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Combiner Box


vulgrim
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Hi everybody, so I got my panels on Friday cannot wait to put them on the roof.

Just a quick overview of what I have.

Axpert 5kva

4 Shoto 150 amp agm gel batteries

6 330 watt Benq solar panels(voc:64volt, vmp:54volt)

Going to run 3 strings of 2 panels (still need to buy 2 more panels)

What I would like some advice with is the location of the combiner box of the panels. Do I put it on the roof and run I single cable to the inverter or do I run multiple cables and combine them before I go to the inverter?

Advice on fuse location would also be very helpful.

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Hi Vulgrim 

 

No problems Energy seems to have tidied up.

 

Rubicon (PE) make up combiner boxes and place the fuse holders inside the combiner box.

 

The placement of the combiner box is a matter of convenience over cable length. 

 

My panels came with cables that are 1.1m long and 4mm2 terminated by an MC4 connector.

 

4mm2 at a 70VDC  and about 9A is good for about 6m (allowing for 1% voltage drop). So find somewhere within 6m of cable where it would be convenient to mount a combiner box. In the normal course of events the only time you actually go to your combiner box is to show your admiring friends your uber-cool solar installation. That being said you don't want to be on a ladder 6m off the ground changing fuses or using a multimeter to test one of your strings.

 

Here is a photo of my combiner box. To illustrate the point your request has resulted in me trimming some morning glory creeper off the combiner box and surrounds. That it the last time I opened it.

 

post-822-0-64028200-1454220683_thumb.jpg

 

If you are going to mount it outdoors try for a metal enclosure. The "weatherproof" plastic boxes may keep out the rain but in my experience just become brittle in the sun and crumble. 

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I have had a quick peek at BenQ's data sheet. If your MPPT only draws you panels to 80DVDC and I see the panels Isc is 7A you may get an extra meter or so. I have seen the combiner box mounted above the inverter with 6 wires (in your case) coming through the ceiling encased in conduit. I would try and keep the 4mm2 as short as possible but within reason.

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Did you use normal 4mm square cable that you got from an electrical supplier ?

 

No solar 4mm2 that I put MC4 connectors on either end crimped and soldered. Depending on the physical placement of your panels you may end up with the "tails" from the panels being uneven in length. So the cable you attach must take this into account. Be careful with MC4 connectors. The male connector's insides are in fact female. I was supplied with connectors male and female in separate packets. Fortunately on inspection I discovered this anomaly and so sorted them out. 

 

Solar cable in fact does not stand up to direct sunshine so into some conduit if it is exposed.

 

We want to see photos when you are finished.

 

Edit: Perhaps I should explain the difference between solar (DC) cable and AC cable. DC is often at lower voltages and higher amperages. Current flow is the flow of electrons, negatively charge particles. This is counter intuitive in that we think of current flowing from positive to negative (but that is a topic for another day). Since the charged particles have the same polarity they repel each other and therefore electrons flow in the skin of each strand of wire in a cable trying to get as far away from each other as possible. If you look at welding cable for instance you will see it is made up of hundreds if not thousands of thin strands. This is the same reason why one is safe inside a motor vehicle in a lightning storm as if the vehicle were to be struck the energy would pass through the thin outside layer of metal of the car before going to ground, called a Edison cage. I hope this is all correct. Perhaps someone whose physical science knowledge starts where my understanding becomes flaky can add to or refute this.

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Chris, for a second there it sounded like you were describing skin effect, that tendency that "electricity" has to run on the outer circumference of the conductor. Now I too stand to be corrected here, but from a bit of study, it seems skin effect is only a problem with high frequency AC. So its a real problem in MPPT charge controllers for example, often running at 100 khz, sending lots of current through a big old wound inductor. For really high current AC (even at 50Hz) I've heard that sometimes pipes are used (hollow core) to save weight, given that the core isn't really "used" anyway :-)

 

For DC, however, there doesn't appear to be that much of a problem. The reason for the multiple small strands where DC is involved is only to increase the total surface area and for mechanical reasons (less likely to break). At least that is what I heard. I too would not mind being corrected here :-)

 

Edit: Spent a bit more time reading. Chris, I think you forgot that though those electrons repel each other, we have a bunch of protons in the nucleus holding up the fort and balancing things out. So for DC, the current will distribute itself uniformly over the surface area of the cable :-)

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Edit: Spent a bit more time reading. Chris, I think you forgot that though those electrons repel each other, we have a bunch of protons in the nucleus holding up the fort and balancing things out. So for DC, the current will distribute itself uniformly over the surface area of the cable :-)

 

I might have it wrong and thought that flow of electricity was electrons "hopping" from atom to the next. Ok one atom receives one or more electrons which excite it and since it now has more electrons than normal and those extra electrons are not part of a covalent bond they are free to move and so electron(s) exit and excite the next atom and so on. The protons, neutrons and existing electrons are the stable matrix that makes up the conductor. Hell a lot of time has passed since I was last in an education institution so everything is a bit rusty. Man Pluto is not even a planet anymore so I am definitely from a bygone era.

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Hi Vulgrim 

 

No problems Energy seems to have tidied up.

 

Rubicon (PE) make up combiner boxes and place the fuse holders inside the combiner box.

 

The placement of the combiner box is a matter of convenience over cable length. 

 

My panels came with cables that are 1.1m long and 4mm2 terminated by an MC4 connector.

 

4mm2 at a 70VDC  and about 9A is good for about 6m (allowing for 1% voltage drop). So find somewhere within 6m of cable where it would be convenient to mount a combiner box. In the normal course of events the only time you actually go to your combiner box is to show your admiring friends your uber-cool solar installation. That being said you don't want to be on a ladder 6m off the ground changing fuses or using a multimeter to test one of your strings.

 

Here is a photo of my combiner box. To illustrate the point your request has resulted in me trimming some morning glory creeper off the combiner box and surrounds. That it the last time I opened it.

 

attachicon.gif20160131_075744.jpg

 

If you are going to mount it outdoors try for a metal enclosure. The "weatherproof" plastic boxes may keep out the rain but in my experience just become brittle in the sun and crumble. 

Hi

 

I also used the Rubicon combiner box.  Its perfect.  I have added an isolator switch and my box i placed inside the house below the ceiling where the strings (1x) come into the house to be routed to the inverter.

 

I also have a dc breaker next to the inverter as a double layer of security.

 

My combiner box is wired for 1 string to the inverter but I think I will rewire when I have more panels so I can have 2x MPPT controllers (or 2x Axperts).
 
PS:  I don't think it is waterproof at all.
 

2015 07 26 11.54.52

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Hi Vulgrim 

 

No problems Energy seems to have tidied up.

 

Rubicon (PE) make up combiner boxes and place the fuse holders inside the combiner box.

 

The placement of the combiner box is a matter of convenience over cable length. 

 

My panels came with cables that are 1.1m long and 4mm2 terminated by an MC4 connector.

 

4mm2 at a 70VDC  and about 9A is good for about 6m (allowing for 1% voltage drop). So find somewhere within 6m of cable where it would be convenient to mount a combiner box. In the normal course of events the only time you actually go to your combiner box is to show your admiring friends your uber-cool solar installation. That being said you don't want to be on a ladder 6m off the ground changing fuses or using a multimeter to test one of your strings.

 

Here is a photo of my combiner box. To illustrate the point your request has resulted in me trimming some morning glory creeper off the combiner box and surrounds. That it the last time I opened it.

 

attachicon.gif20160131_075744.jpg

 

If you are going to mount it outdoors try for a metal enclosure. The "weatherproof" plastic boxes may keep out the rain but in my experience just become brittle in the sun and crumble. 

Chris

You may have to watch for overheating on your terminals, looks like 32A AC rated with 16mm2 cable to the charge controller, short loop 4mm2 ? 

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Chris

You may have to watch for overheating on your terminals, looks like 32A AC rated with 16mm2 cable to the charge controller, short loop 4mm2 ? 

 

Morning Eugene

 

You made me panic! I have woken up slightly now. I checked on the armoured cable where it enters the house. No discolouration and no deformation of the insulation. The cable use to bring 32V DC into the house from batteries in the shed. When I had an electrical engineer helping me he OK'ed it. After he left me to my own devices, I never gave it a second thought. It is 25mm2. The cabling inside the combiner box ditto I did not check on but the earth wire is about 10mm2 (perhaps everything looks smaller in the photo). Everything was too big to get into the terminals and was a fight. I eventually swapped the earth and live terminal around to get everything in place. 

 

Now for a cup of coffee.

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HI Eugene 

 

I went to confirm and looked at the leftover piece of cable it is 25.0 x 2 That is within spec at 2% loss but does not have the nice thin strands a DC cable has. I used it because it was here perhaps I shoud replace it. Not that they are right but the Axpert's PV terminals are designed to only accept 10mm2. So I have 25mm2 into the house and then 16mm2 battery cable from the DC circuit breaker into the 10 mm2 terminals. Fortunately my wife and kid were away when I did the install because that process was the source of some very flowery language. If I upgrade my inverter I will definitely try and get an inverter that accepts a higher PV voltage. Just over 100V DC is just a pain.

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I might have it wrong and thought that flow of electricity was electrons "hopping" from atom to the next. Ok one atom receives one or more electrons which excite it and since it now has more electrons than normal and those extra electrons are not part of a covalent bond they are free to move and so electron(s) exit and excite the next atom and so on. The protons, neutrons and existing electrons are the stable matrix that makes up the conductor. Hell a lot of time has passed since I was last in an education institution so everything is a bit rusty. Man Pluto is not even a planet anymore so I am definitely from a bygone era.

That's what I recall from high school as well. I went as far as doing Physics in my first year at uni, then switched courses when I realised that the one I was one meant two more years of that stuff!

 

So the electrons do jump from atom to atom, so as I see it, when you have no current going through the cable you have a bunch of copper/whatever atoms sitting in balance, and then when you start pushing some current through it, you force them to jump from atom to atom, but they are still swinging around the protons that are distributed evenly through the conductor, and so the current spreads out across the surface of the conductor.

 

With AC, there is this back EMF thing that causes the center of the conductor to have a higher resistance and this causes the current to migrate to the outer layers, so you get skin effect. This means that you have to oversize wires at higher frequencies. With these new-fangled MPPTs we use nowadays, that gets interesting, the higher you make the frequency, the smaller the core of your inductor has to be, but the skin effect is higher, the lower you make it, the larger the core needs to be, but you have less skin effect. So if you compare the inductor in the Microcare with the one in the Victron BlueSolar you clearly see the trade-off. The Microcare runs at a relatively low 40khz :-) See attached.

 

post-157-0-28357700-1454314069_thumb.jpg

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Your wife lets you test things on the freezer!!! How did you get that right!?

As long as I put it away by the next time it has to be opened, no problem. Seriously though, you can't do heavy work on the freezer, that lid is ridiculously flimsy :-)

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Hi

 

I also used the Rubicon combiner box.  Its perfect.  I have added an isolator switch and my box i placed inside the house below the ceiling where the strings (1x) come into the house to be routed to the inverter.

 

I also have a dc breaker next to the inverter as a double layer of security.

 

My combiner box is wired for 1 string to the inverter but I think I will rewire when I have more panels so I can have 2x MPPT controllers (or 2x Axperts).
 
PS:  I don't think it is waterproof at all.

 

The IP rating should be on the enclosure, looks like IP65 - Complete protection from dust and low pressure water spray from all directions, unless the gasket has been removed,

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Yipp the box i used is also ip65 rated. Water and dust proof. My 6 panels are on the roof busy getting everything nice and neat. What did you guys do with the panel wires underneath the panels.

Use some black cable ties, they should last a few years. Don't use white, they are less UV resistant. For the rest of the cable runs, 40 mm grey waste pipe and bends work beautifully! Make up some P-shaped clamps with galvanised strapping, the type used to tie roof woodwork.

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  • 1 month later...
On 2/1/2016 at 10:08 AM, plonkster said:

That's what I recall from high school as well. I went as far as doing Physics in my first year at uni, then switched courses when I realised that the one I was one meant two more years of that stuff!

 

So the electrons do jump from atom to atom, so as I see it, when you have no current going through the cable you have a bunch of copper/whatever atoms sitting in balance, and then when you start pushing some current through it, you force them to jump from atom to atom, but they are still swinging around the protons that are distributed evenly through the conductor, and so the current spreads out across the surface of the conductor.

 

With AC, there is this back EMF thing that causes the center of the conductor to have a higher resistance and this causes the current to migrate to the outer layers, so you get skin effect. This means that you have to oversize wires at higher frequencies. With these new-fangled MPPTs we use nowadays, that gets interesting, the higher you make the frequency, the smaller the core of your inductor has to be, but the skin effect is higher, the lower you make it, the larger the core needs to be, but you have less skin effect. So if you compare the inductor in the Microcare with the one in the Victron BlueSolar you clearly see the trade-off. The Microcare runs at a relatively low 40khz :-) See attached.

 

post-157-0-28357700-1454314069_thumb.jpg

A few years ago I had this discussion with a battery supplier (I mean, surely they should know this stuff....?) and he said (aparantly he studied electrical engineering) that it's a bad idea to use bus bars on battery banks and cable should be used instead. His explanation came down to the "skin affect" as well. Since I don't have a degree in anything, but did have Electrical engineering as a school subject, I also remember learning about it but didn't want to argue with him. 

Now, some time later it did occur to me, why do people use cables and not bus bars for battery banks? Can anyone share some hard evince why it would be a bad idea?

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