Will this string configuration work?

softdown · January 2015

Configuring strings has got to be the most mind numbing exercise that I have participated in. This is my third(!) thread (2 here) trying to get my strings configured properly.

Equipment: 18 Grape Solar 190 watt panels 45 Voc 36 Vmp 5.5 Isc
Outback MPPT FM80 charge controller 150 VDC
Minimum winter temperature -32F (butt cold)

The challenge comes from:
1) 18 panels is a bit quirky. 16 would have been easier.
2) 45 Voc in a very cold climate limits me to two panels in series (unless one uses a Midnite CC made for Hyper Voc).

Being a relative newbie, I somehow assumed that my Charge Controller would have multiple inputs for multiple strings. Well....it has one. I was sick a couple weeks ago and balked at the idea of wiring up a 12 string combiner with 9 strings by myself. Pre-wired 12 string combiners costs $600 - well over my budget. So I bought five pre-wired 2 string combiners. Thinking that my charge controller would have room for at least six incoming wire pairs. There are some advantages to several small string combiners so I believe that may work out.

Four of the combiners will handle four panels/combiner. The fifth combiner will handle two panels and leave expansion room for two more future panels.

Now I still have to combine the output from five small combiners into a single combiner box with one 2AWG output. My math shows that each combiner box will feature two panels in series and two parallel strings. So the output Voc is 90 (Vmp is 72). Looks OK for a 48 volt battery array.

The fab shop thinks this is going to work. I just got a clamp style voltage meter so I can double check as I proceed.

I'm sure that there was probably a better way to do it. Like getting a 12 string combiner box and wiring it myself. But I was sick and didn't feel comfortable doing that.

Do you think this will work? This is a look at the six string combiner box under consideration. http://www.ebay.com/itm/Solar-Panel-Combiner-Box-6-String-PV-Power-Combiner-Pre-wired-/141544800772?ssPageName=ADME:X:RTQ:US:1123

westbranch · January 2015

Re: Will this string configuration work?

this one is about $20 cheaper http://www.solar-electric.com/installation-parts-and-equipment/midnite/pvarco/mnpv6.html

softdown · January 2015

Re: Will this string configuration work?

Hah...funny you mentioned it. I have this box. Problems:
1) Needs circuit breakers or fuses. Costs about $100. I already have fuses in my smaller boxes.
2) It needs to be wired with MC4 connectors. Not sure why but I don't feel comfortable wiring combiner boxes.

I'll sell my new MNPV6 combiner box for $60 plus shipping....now that you mentioned it. Still have the box it came in.

westbranch wrote: »

this one is about $20 cheaper http://www.solar-electric.com/installation-parts-and-equipment/midnite/pvarco/mnpv6.html

westbranch · January 2015

Re: Will this string configuration work?

But if one of those white CB's fails you lose 3 arrays... as oppose to one array in the MN box

softdown · January 2015

Re: Will this string configuration work?

westbranch wrote: »

But if one of those white CB's fails you lose 3 arrays... as oppose to one array in the MN box

I don't see any moving parts to fail. It is also weatherproof. Maybe I should get a couple extra of those white thingies in case there is a failure and replacement proves difficult.

I don't see a need to have two separate series of fuses and/or circuit breakers.

Seems to be pros and cons to most everything.

Photowhit · January 2015

Re: Will this string configuration work?

westbranch wrote: »

But if one of those white CB's fails you lose 3 arrays... as oppose to one array in the MN box

Those White "CB's" I think are just the new power distribution blocks, NEC in their infinite wisdom(or infant wisdom?), has endeavored to protect us from our selves no longer allows the side by side power distribution blocks;

Attachment not found.

Now they must have greater spacing or be covered, so I think you're seeing a hunk of aluminum with set screws covered by a piece of plastic/insulation.

Wiring a combiner box is pretty straight forward, it will get very expensive and in my opinion, less safe since you have more connections to fail, if you wire it that way. On top of that you will have more expensive wires since you will have to buy or make wires with MC4 connectors on each end. It will take a little time, but buying water proof wire restraints and wiring directly will save you a lot of money and as I said, IMO safer.

Attachment not found.

Wire strain releifs;

Here's my box wired pre new code on the distribution blocks, hope they aren't waiting on my to change it!
PS - This feeds 2 different charge controllers!

Attachment not found.

softdown · January 2015

Re: Will this string configuration work?

Congrats Photowit - that looks good.

As Cariboocoot (sp?) previously pointed out - things become simple after you have finished the job. I can only imagine the disasters if everybody tried to do their own solar install using a "little of this and a little of that" accumulated over the years.

Photowhit · January 2015

Re: Will this string configuration work?

I know I've done several, but this never confused me. If you want to you can just deal with all the positive lines and the all the negative lines. As perhaps you suspected I picked up pieces along the way, I think, I would be much more confused wiring so many different elements.

...and I am cheap, I can see huge extra expenses in dollars for you and time and a bit of money for me, in all the extra MC4 connectors or prebuilt cables!

Think of all the extra failure points as well! Each MC4 connection is really 3 connections, wire to connector, connector to connector, and connector to wire.

If I follow your path correctly, you will have a 2 panels connected together, then from the MC4 connector you will have an extension cable, with an MC4 connector on either end connecting to an MC4 connection at the combiner box, which goes to a breaker, back to a MC4 connector, to another mc4 extension, to another combiners MC4 connector, to a distribution block and out to another MC4 connector, to another extension, then to you charge controller.... Scares the heck out of me!

I'd much rather have from the panels, a single MC4 extension to a breaker (or fuse) and out to the charge controller. To me a much easier wiring as well. I am confused just writing your proposed setup!

softdown · January 2015

Re: Will this string configuration work?

I thought most everybody would get confused over my explanation. My system will actually cost $200 less that a pre-wired 12 string combiner box. More parts? Sure. But more flexibility as well.

I spent a day searching for a no frills pre-wired 12 string box. Nada. Zip.

I'm not sure why folks keep saying that I should wire my own combiner box when I keep saying that I just don't feel comfortable doing it. It was easy for them. Super - Congrats. Everybody has different comfort zones. I know several electricians and plumbers. None of them can do carpentry. I'm a natural at carpentry.

westbranch · January 2015

Re: Will this string configuration work?

SD, why don't you get an electrician that you know and do the 'connections' for you and you 'pull the wire' just as I have done for the AC on our build job.

Ps I like PW do not like MC4s due to the added cost when a wire stripper and a screw driver work just as effectively and a lot cheaper. They are particularly tough to get apart without a special tool! http://www.solar-electric.com/catalogsearch/result/?q=mc4+tool&cat=0

Vic · January 2015

Re: Will this string configuration work?

Hi again softdown,

This has been discussed before. Believe that the approach that you are describing is to use "T-Branch MC-4 Connectors" . Think that this is what you are calling your "small combiner". Combiners really have Over-Current Protection Devices -- a circuit breaker or fuse, one for each string of PVs gemerally, when there are more than two strings of PVs. The T-Branch connectors are only suitable for paralleling two PV modules or two strings of PV modules, IMO.

Think that you are planning on using the VK tech T-Branch connectors (as seen on Amazon, etc).

What about using two MNPV-6 real Combiners? This would allow for some expansion capability.

OR, as you mentioned before, finding a reliable off-grid PV system installer to do just the things that you are not comfortable doing, and perhaps to consult on the rest of your new system?

Here is a Link to a discussion of Using MC-4 connections in various types of systems:
http://www.solar-electric.com/how-to-use-mc4-connectors-cables.html/

Have Fun, Vic

Photowhit · January 2015

Re: Will this string configuration work?

softdown wrote: »

I thought most everybody would get confused over my explanation. My system will actually cost $200 less that a pre-wired 12 string combiner box. More parts? Sure. But more flexibility as well.

I'm Cheap! I have another array to do yet and purchased a Outback PSPV(designed much like the MNPV12, by the same people), a doz 15 amp DC CBI breakers, and 10 - 3 wire strain reliefs for under $175 shipped to my door, no extra MC4's to buy and crimp, your way, to me, would either be more expensive, buying all the extra 'to size' cables or more time consuming and expensive, building all the cables... and as I said before more confusing!

softdown wrote: »

I'm not sure why folks keep saying that I should wire my own combiner box when I keep saying that I just don't feel comfortable doing it. It was easy for them. Super - Congrats. Everybody has different comfort zones. I know several electricians and plumbers. None of them can do carpentry. I'm a natural at carpentry.

I'd rather you stay within your comfort zone, I like Westbrach's idea, if you're a carpenter, you likely know several people who could wire this up for you! I was looking at the available 2 string boxes and there is a bit of variety, DC breakers can be confusing, since they are polarized. Fuses are simpler. I hope your combiner boxes use proper sized fuses. Auto fuses don't cut it!

gww1 · January 2015

Re: Will this string configuration work?

Whitt

was looking at the available 2 string boxes and there is a bit of variety, DC breakers can be confusing, since they are polarized. Fuses are simpler. I hope your combiner boxes use proper sized fuses. Auto fuses don't cut it!

I agree with the polarity issue but I must say that after everything is installed that breakers are really nice. I have fliped mine on and off in full sun and never had an arking issue and they are just handy when checking issues. Since I have zero experiance with anything else, I might be prejudiced towards them cause it has serve my needs well.
gww

softdown · January 2015

Re: Will this string configuration work?

I am not using T-branch connectors. I am using 5 pre-wired, fused combiner boxes that are sitting right here. Somehow had the impression that charge controllers had multiple inputs - which they do not.

I do my own carpentry, not for others. I'm too slow to work for others. Only know a very few tradesmen. Most are overcome with wicked envy when they see what I built. Seems they can't go a day without telling me that I may get robbed. Some have robbed me of valuable items. Others have robbed me of smaller things.

The roof is 44' wide and 14' deep. I am planning to build an astronomy tower in the middle of it. Already bought 20 pairs of 25' USE-2 MC4 12 AWG wire. We all know that it always seems to take more wire length than we thought it would. I don't think that 25' wire runs would allow me to go from front roof edges all the way to back roof center while going around the astronomy tower. So the small combiner boxes allow real flexibility. The roof ice does not want to melt so I wait for warmer weather.

Another 10' of USE-2 MC4 10 AWG wire will provide the needed length to get to the pre-wired six string combiner box. So I took a slightly circuitous route due to pre-purchasing before I knew exactly what was needed. I had originally thought that the CC, inverter, and batteries would sit at the front of the sun room. Bad idea due to heat. Placing them in the back of the sunroom/down in the root cellar staircase requires more USE-2 wiring.

If I had it to do over again - sure, things would be done differently. A lot of us can relate to that sentiment. What I was trying to determine was: "Will this string configuration work?" What I seem to be getting is well meaning "This is what I favor" or "This is how I would do it" Those sentiments would be great if I had not purchased my equipment already.

Overriding fear is creating a HyperVOC situation with butt cold weather and too many panels in serial strings. To be frank, I still can't wrap my head around how eventually combining eigtheen 45 Voc panels will not create more than 90 Voc at the charge controller. I read several solar books and studied serial and parrallel strings as best I could. Even made a thread asking for recommendations on a great explanation of strings.

I'm pretty sure I will look back at it and realize how simple it is. So long as you don't step into 1 of the 1001 pitfalls awaiting the do-it-yourself solar installation. As for having someone else doing it? I have loads of time and almost no money at this point in time. In a few years, I can sell my modest Denver home, pay off some debts, and maybe relax a bit. I want to do this myself. That way I can repair it and do a job that is up to my quality standards.

Thanks for all of the ideas to date...I was afraid that nobody would understand what I was saying. Not sure a beginner can usually present a competent explanation of their proposed strings.

Photowhit · January 2015

Re: Will this string configuration work?

gww1 wrote: »

I have fliped mine on and off in full sun and never had an arking issue and they are just handy when checking issues.

You are correct, you don't want to pull a fuse under load, if the need arises, I and I would assume 'Softdown' could throw the breaker ahead of the charge controller, breaking the circuit before pulling the fuse.

We haven't discussed the power center and breakers ahead of and behind the charge controller and main fuse from the batteries, and AC breakers for the run to the house breaker box...

gww1 · January 2015

Re: Will this string configuration work?

soft

If I had it to do over again - sure, things would be done differently. A lot of us can relate to that sentiment. What I was trying to determine was: "Will this string configuration work?" What I seem to be getting is well meaning "This is what I favor" or "This is how I would do it" Those sentiments would be great if I had not purchased my equipment already.

Since you have not did it yet, it is good that you get the other options presented as you may decide that some thing else is better and it is much easier before install then after. You may decide that what you have after, hearing everything, fits your needs. Some opinions may even have enough merit to put stuff that was already bought back up for sell and to get something differrent, then again maby not. I had thought I saw you offer up something you had already bought for sell in a differrent thread.

When you do things for the first time it slows you down cause it takes research to know how to do it. When I offer advice it is from my experiances and they may be flawed or not as well informed as others. I agree that doing it yourself put you ahead for later when using it.

I also think that as far as you comfort level, It takes a bit of patiants and research and you comfort level will increase as you do it. Soon you will be trying to help others who don't have a good comfort level doing what you have done. Combiner boxes are just like the house you helped wire. there is a termanal screw and a striped wite that you place in it and screw it down to the proper lbs. Hopefully with a simatic to show where things go. There are a million ways to skin a cat and if I wanted to skin one, I want to here them all. Also it may be helptull to get others thoughts prior to buying if there is question on weather it will work or work best.

I truly believe you will get the install done and be smarter because of it. It may take a bit longer then you think. You may find that you could have did things differrent the the advce you end up following. I did on a couple things but couldn't have done it with out the help I got here. So add it all up I am still better off then not doing it and will be better next time.

I know I didn't answer your question on what you already have but have told you my experiances on what I have done for you to take the portions you want to from.

The panel voltage is just math. The back of the panel gives you the numbers and the weather station tells you what to put those numbers to. I am lazy so I know I get to zero or below here. I just used the voc times 1.25. Not correct for every panel but if you leave a bit of head room at the top, it is close enough for gov work.

I wish you success.
gww

Photowhit · January 2015

Re: Will this string configuration work?

softdown wrote: »

I am not using T-branch connectors. I am using 5 pre-wired, fused combiner boxes that are sitting right here. Somehow had the impression that charge controllers had multiple inputs - which they do not.

you might have picked that up in discussion here for grid tied inverters which often have multiple panel inputs.

softdown wrote: »

The roof is 44' wide and 14' deep. I am planning to build an astronomy tower in the middle of it. Already bought 20 pairs of 25' USE-2 MC4 12 AWG wire.

Is that too many? you have 9 strings so will need 9 pairs (one for positive and one for negative ), to the first combiner box, and 5 pairs to the 2nd combiner box. Now we have an addition problem if you plan on having 25 feet to the first fused combiner, and another 25 feet from the fist combiner to the 2nd combiner and then 10 feet to the charge controller. You have a lot of room for voltage drop.

You have very minimal 'head' room with your panels already, with long DC voltage runs you have voltage drop and you would have 60+ feet one way. Since you intend a 48 volt system, you need to worry that you have high enough voltage coming into the charge controller.

I ran the numbers and you're ok on voltage drop, but...

softdown wrote: »

Another 10' of USE-2 MC4 10 AWG wire will provide the needed length to get to the pre-wired six string combiner box. So I took a slightly circuitous route due to pre-purchasing before I knew exactly what was needed. I had originally thought that the CC, inverter, and batteries would sit at the front of the sun room. Bad idea due to heat. Placing them in the back of the sunroom/down in the root cellar staircase requires more USE-2 wiring.

If I read this correctly you have figured out;

...for the 3rd run of 10 feet you will have the potential of 45 amps so you just squeak under the voltage drop, but 10 gauge wire is too light for 45 amps, I think you will need 6 gauge wire for that run.

softdown wrote: »

Overriding fear is creating a HyperVOC situation with butt cold weather and too many panels in serial strings. To be frank, I still can't wrap my head around how eventually combining eigtheen 45 Voc panels will not create more than 90 Voc at the charge controller. I read several solar books and studied serial and parallel strings as best I could. Even made a thread asking for recommendations on a great explanation of strings.

I'm pretty sure I will look back at it and realize how simple it is.

Most of the series and parallel calcs are, in parallel the amperage adds and voltage remains the same, in series voltage adds and amperage remains the same.

softdown wrote: »

So long as you don't step into 1 of the 1001 pitfalls awaiting the do-it-yourself solar installation. As for having someone else doing it? I have loads of time and almost no money at this point in time. In a few years, I can sell my modest Denver home, pay off some debts, and maybe relax a bit. I want to do this myself. That way I can repair it and do a job that is up to my quality standards.

Most of us here are diy types. You'll be helping others very shortly.

softdown · January 2015

Re: Will this string configuration work?

Let me try better this time: Four small combiner boxes will handle four panels/box. Two panels in series and two series per box. There are 18 panels so at least 18 USE-2 wires are required. Last two panels will go to fifth combiner box - which is only 50% utilized.

Now we employ 5'-10' runs from the five small combiner boxes to the final six string combiner box. #2 or #4AWG to charge controller with disconnect in between.

I plan to have an electrician take the power from the inverter to the circuit breaker box.

This would been easier with 16 panels instead of 18. Much easier with approx 25-35 Voc instead of 45 Voc.

What I should probably do is sell some stuff and buy a few more panels while they are so cheap. All good things come to an end.

Photowhit · January 2015

Re: Will this string configuration work?

softdown wrote: »

Now we employ 5'-10' runs from the five small combiner boxes to the final six string combiner box. #2 or #4AWG to charge controller with disconnect in between.

I read that you had "20 pairs" or 40 wires, so perhaps I misunderstood, MC4 wires are sometimes described as pairs as they are often used by buying a length 2x as long as you need and cutting in half since many/most combiner boxes aren't setup for MC4 connections.

softdown wrote: »

I plan to have an electrician take the power from the inverter to the circuit breaker box.

This is a battery based system so you will have other breakers needs, Midnite has been making an Epanel that provides a main DC breaker and a shunt and places for breakers for incoming from array, between charge controller and battery, and AC breaker between the inverter and the breaker box. Sounds like you'll house batteries inside which should be vented outside if flooded batteries. Link to Midnite Epanels for Outback.

Typically you would want to make the new panel, the main panel and bond your neutral and ground here, near your power supply and make your inside breaker box a sub panel. The electrician will understand this.

gww1 · January 2015

Re: Will this string configuration work?

whitt

I think he is installing the outback flex power one. I thought that it had the dc breaker box as part of the system.
Cheers
gww

softdown · January 2015

Re: Will this string configuration work?

I am gleefully reading up on the proper grounding of PV systems. I have 8' copper clad rods (but have misplaced the lugs). I keep having to tear things up each time I expand. Looks like it will be a few months before things are hooked up.

As for venting lead acid batteries. Everything says to build a cabinet around them and vent hydrogen gas outside. This is code but sometimes code is crazy. A cabinet creates a situation where the gasses may very well build up to a point of volatility. Some hydrogen gas is not dangerous to breathe - but it is volatile when confined. So I'm not sure about the cabinet since it is, in essence, creating a potentially volatile environment. No kids here to place tools on he battery terminals.....

gww1 · January 2015

Re: Will this string configuration work?

Soft
I forget where you said you were going to place the battery. I have mine in the open in a garage.

I have a wood stove on the right about 7' away. I did not know it was code to incase a battery. What I thought was code, was either conduit or a dedicated locked area. I may be wrong and don't get inspected here. I have had no issues but would be carefull that your equiptment is not directly above the battery due to sparks and corrosion. The battery is going to have an acid mist and I put mine on rollers to be able to spray it off once a year but in practice I have never moved it and spray it there and then dilute or throw baking soda on the floor and then mop it up.

My garage is insulated but has lots of air leakage and I don't worry about it. I was origionally going to inclose it but have found no real need to.

What battery did you end up with. The forktruck has all the lead post covers and the only time I ever get scared is when messing with the leads and I try to never have one of them uncovered at a time (which is why I mop in place). If my place was more sealed, I believe a hood type setup vented out side would be enough. If it was in a finished part of my basement I might think differrent.

Good luck.
gww

Vic · January 2015

Re: Will this string configuration work?

softdown,

Thanks for clearing up nature of your small combiners ... on another Forum some weeks ago, you did refer to those 'T-Branch' parallel string connectors, and that was the reason that I had felt that those were your small combiners.

Hydrogen is a very light, and "loose" gas. It really wants to rise and get away from containment. It is EXPLOSIVE over a very wide concentrations in air, and in oxygen, even more so.

Often, one places batteries in a box of some sort, with one or two air inlets from outside (low in the container), and vents the box to the outside, at the highest point..
BUT, this container traps heat and makes cooling the battery difficult in warm months.

Have built insulated plywood battery boxes here, but now keep the hinged box lid always open. The insulation does help keep the battery temperatures fairly consistent, and undesirable fumes and Hydrogen exit through a 4-6 inch opening in the ceiling that vents outside. Not all of the electrolyte vapors exit the room, but a bit of natural convection is a good thing, and clears Hydrogen well.

One should be observant of the gassing stages (Absorption and EQ for Flooded batteries) and the increased risk that a spark could possibly cause an explosion. But, it is not very often that one hears of an explosion from an off-grid battery .... knock, knock.

And so on ... your adventure continues. Have Fun, Vic

softdown · January 2015

Re: Will this string configuration work?

The CC and inverter will probably be about 4' to the right and 4' lower than the batteries. These is a strong wall that stays cool at the end of the staircase to the root cellar. Looks good for the electronics.

I am still using 8Ds. Decided to start out cheap because there was a lot of learning to do. Not sure that battery superiority claims should be believed much. Looks like one should endeavor to get a LOT of thick plate lead at a good price. The key is proper mtce....not proper manufacturer. I have a suspicion that car batteries would work ok if the voltage was kept above 12.4V. I keep hoping for a deal on slightly used traction/forklift batteries.

EDIT: I recently made a thread regarding possible battery destruction after the inverter took the string down to 10.5 volts for awhile. That string had four batteries and two of the batteries are currently undergoing a de-sulfation process. Currently using two 8Ds that did not get discharged.

Will this string configuration work?

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