fail.
Click to expand...
yet :)

I’ve been doing 12V automotive, off-road, trailer, and marine wiring for a very long time in NH and VT. I’m opinionated- sorry.

Just with all the NaCl salt on the roads in winter and CaCl salt on the dirt roads in summer (to keep dust down) you get rapid results on finding the methods that don’t work well.

Granted, nearly zero solar connectors will ever see that corrosive environment, but lack of “failure” doesn’t mean it’s waterproof. It’s just so much slower to corrode without electrolytes.
Typical trailer brakes have s Y-splice or two depending on number of axles. Other than ‘dipped’ splices, most shrink-tubed Y-splices only last a few years around here.
 
In the end, I went with crimped connections. I waterproofed them by using a rubber splicing tape - self amalgamating stuff. Some wraps went between the parallel cables, effectively filling the space. This was then wrapped with good quality electrical tape, mostly for UV resistance, but it also provides additional compression to help the rubber tape seal onto the joint. The whole joint is tucked up under the panel, so it shouldn't see much water or UV anyway. Seems to be working so far, I'll update in a decade.
 
12VoltInstalls said:
Just with all the NaCl salt on the roads in winter and CaCl salt on the dirt roads in summer (to keep dust down) you get rapid results on finding the methods that don’t work well.
Click to expand...

Guess you’ve never worked in mining?

Green death is very common in connectors.
Well assembled Deutsch connectors are OK, but still fail occasionally. Good adhesive lined crimps i’ve still yet to see fail. Love to see pics showing otherwise though, always room for improvement.

Sounds like the OP has it covered (literally) in any case.
 
I had a MC4 connector fail recently. However, it did make troubleshooting and repair very easy. OTOH, one could say I never would have had to troubleshoot in the first place had I used solid splices.
 
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