Off-Grid Install/Classroom+Lab Project Questions

divonbriesen · September 2010

I have a related series of questions- and debated posting a new thread. First post on site, so forgive me for not yet gauging the climate.

I'm working on a container project - you can read about it at http://ecobox.me - in short, an off-grid dwelling for 2 people out of a 40' container.

We're also using this as a teaching lab for a community college, so some things are a bit unorthodox, and we're on a pretty strict budget (read: lots of trips to restore, salvage, and donations, plus some expensive stuff from random grants).

So it goes like this: currently 4 75v 100w panels on the roof (sencera branded, but made overseas), on loan. In parallel, using 6 y-connecters: http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=300442631363&rvr_id=139707658660&crlp=1_263602_263622&UA=WVS%3F&GUID=ca43f5fc1220a0437230d320fff5841f&itemid=300442631363&ff4=263602_263622

This saves us running 4 pairs of wires 40'- but then i get to the midnight solar combiner box, and there's nothing to combine! I don't know whether to laugh or cry. In that combiner we'll have the DC lighting arrestor, a DC breaker for the array (now 1 instead of the planned four i guess), the GFI (63+2amp or whatever it is).

So we need a cutoff- intuitively it seems that cutoff would go before the combiner, but then you'd have to have a cutoff for each set of wires, which makes no sense, plus the box has the breakers, which function as a cutoff.

So then we wanted a cutoff between the combiner box and the outback mppt (60) - i found a 240v ac blade cutoff (big red lever) - but it's not ac rated. It's just big metal blades... should I care? This allows me by pulling the lever (as opposed to opening the combiner box) to cut the panels off from the mppt.

Then we go to the outback mppt, and from that into a midnight epanel. Now our supplier likes these- I think they're nuts, and the diagram inside affirms that for me. What's that pv in bus for? Why is the breaker mount for the incoming mppt current in such a %^&*( place - we're running wires all over the place. But I digress.

From there we go the batteries, but have double wires (thick wires!) leading to two different inverters - a nice big sine wave outback, and a cheesy mod. sine wave xantrex. The boss wants me to able to charge the house with pure sine or not as a teaching device- so i needed a switch for the ac. Bought a $100 on off on switch (way bigger and bulkier than i want or think i need) and have the inverters coming in to the outs and the in leading to the subpanel for the whole place.

I guess i should provide pictures...

So my question:

What's the right UL/NEC way to do a cutoff between the combiner and the mppt?
Is it bad that i'm doing my parallel connections under the panels using MC4 Y connectors? If so, why? (they're rated to 30amps).

I noticed that midnight sells these mini boxes with rails... i have a whole bunch of different dc breakers from some bad orders (7amp, 15amp, 2amp, 30amp) in the past. But i'd feel better having something i could hit without having to open a box (like the big red switch on the epanel).

Finally, the epanel has this cool inverted breaker setup so you can switch from generator to inverter - it turns one on while it turns the other off. Could i do something like this instead of that big ol' switch box I have? If so, what parts should I order? I realized i could probably just route the outback inverter output into where the epanel wants the generator to go, but that'll make teaching the thing even harder.

I suppose i could also never mind the cutoff, and run the line from the combiner into the epanel, mount a few more dc breakers on the vacant side slots, and just use those (since they're exposed on the outside) as my cutoffs. Is the epanel meant to be (or can it be turned into) a combiner as well? It seems to have lots of fiddly bits that i'm not using...

Have I confused you yet? I should take some pictures...

d.i.

BB. · September 2010

Re: AC vs DC Panels and Breakers

d.i.,

If I understand your question correctly... First, you really do not need another cutoff switch between the array and the charge controller. The 63+2amp GFI will do that for you.

The reason you cannot figure out where to put the combiner box (should have at least 4 breakers or fuses inside) is because you have used the "Y" MC4 connections.

At best, according to code, you can only use the "Y" connectors if you have two solar panels in parallel. If you have three or more panels (or series strings) in parallel, you need a series protection fuse (or breaker) in the positive lead of each parallel connected panel string.

The theory behind this is--With one or two panels in parallel, there is not enough current to cause a problem if there is a short in the array wiring or solar panels to cause a fire. However, if you have three or more parallel connections, then if there is an array short somewhere, the other three panels (in your case) can supply enough current to over heat the shorted wire or solar panel and cause a fire.

So--You need to remove the "Y" connections from the positive side and run each "+" wire to its circuit in the combiner box. Then run the single cable output from the combiner box (and the "-" combiner) back to the epanel 63 amp breaker.

Also, I am not sure that you can use "Y" combiners in the array negative connection... Depending on the current rating of the panels, the sum of the 4 panel current output may exceed the current rating of the "Y" cable/connectors. You may need to use a standard bus bar to combine the solar panel negative connections. Remember, from code, you need to:

Isc of panels * 1.25 NEC solar current safety factor * 1.25 NEC safety factor for wire/protective device deratings
4x*Isc of panels * 1.5625 = "Y" and single cable wire rating.

To be honest, from an NEC or installation point of view--It does not matter if you run 4 pairs of wires from the array to the combiner at the edge of the array, or 40' away at the input to the epanel... It is your choice. Which ever works out best for you (I like to have breaker boxes installed out of direct sunlight and shielded from rain/snow if possible--hopefully, you will have fewer problems over time).

Regarding the whole theory behind the DC GFI setup (solar array ground goes through 2 amp breaker, if there is too much current--such as a positive to ground array short--The 2 amp breaker will open and also switch off the paired 63amp breaker. I am not sure that this whole setup is a good thing, however it is what code requires. That "DC ground fault interrupt" function is why you have those confusing in/out/in connections for the PV panels and charge controller (at least as I understand your question).

And, if you think you want a "panic switch"--think of placing one at the battery + bus instead... The PV Array is pretty much fixed in its maximum current/power output and is not really a source of excess current.

It is the lead acid battery bank that can output 1,000's or 10,000's of amp into a short circuit... In many ways, a big battery bank feeding a short is more of a hazard than the power that comes into your home from the utility drop.

In reality, a battery bank is frequently overlooked as to providing proper fusing/breakers/cutoff switches. People are used to the battery in an automobile (no obvious over current protection). However, that is not a good model for off-grid solar... First, many cars include a "fusealble link" for the first connection from the battery to the car's electrical system. And to the starter, the cable is large enough to take 100% of the short circuit current from the single battery without overheating/causing a fire (the battery may melt/catch fire/explode--but that is another issue:roll:).

Storage batteries for off grid systems are generally many times larger than a single car battery (both in current and voltage)--and it is not possible to use heavy enough wire to prevent overheating. You are looking at wiring/fuses/breakers that can handle hundreds of amps (or possibly more) depending on wiring connections (series/parallel/battery bank size) and loads.

And make sure you respect the AC and DC voltage/current ratings of switches/fuses/breakers... DC current is much more difficult to interrupt. DC tends to sustain arcs very well (when compared with AC power). So, DC devices tend to be much heavier/larger than their AC equivalent (or, many devices have AC and DC ratings--and the DC ratings are much less).

Each wire that leaves the battery bank should have its own, properly rated, fuse/breaker installed (close to the major current source--i.e., the battery bank).

-Bill

BB. · September 2010

Re: Off-Grid Install/Classroom+Lab Project Questions

By the way, I did move this to its own thread from the AC vs DC Panels and Breakers thread...

Generally, it is easier for every installation/poster to have its own thread. Keeps people (like me

) from getting confused as to what is getting answered (the original thread/questions or about your specific system).

Safely wiring up the DC/Battery side of an Off-Grid solar system is one of the major issues out there... Even electricians.installer sometime ignore the differences between normal AC home/business wiring and the requirements of low voltage DC wiring (typically, installing way too small AWG of wire, and neglecting proper fuses/breakers around the battery bank).

Unfortunately, doing the DC side of the system is expensive and difficult to do properly. And when done improperly, frequently there is no problems for years--but there is a "time bomb" left behind if something "unplanned" goes wrong (i.e., see Murphy's Law--along the lines of--If it can go wrong, it will go wrong, in the worst way possible--There is also the corollary--Murphy was an optimist).

And welcome to the forum.

-Bill

divonbriesen · September 2010

Re: AC vs DC Panels and Breakers

bb. wrote: »

d.i.,

if i understand your question correctly... First, you really do not need another cutoff switch between the array and the charge controller. The 63+2amp gfi will do that for you.

++But that unit is inside- under the combiner box cover plate- so you'd have to open it with a screwdriver to get to it in a hurry.

The reason you cannot figure out where to put the combiner box (should have at least 4 breakers or fuses inside) is because you have used the "y" mc4 connections.

++Right. Got that.

At best, according to code, you can only use the "y" connectors if you have two solar panels in parallel. If you have three or more panels (or series strings) in parallel, you need a series protection fuse (or breaker) in the positive lead of each parallel connected panel string.

The theory behind this is--with one or two panels in parallel, there is not enough current to cause a problem if there is a short in the array wiring or solar panels to cause a fire. However, if you have three or more parallel connections, then if there is an array short somewhere, the other three panels (in your case) can supply enough current to over heat the shorted wire or solar panel and cause a fire.

++Wouldn't this depend more on the size of the panels than the actual count? How is this done when you have a really big array? Shouldn't the total amps be the measure? The connectors are rated to 30amp.

So--you need to remove the "y" connections from the positive side and run each "+" wire to its circuit in the combiner box. Then run the single cable output from the combiner box (and the "-" combiner) back to the epanel 63 amp breaker.

++Clarification- there's a 63amp/2amp gfi breaker combo inside the combiner box- then there's another 63amp breaker in the epanel- between the outback mppt and the batteries it's charging.

Also, i am not sure that you can use "y" combiners in the array negative connection... Depending on the current rating of the panels, the sum of the 4 panel current output may exceed the current rating of the "y" cable/connectors. You may need to use a standard bus bar to combine the solar panel negative connections. Remember, from code, you need to:
isc of panels * 1.25 nec solar current safety factor * 1.25 nec safety factor for wire/protective device deratings

4x*isc of panels * 1.5625 = "y" and single cable wire rating.

to be honest, from an nec or installation point of view--it does not matter if you run 4 pairs of wires from the array to the combiner at the edge of the array, or 40' away at the input to the epanel... It is your choice. Which ever works out best for you (i like to have breaker boxes installed out of direct sunlight and shielded from rain/snow if possible--hopefully, you will have fewer problems over time).

++I'm actually going about 40' from the panels to the combiner, then a foot to the mppt, then a foot to the epanel (all 3 are inside). I thought it was more elegant to run 1 pair than 4 pairs of wires (and a bit cheaper).

++So... Can i lose the y connector that joins the two y connectors on the positive side... And run 2 positive wires and the existing negative so i have 3 wires coming in? Then i'll have 2 breakers on + and the bus for negative.

Regarding the whole theory behind the dc gfi setup (solar array ground goes through 2 amp breaker, if there is too much current--such as a positive to ground array short--the 2 amp breaker will open and also switch off the paired 63amp breaker. I am not sure that this whole setup is a good thing, however it is what code requires. That "dc ground fault interrupt" function is why you have those confusing in/out/in connections for the pv panels and charge controller (at least as i understand your question).

And, if you think you want a "panic switch"--think of placing one at the battery + bus instead... The pv array is pretty much fixed in its maximum current/power output and is not really a source of excess current.

++Actually the epanel has a huge switch on it for that already- cuts off the batteries from the inverters - and it's on the outside left of the box.

It is the lead acid battery bank that can output 1,000's or 10,000's of amp into a short circuit... In many ways, a big battery bank feeding a short is more of a hazard than the power that comes into your home from the utility drop.

In reality, a battery bank is frequently overlooked as to providing proper fusing/breakers/cutoff switches. People are used to the battery in an automobile (no obvious over current protection). However, that is not a good model for off-grid solar... First, many cars include a "fusealble link" for the first connection from the battery to the car's electrical system. And to the starter, the cable is large enough to take 100% of the short circuit current from the single battery without overheating/causing a fire (the battery may melt/catch fire/explode--but that is another issue:roll:).

Storage batteries for off grid systems are generally many times larger than a single car battery (both in current and voltage)--and it is not possible to use heavy enough wire to prevent overheating. You are looking at wiring/fuses/breakers that can handle hundreds of amps (or possibly more) depending on wiring connections (series/parallel/battery bank size) and loads.

And make sure you respect the ac and dc voltage/current ratings of switches/fuses/breakers... Dc current is much more difficult to interrupt. Dc tends to sustain arcs very well (when compared with ac power). So, dc devices tend to be much heavier/larger than their ac equivalent (or, many devices have ac and dc ratings--and the dc ratings are much less).

++Ok, so i'll remove the ac blade disconnect- bummed that i can only find big dc switches for over $100- seems crazy for simply opening and closing a circuit. For ac (up to 277v) it's a pretty small switch.

Each wire that leaves the battery bank should have its own, properly rated, fuse/breaker installed (close to the major current source--i.e., the battery bank).

++Really? So the big breaker isn't enough? (it's on positive)- the negative goes thru a shunt (that brass thing with the 4 thin pieces) - if i needed it, wouldn't that already be in the epanel (prewired)?

++feel free to address in separate replies, if you have time... I should have known better than to try and hit this all at once.

divonbriesen · September 2010

Re: Off-Grid Install/Classroom+Lab Project Questions

I've just uploaded some shots of this- note that halfway between these, i mounted the outback inverter and the switchbox- and moved the subpanel over-

http://www.flickr.com/photos/divb/sets/72157625001268262/

so you see combiner box->switch box-> outback mppt-> epanel with xantrex inverter on top, outback inverter with both leading to switch box, which leads to subpanel.

Note also panels on roof. Here's a picture of the plate on the panels- we have 4 of them. http://www.flickr.com/photos/divb/4679415019/in/set-72157624101635339/

Per Bill's post, i'm thinking:

lose the y connector that joins the 2 parallel arrays to each other, and bring those 2 pairs down to the combiner (not sure if i can leave the one on - or not - probably better to remove for clarity).

Lose the AC disconnect (neither the combiner nor the disconnect are hooked in right now- the wires are going right into the mppt) and bring wires to combiner, then to mppt, and don't worry about external cutoff.

what else?

BB. · September 2010

Re: AC vs DC Panels and Breakers

divonbriesen wrote: »

++Wouldn't this depend more on the size of the panels than the actual count? How is this done when you have a really big array? Shouldn't the total amps be the measure? The connectors are rated to 30amp.

The breakers are to protect each solar panel string from excessive current if it got shorted. The excessive current would come from the other paralleled solar panels, not from the charge controller/battery bank.

++Clarification- there's a 63amp/2amp gfi breaker combo inside the combiner box- then there's another 63amp breaker in the epanel- between the outback mppt and the batteries it's charging.

Yea--this is one of the things I disagree with NEC about their use of "Ground Fault Protection" for DC circuits. They are trying for other methods of safety through complexity. :roll:

++So... Can i lose the y connector that joins the two y connectors on the positive side... And run 2 positive wires and the existing negative so i have 3 wires coming in? Then i'll have 2 breakers on + and the bus for negative.

You can do it--make sure you watch the gauge of the negative wire--It will need to carry 2x the current (~3 AWG larger diameter) for the two strings.

++Ok, so i'll remove the ac blade disconnect- bummed that i can only find big dc switches for over $100- seems crazy for simply opening and closing a circuit. For ac (up to 277v) it's a pretty small switch.

DC makes a marvelous arc welder--does a really good job of sustaining arcs. And those "little" lead acid batteries can supply as much (or more) current than those 10,000 amp limited utility transformers outside your home.

++Really? So the big breaker isn't enough? (it's on positive)- the negative goes thru a shunt (that brass thing with the 4 thin pieces) - if i needed it, wouldn't that already be in the epanel (prewired)?

The big breaker to the Inverter is sized to the wire gauge to the inverter... XXX Amp inverter and X/0 wire... If you run some other wire to run a DC fan, DC lighting, DC radios, DC Fridge, etc... Then each of those wires is only a fraction of the size of the inverter wiring... And therefore each of those wires need their own fuse/breaker to prevent them from catching fire if they get shorted (i.e., NEC would use 15 amp breaker for 14 AWG wire; 20 amp for 12 AWG; 30 amp for 10 AWG; etc.).

++feel free to address in separate replies, if you have time... I should have known better than to try and hit this all at once.

I think I got all of your above questions...

-Bill

divonbriesen · September 2010

Re: AC vs DC Panels and Breakers

BB. wrote: »

The breakers are to protect each solar panel string from excessive current if it got shorted. The excessive current would come from the other paralleled solar panels, not from the charge controller/battery bank.

As I understand it, all this stuff is made to tolerate up to 600v short without damage... so isn't it the case that any combination of these 4 <100v panels couldn't cause an equipment damaging short?

Having said that, I did have to splice the wires on one panel- I think for a demo someone left them in the sun (just for show) but had the male/female plugged into each other to keep them from flopping around- they sort of melted into each other - so this puts my 'tolerate damage' theory into question.

BB. wrote: »

You can do it--make sure you watch the gauge of the negative wire--It will need to carry 2x the current (~3 AWG larger diameter) for the two strings.

I'm using the mc4 extensions (ended up with a bunch of different sizes, including 100') - again (perhaps mistakenly) thought these would do fine with the numbers I'm working with. Since we are using this for teaching, my gut is to just bring down 2 pair- so as not to have to confuse newbies with some odd configuration. Ultimately would like to bump up to 8 or 10 panels, or may have to switch panels (these are on loan) and reconfigure anyway.

On a related note, I have 3 leftover sharp panels: http://www.flickr.com/photos/divb/4679418833/in/set-72157624101635339/ (or are they GE) - these are 12v nominal right? Is there any way to combine them with these on the MPPT: http://www.flickr.com/photos/divb/4679415019/in/set-72157624101635339/ or is that like mismatching batteries in a toy? I'm thinking it is....

The big breaker to the Inverter is sized to the wire gauge to the inverter... XXX Amp inverter and X/0 wire... If you run some other wire to run a DC fan, DC lighting, DC radios, DC Fridge, etc... Then each of those wires is only a fraction of the size of the inverter wiring... And therefore each of those wires need their own fuse/breaker to prevent them from catching fire if they get shorted (i.e., NEC would use 15 amp breaker for 14 AWG wire; 20 amp for 12 AWG; 30 amp for 10 AWG; etc.).

I'm not doing any DC at all - though I did buy an aaooogahhh horn at northern that I'm tempted to plug in. I really hate the 12v plugs (car cigarette outlets) - is there any more friendly/standard 12 plug? I saw the honda generator (like yours i believe) has a cool little plug with one prong at a 90 degree to the other - but many walks down the electrical aisle show me only an overbuilt version of the same with a ground built-in (and expensive).

I think I got all of your above questions...

-Bill

you rock.

and thanks for tidying up my earlier post- I didn't realize how the quote tags worked.

d.i.

ps- do you use the epanel switch to go between your generator and battery bank? Any notion about how one could buy that switch separately? (there's an inverted breaker, so when you turn on on you turn the other off since they're attached by a plate)

icarus · September 2010

Re: Off-Grid Install/Classroom+Lab Project Questions

You can use Anderson connectors for any 12vdc outlets. Polarized, easy to plug and un plug. Do a search on the NAWS site. (My bandwidth stinks this morning) Come in many sizes.

Tony

BB. · September 2010

Re: AC vs DC Panels and Breakers

divonbriesen wrote: »

As I understand it, all this stuff is made to tolerate up to 600v short without damage... so isn't it the case that any combination of these 4 <100v panels couldn't cause an equipment damaging short?

Having said that, I did have to splice the wires on one panel- I think for a demo someone left them in the sun (just for show) but had the male/female plugged into each other to keep them from flopping around- they sort of melted into each other - so this puts my 'tolerate damage' theory into question.

The 600 Volt system rating is a standard for insulation "punch through" resistance (type and thickness of insulation, during manufacture, the components are subjected to 3x the rated voltage, 1,800 VAC in this case, to ensure they were manufactured without error)... Current does not really matter (for this specification).

Interesting about the melted plugs... On solar panels, that should not happen (melting by shorting a single panel output). Solar panels are current type energy sources and output (pretty much) a constant current related to the amount of sunlight on the panel, and is not dependent on load (between 0 volts and ~Vmp).

I don't know why they melted--They would have melted under normal operation too... Bad connector/not fully connected/plugged and unplugged a bunch and got worn out/dirty/pitted from plugging under load, exposed to heat from some other source, wrong rating of plug--Don't know.

I'm using the mc4 extensions (ended up with a bunch of different sizes, including 100') - again (perhaps mistakenly) thought these would do fine with the numbers I'm working with. Since we are using this for teaching, my gut is to just bring down 2 pair- so as not to have to confuse newbies with some odd configuration. Ultimately would like to bump up to 8 or 10 panels, or may have to switch panels (these are on loan) and reconfigure anyway.

It is easy to make a confusing array connection... Many people have a mix of different panels collected over time which can make the whole series/parallel/fusing/breaker/combiner configuration issues even more confusing.

Panels are getting less expensive (not too many years ago whey were $10 per watt and now we are at or below $3 per watt for the larger wattage panels). In some cases, it is better to just remove the collection of small/odd sized panels and EBay them... Then replace with uniform new panels to keep things sane (for example the series protection fuse/breaker of the combiner is based on each panel's UL safety rating is is roughly 2x Isc--mixing other panel ratings can require other fuses).

On a related note, I have 3 leftover sharp panels: http://www.flickr.com/photos/divb/4679418833/in/set-72157624101635339/ (or are they GE) - these are 12v nominal right? Is there any way to combine them with these on the MPPT: http://www.flickr.com/photos/divb/4679415019/in/set-72157624101635339/ or is that like mismatching batteries in a toy? I'm thinking it is....

About the only two ways I see... 1) combine 5x of the Sharp panels in series (Vmp=5x16.2v=81v) then parallel with the Vmp=73v of the Nextpower panels... Will not be perfect, but close enough (behind a single MPPT charge controller charging a 12/24 volt battery bank--48v battery bank may work OK too if panel are not too hot).

Or 2), get a second MPPT charge controller and put the three Sharp panels in series. Then parallel the second MPPT controller to your battery bank +/- bus connection (paralleling charge controllers is OK as long as you run the charge controller wiring directly back to the battery banks--for accurate voltage monitoring).

I'm not doing any DC at all - though I did buy an aaooogahhh horn at northern that I'm tempted to plug in. I really hate the 12v plugs (car cigarette outlets) - is there any more friendly/standard 12 plug?

Anderson Power Pole connectors are used quite a bit... They are very neat and can be color coded/stacked for polarity protection.

I saw the honda generator (like yours i believe) has a cool little plug with one prong at a 90 degree to the other - but many walks down the electrical aisle show me only an overbuilt version of the same with a ground built-in (and expensive).

I think the Honda 12 VDC plug has each blade rotated by something like 39 degrees. The Vertical/Right Angle blade (if I recall correctly) is a a standard 120 VAC 20 Amp rated wall plug (standard US 120 VAC plugs are rated at 15 amps).

and thanks for tidying up my earlier post- I didn't realize how the quote tags worked.

d.i.

You are very welcome--And you caught on very quickly :cool:;). We all started posting with zero experience. :roll:

ps- do you use the epanel switch to go between your generator and battery bank? Any notion about how one could buy that switch separately? (there's an inverted breaker, so when you turn on on you turn the other off since they're attached by a plate)

My system is a Grid Tied system and I have a very simple manual transfer switch made for connecting a generator to selected house hold circuits.

Here is a nice writeup on AC Transfer Switches.

Here is a sample of Manual Transfer Switches.

And here are a couple manual and automatic AC transfer switches from our host.

-Bill

dwh · September 2010

Re: Off-Grid Install/Classroom+Lab Project Questions

The 12v receptacle on the Honda generator seems to be pretty common on portable generators. I've never seen anything that plugs into it except small jumper cables for battery charging.

http://ep.yimg.com/ca/I/yhst-12532106308829_2120_16995699

There are other types of low voltage receptacles, such as trolling motor plugs and trailer plugs.

I met a guy years ago who had a shack out in the desert and the whole thing was wired 12v. He had these things for his receptacles - but they are actually 15a 240 VAC receptacles and I don't know what they would be rated at for DC:

divonbriesen · September 2010

Re: AC vs DC Panels and Breakers

BB. wrote: »

I don't know why they melted--They would have melted under normal operation too... Bad connector/not fully connected/plugged and unplugged a bunch and got worn out/dirty/pitted from plugging under load, exposed to heat from some other source, wrong rating of plug--Don't know.

perhaps they were not fully plugged, but in far enough to create some kind of arc? I don't even know when it happened, just that when i had it up on the roof and was ready to plug in they were basically stuck together (these are the permanently wired mc4 connectors).

About the only two ways I see... 1) combine 5x of the Sharp panels in series (Vmp=5x16.2v=81v) then parallel with the Vmp=73v of the Nextpower panels... Will not be perfect, but close enough (behind a single MPPT charge controller charging a 12/24 volt battery bank--48v battery bank may work OK too if panel are not too hot).

Or 2), get a second MPPT charge controller and put the three Sharp panels in series. Then parallel the second MPPT controller to your battery bank +/- bus connection (paralleling charge controllers is OK as long as you run the charge controller wiring directly back to the battery banks--for accurate voltage monitoring).

i only have 3 of them, so five is not an option, but i did pick up a phocos 30amp mppt, so option 2 might work. I guess the incoming charge just does what it can and either one jumps in when the batteries need it? (either mppt i mean)

Anderson Power Pole connectors are used quite a bit... They are very neat and can be color coded/stacked for polarity protection.

wow, these aren't cheap. wouldn't be practical for usage on 12 outlets and appliances... but maybe good as battery disconnects i guess. what's the standard way to cut off batteries? I have vague memories of seeing a part at the auto store that's basically a big knob- you turn it (90 degrees?) to disconnect. In my case i'm wondering what plug/outlet system one could use for 12v system regularly (i.e. for the wall outlets) other than the standard cig lighter.

I checked out the transfer switch:

http://store.solar-electric.com/miso60amp240.html

and am wishing i'd bought this instead of the big beast i have: http://www.flickr.com/photos/divb/5010788562/
this thing is 14" or so high- annoyingly large.

However, the smaller one here on this site shows "not rated" - can i assume that means not UL? I don't care, but my bosses might.

The epanel already has this setup, so i toyed with the idea of just using the 2nd inverter as my "generator in" but eventually would like to have a little generator as part of the unit, so wanted to leave that available.

A new question:

i've currently got the wires from the panels just hanging, and need to put them into conduit- what "entry point" should i use? I presume you can't just run them right into the conduit- do i use one of those openings like at your house (only smaller) that has downward facing holes to bring in wires from the pole and lead off from there? Since these long cables are black, where do I have to start making the negative white? Can i just put some white tape on each end of that long run? The shorter cables I've got white/black/green number 6.

best,

d.i.

divonbriesen · September 2010

Re: Off-Grid Install/Classroom+Lab Project Questions

dwh wrote: »

The 12v receptacle on the Honda generator seems to be pretty common on portable generators. I've never seen anything that plugs into it except small jumper cables for battery charging.

http://ep.yimg.com/ca/I/yhst-12532106308829_2120_16995699

There are other types of low voltage receptacles, such as trolling motor plugs and trailer plugs.

I met a guy years ago who had a shack out in the desert and the whole thing was wired 12v. He had these things for his receptacles - but they are actually 15a 240 VAC receptacles and I don't know what they would be rated at for DC:

yeah, this is what i was just looking at at northern- they don't seem very common though- seems like they'd be ideal for 12 v appliances and such - similar to household plugs, but not interchangeable.

as an aside, i LOVE american household plugs (as opposed to europe/africa's 2 round plugs). What i've seen and used in europe/africa sucks by comparison just in terms of size and fixtures. On the flip side, I wish we were 220v...

d.i.

divonbriesen · September 2010

Re: Off-Grid Install/Classroom+Lab Project Questions

icarus wrote: »

You can use Anderson connectors for any 12vdc outlets. Polarized, easy to plug and un plug. Do a search on the NAWS site. (My bandwidth stinks this morning) Come in many sizes.

Tony

I can't seem to figure out how these work- i see the hermaphroditic design, but wouldn't flipping the other side over to get it to fit revers the 2 wires?

I have a big UPS that the telco dept gave me that was designed to charge an exterior battery bank of 120 VDC... and it used a triple connection like this- but I think it does have male female.

Anybody have any ideas what I should do with the unit? It's about 200-300 lbs, and there's a separate rack for 12 batteries- half of which are no good anymore. I think there must be a built in battery too... since it's so heavy.

i'm only doing 110, so can't think of a use for it at the moment.

I also have about 96 small 12v batteries that were part of some APC banks for the data center- they were wired in strings of 8, each in it's own metal cartridge, that slides into the racks... kinda cool... but not sure what i can do with them, and not sure what condition they're in (end of 3 year life that APC gives but you know how that goes).

d.i.

BB. · September 2010

Re: Off-Grid Install/Classroom+Lab Project Questions

The Anderson connectors are expensive... The way they stack together, you cannot flip them over to reverse polarity. And you can stack 2 to many in one stick... For DC you would only need two or three (power+ground?).

Using "unusual" AC plugs (240 VAC, twist locks, etc.) is very common for people to use for DC power plugs (use appropriately rated current levels). The larger plugs start getting pretty pricey too--And does not comply with the electrical code.

Regarding the transfer switch--Yes, you would need it to be UL/NRTL listed... You might call Midnite Solar and ask them directly.

Regarding the MC4 connectors--There is a set of tools that make them easier to unlock--I assume you saw them melted vs just difficult to unlock.

Yes, you can tape/color code just the ends of the wire (wrap down to where wire comes into the box if possible).

The old UPS batteries--Probably not worth trying to salvage... If you want to reuse the trays for your system--may be interesting.

Regarding a battery disconnect--Look at Marine applications/vendors... NAWS does have one:

Blue Sea Battery Switch 1-2-OFF 350 Amp

Blue Sea 9001e 350 Amp Battery Switch Make-before-break contact design allows switching between battery banks without power interruption Ignition protected - Safe for ... mounting Meets American Boat and Yacht Council (ABYC) requirements for battery switches 3/8"-16 tin-plated copper studs

-Bill

divonbriesen · September 2010

Re: Off-Grid Install/Classroom+Lab Project Questions

BB. wrote: »

Regarding a battery disconnect--Look at Marine applications/vendors... NAWS does have one:

Blue Sea Battery Switch 1-2-OFF 350 Amp

Blue Sea 9001e 350 Amp Battery Switch Make-before-break contact design allows switching between battery banks without power interruption Ignition protected - Safe for ... mounting Meets American Boat and Yacht Council (ABYC) requirements for battery switches 3/8"-16 tin-plated copper studs

-Bill

So this is interesting, and not too pricey- so why couldn't you also just use this as a DC cutoff for the PV, or for that matter, anywhere on the DC line (i.e. between the MPPT and the batteries)?

It's a heckuva lot cheaper and more elegant than the DC blade disconnects... so i wonder what I'm missing.

Also, am I correct in thinking you'd only need one of these (say, on the positive side)?

d.i.

BB. · September 2010

Re: Off-Grid Install/Classroom+Lab Project Questions

Yes, you can use the switch for other DC loads/connections--as far as I know.

I would avoid switching things "under load" if possible--DC switches tend to arc (worst that AC switching) and you probably will erode the contacts over time.

Here is the Mfg.'s website--lots more choices:

Blue Sea Systems

Yes, with proper planning (making sure ground/negative/return cables are rated for switched loads)--Should not be a problem (common ground system between all DC devices).

-Bill

mike95490 · September 2010

Re: Off-Grid Install/Classroom+Lab Project Questions

BE AWARE - there are 2 versions of these switches,

Break before Make (you get 0V for a split second)

Make before Break (2 systems are shorted together for a split second)

Chose wisely for your application

Off-Grid Install/Classroom+Lab Project Questions

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