Wiring Choices for PV to Combiner Box

sub3marathonmansub3marathonman Solar Expert Posts: 300 ✭✭✭
There is no clear answer that I can see, so maybe some people here with practical knowledge can help.

There are two roughly equal choices.

I can have the Combiner Box right up on the garage roof with the PV panels. The benefit is that there aren't many expensive MC cables to buy. The drawback is that then there is a longer run of wire for the combined output, although the cost for that isn't as bad as the MC cables. In addition, there would need to be conduit, which would cost a bit and offset some of the wire cost savings, and also more roof penetrations to hold the conduit. However, after about 80 penetrations I don't know if a couple more are enough to worry about when a qualified roofer is doing it.

Or, I can have the Combiner Box inside the house, where it would stay nice and dry. Of course, the Combiner Box is made for outdoor use, so I don't know how much that aspect should enter into the decision. Of course, if there was ever a need to reset a breaker or to turn off the PV output, it would be much easier to do that from inside the house instead of up on the garage roof.

And I'm not sure which way is really the most efficient. It would be 50' of #10 MC wire for some panels, with as little as 5' for others, and I'm not sure about such a discrepancy between panels. Otherwise, it would probably be about a maximum of 20' for all panels, with some as little as 2'. Of course, then after the combiner box there would be probably 20 extra feet of either #6 or #4 wire.

Is it really 6 of 1, half a dozen of another, or am I overlooking something? Right now I'm leaning toward the Combiner Box on the garage roof.

Comments

  • BB.BB. Super Moderators, Administrators Posts: 31,061 admin
    Re: Wiring Choices for PV to Combiner Box

    I like to keep as much hardware out of direct sun and rain as possible... This could be as simple as mounting on a north facing wall and/or under an overhang/behind tree/landscaping. If you can reduce the need to drag out a large ladder for servicing/debugging--another plus.

    I have not seen the box you are talking about--but just about anything mounted in direct weather will collect moisture and it is usually highly recommended that a small weep hole be drilled in the lowest corner once mounted (and remember conduit does a very nice job of bringing rain water into the home/junction boxes--been there, done that).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • arkieoscararkieoscar Solar Expert Posts: 101 ✭✭✭✭
    Re: Wiring Choices for PV to Combiner Box

    You'll be going up there periodically to check on the panels, so it wouldn't be a problem if the combiner was up there with them and you checked for problems in it while you were there. The difference in length of the conductors shouldn't be an issue. It would take lab tests to detect any voltage drop in such short runs.
  • BB.BB. Super Moderators, Administrators Posts: 31,061 admin
    Re: Wiring Choices for PV to Combiner Box

    You can also use the Solar Wiring Voltage Drop calculator (Excel spreadsheet download) to check your voltage/power drops and see what works out best for you.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Wiring Choices for PV to Combiner Box

    that is a dilemma to have to choose between the convenience of seeing breakers/fuses in an easy location with having to run long lengths of expensive mc cables that may be presenting excessive voltage drops for the long lengths and climbing to your roof to reset a breaker or replace a fuse. what i would consider doing is cutting the mc cables and still putting them into a box up on the roof, but only to join each of the mc cables with another wire to continue its individual run even though in conduit and then place the fuse/disconnect box into a more convenient place. this also would allow you to have larger wire to lower the voltage drops on individual strings. it is still advisable to have a durable enclosure for joining those wires with solid connections inside and the weep hole on the bottom is also a very good idea.
    something along this line would be good inside that box. http://store.solar-electric.com/podibl2x4ci.html
    it does not have to be even something so elaborate as even split bolts come to mind to be used, but i'd worry of the wires being able to be pulled apart. if the box being used has any knockouts then some standard electrical strain reliefs on the knockouts may solve that. how these would be weather proofed would be another set of problems.
  • sub3marathonmansub3marathonman Solar Expert Posts: 300 ✭✭✭
    Re: Wiring Choices for PV to Combiner Box
    niel wrote: »
    what i would consider doing is cutting the mc cables and still putting them into a box up on the roof, but only to join each of the mc cables with another wire to continue its individual run even though in conduit and then place the fuse/disconnect box into a more convenient place. this also would allow you to have larger wire to lower the voltage drops on individual strings. it is still advisable to have a durable enclosure for joining those wires with solid connections inside and the weep hole on the bottom is also a very good idea.


    Wouldn't that open up a whole new dilemma with many additional wire to wire connections? I guess I could use additional combiner boxes and have shorter MC cable runs, but isn't the cost going to negate any savings?

    What I calculated, strictly for PV output, was that over 50 years it would be about $120 savings (at $0.20/kwh) going with the shorter MC cable, and larger wire. There would be additional savings from using the shorter MC cable too, I figured about $200. But some of that would be offset buying the connectors and additional conduit.

    Is it possible to put the combiner box in another larger box to protect it somewhat, or is that an NEC code violation? I was thinking even under the solar panels, but I'm not sure it would fit.

    I also appreciated the practical advice from B.B. about the water issues.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Wiring Choices for PV to Combiner Box

    after so much has already been spent you are now going to be thrifty with the wire? i believe it is a violation to have the disconnect located on the roof anyway as that makes it too inaccessible. it is also not convenient to switch a breaker back on or replace a fuse located up there. many people do connect to the pvs with a short mc cable and splice the wire of their choice to it to make the main run along side any other like strings to the combiner/circuit breakers and main disconnect. to make myself clear, i am not saying to cut the mc cable that is on the pv itself, but obtain another short mc cable that will be cut in 2 so that it can then be plugged into the pv with the mc connections (keeping with pv warranty) on the 1 end and your home run wire to be attached to the open end of the mc cable to go down to the more convenient location for the disconnect/breakers and this allows for heavier wire to be used also keeping resistive losses down. i hope you're not also the type to buy an expensive stereo and put $10 speakers on it. i appologize if this sounds harsh, but don't skimp on the wiring or safety and you certainly don't want to compromise on an expensive pv system or am i misunderstanding something here?
  • sub3marathonmansub3marathonman Solar Expert Posts: 300 ✭✭✭
    Re: Wiring Choices for PV to Combiner Box
    niel wrote: »
    after so much has already been spent you are now going to be thrifty with the wire?
    i hope you're not also the type to buy an expensive stereo and put $10 speakers on it. i appologize if this sounds harsh, but don't skimp on the wiring or safety and you certainly don't want to compromise on an expensive pv system or am i misunderstanding something here?


    Well, yes it does sound a bit harsh, especially in this forum as opposed to the Advanced Solar forum. I'm more uninformed than stupid. That is why I'm asking the questions. And I appreciate the people, including niel, who take the time to answer and share their expertise. The local electrician I thought I could rely on has proven to be not so reliable, so I'm having to figure everything out myself currently.

    And yes, I thought the idea of joining a short MC cable with a regular cable was a great idea. It is just that I don't know how to do that or what the repercussions are. People have said that all connections will go bad eventually, yet that would be adding 14 more connections. It was said to use a "splice block," but I'm not sure which splice block is right. Is it OK to join different size wires in a splice block? I don't know and from reading the literature I still don't know. There would be more small electrical boxes, so therefore more roof penetrations for that. But that may just be my worry, since the roofer isn't worried about doing about 80 he shouldn't worry about 8 more.

    Also, I don't understand why everyone doesn't do it that way if it is a great idea and obviously more efficient for the PV system. It seems like it would be less costly too, I don't understand why the MC wire has to be so expensive, especially for the longer runs. SolarGuppy ran long lengths of MC cable through the roof and down to the combiner box, and he is very knowledgeable. So that is confusing to somebody like me who is uninformed.

    And yes, I am trying to be intelligently thrifty, since everything has an impact on the payback time. I even thought about using 316 stainless bolts instead of 304 stainless bolts to hold the mounting hardware on the roof because they are supposed to be stronger, and I would have paid the extra money for the added safety, but then I guessed that either bolt would probably pull out of the rafter before it snapped. But I also agree that it is better to buy a #4 wire instead of a #8 or #10 wire, because there is a benefit to the efficiency and it will pay after some time period, even if it is 20 years it is better than never. And what usually happens when you cut corners is that you then have to go back and do it right, thus spending more money than if you do it right the first time.
  • Solar GuppySolar Guppy Solar Expert Posts: 1,963 ✭✭✭
    Re: Wiring Choices for PV to Combiner Box

    I ran MC's on the roof to a PVC box. There I wire-nutted to THWN one to one into the sturcture using emt for the conduit. The THWN is 10 AWG 500ft rolls at home depot for 100 bucks roll.
  • sub3marathonmansub3marathonman Solar Expert Posts: 300 ✭✭✭
    Re: Wiring Choices for PV to Combiner Box
    niel wrote: »
    i believe it is a violation to have the disconnect located on the roof anyway as that makes it too inaccessible.

    I was wondering about that too. But according to the May/June 2007 article John Wiles wrote for IAEI news it seems to be OK.

    "There are some PV installations, both residential (flat roofs) and commercial, where the inverters are mounted near the PV arrays on the roof in not-readily-accessible locations. NEC 690.14(D) addresses these systems and requires ac and dc disconnects at the inverters and an additional ac PV disconnect at ground level. Figure 4 shows this system where all of the equipment is outside the building." Here is the link for Figure 4: http://www.iaei.org/subscriber/magazine/07_c/wiles_figure4.gif
    What is notable is that there is only one DC disconnect by the PV array on the roof.

    So to me that seems to be OK with the NEC to have the disconnect on the roof. But maybe it can only be OK on a flat roof? The NEC also says that you are supposed to have the disconnect where the wire enters the building, and apparently some inspectors want the disconnect on the outside, which would be the roof. I don't know how anybody would know that either unless the specific inspector tells you in advance where the disconnect should be.
  • sub3marathonmansub3marathonman Solar Expert Posts: 300 ✭✭✭
    Re: Wiring Choices for PV to Combiner Box
    I ran MC's on the roof to a PVC box. There I wire-nutted to THWN one to one into the sturcture using emt for the conduit. The THWN is 10 AWG 500ft rolls at home depot for 100 bucks roll.

    So I apparently misunderstood, it really only was a short MC cable before it went into the PVC box.

    But then too, I thought those wire-nuts were frowned upon.
  • Solar GuppySolar Guppy Solar Expert Posts: 1,963 ✭✭✭
    Re: Wiring Choices for PV to Combiner Box

    Not sure where you got wire nuts are bad , They just have to be in a dry environment ( like a PVC or metal gang box sealed from the wire entrance from the roof )

    Its code compliant and I have never had a well torqued nut fail, ever
  • mike95490mike95490 Solar Expert Posts: 9,021 ✭✭✭✭✭
    Re: Wiring Choices for PV to Combiner Box

    Combiner box - I missed how many strings you are combining ? Maybe you don't need breakers/fuses
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • Solar GuppySolar Guppy Solar Expert Posts: 1,963 ✭✭✭
    Re: Wiring Choices for PV to Combiner Box

    He is doing a 48V XW6048 system Mike, not a HV gridtie system
  • sub3marathonmansub3marathonman Solar Expert Posts: 300 ✭✭✭
    Re: Wiring Choices for PV to Combiner Box

    Solar Guppy has been a gigantic help with this project.

    He even saved the day by sending me to a great Physical Engineer (Robert Haug) when the Conergy mounting system I bought thinking it was certified for Florida turned out to need certification from a PE in Florida.

    Also, I thought if the boxes are on the garage roof close to the panels it would be needing to be code compliant for a wet environment, in addition to a high heat environment. (That was my understanding of niel's idea, just a short MC cable connected to a larger regular cable in the electrical box with those wires then going in the conduit to the charge controller.)

    I'm still trying to figure out if those power distribution blocks that niel mentioned would be good. It seems that the one mentioned by him takes one input and has three outputs, so that wouldn't be necessary, I would only need a one to one block. I think I have found one that might work good, but I'm not 100% sure it is OK for the NEC. Also, once again, they seem pretty expensive. It would end up at about $18 / wire. I don't know if there are multiple single wire to single wire blocks that would be more reasonable. I think the split bolt would probably be a lot more reasonable, but I'm not sure they are OK either.

    There will be 21 panels total, Sharp 224U1F, and arranged as 3 strings of 3 panels and 4 strings of 3 panels. So I will have a total of 14 wires to figure out.
  • Solar GuppySolar Guppy Solar Expert Posts: 1,963 ✭✭✭
    Re: Wiring Choices for PV to Combiner Box

    Wire nut the MC to THWN in the roof box ( any weather tight box, I have used PVC and the gang boxes for outside lights that have the scew in plugs)

    5 bucks max for wire nuts and the connections will outlive you. use the thwn from the wirenut down to the combiner in the garage, run one to one, don't do the combining on the roof

    In both systems I've done, never had a single issue with the wire nuts, just make sure you get about 2-3 full twists on both wires, this indicates the nuts on tight
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Wiring Choices for PV to Combiner Box

    sorry i didn't mention the wire nuts, but they're fine too. i do recommend rechecking the tightness on wire nuts after a period of time as occasionally i have personally seen some loosen up a tad. maybe it has to do with the initial spring tension being released somewhat for some of them, but once rechecked i've had no troubles.

    i guess i should mention in using wire nuts, being you admit to knowing very little, that the wires must be twisted in a clockwise fashion or the turning of the wire nuts while putting them on will untwist the wires. also, watch that you get the right amount of insulation stripped as too little will interfere with the contact area and too much could allow a short to another wire set or the box if made of metal.

    with smaller wires it is possible to slightly knot the wire and give it some strain relief. most use that over under thing we do on shoes before before the final loop knot. some shrink tubing can also provide a small amount of relief, but it would help in the weatherization of the connection moreso.

    i'm sorry for my over reactions as i get that way when seeing somebody possibly headed down a wrong path at times. my goal is to try to help you as is everybody else. btw, connections can fail, but using a good connection method and inspections by yourself every few years should insure your system will go on functioning well. sometimes these connections aren't easily accesable and it's ok to trust them when seeing the results are still good. (as in good output levels) now a loose or bad connection isn't the end of the world, although it is wasteful of expensive pv energy and if not caught in time can have repercussions to any batteries in battery systems.

    one more note here that i might bring up is the negative leads from your pv strings. those can be combined and sent down the line in one larger wire if you wish as there aren't any disconnects or fuses/breakers to be in the negative side that would restrict you from doing so. (at least for negative grounded systems anyway)
  • sub3marathonmansub3marathonman Solar Expert Posts: 300 ✭✭✭
    Re: Wiring Choices for PV to Combiner Box
    niel wrote: »
    one more note here that i might bring up is the negative leads from your pv strings. those can be combined and sent down the line in one larger wire if you wish as there aren't any disconnects or fuses/breakers to be in the negative side that would restrict you from doing so. (at least for negative grounded systems anyway)

    Wow, I didn't know you could do that! Do you then have to make that a much larger wire than the three or four positive wires? That really means that you could use the combiner boxes just for combining the positive wires and with the breakers.

    Also, a really basic question, then how much voltage and amperage do you calculate is going on the negative wire?

    And can you use these wire nuts or even split bolts to connect wire of different sizes without violating any NEC rules?
  • BB.BB. Super Moderators, Administrators Posts: 31,061 admin
    Re: Wiring Choices for PV to Combiner Box

    Yes, you only need to put breakers/fuses in the positive side. The negative circuits are just all combined together and sent down one large awg wire.

    How much current in the negative lead... The sum of all of the positive (paralleled) solar panel currents into the positive bus bar to the common positive lead going back.

    Regarding the voltage on the negative wire--technically, there is almost no voltage (assuming negative grounded system) other than the voltage drop caused by the current flow... Practically, the NEC (as I dimily remember) requires all of the wire insulation to be of the same rating when in the same conduit... For example, the green safety ground wire and the white neutral wire in Home AC wiring both have near ground potential--but they all must have the same 600 VAC insulation rating as the Black/Red hot wires. However, you can run a bare safety ground wire with no insulation just fine (not sure if bare wire is allowed in conduit--would probably be pretty difficult to pull in a long/curvey run).

    Regarding adding, for example, 2 wires in parallel to make the next 3 awg larger equivalent wire... I don't think NEC allows you to parallel wires--and I would not do it anyway. It is very difficult to parallel wire runs and get balanced current between them... There is always an extra bit of resistance some where (plug, connector, connection, lug, wire nut, etc.) which reduces current flow in one of the parallel legs, and causes more current to flow in another leg (or, in a group, one leg has lower resistance and "hogs" more of the current flow). In either case, the P=I^2*R really kills you... Double the current flow, and you get 4x the heating effect.

    Paralleling of wires is frequently done in electronic/computer equipment--and, many times, it turns out to be a real weakness for long term reliability (typically a burned contact or so out of 4+ shared current connections--and eventually all the remaining contacts burnout and the device fails).

    You can "parallel" your negative (and positive) leads (use, for example, 6x 12 awg wires--one for each panel string), but do not combined them into "one circuit" until you get back to the main bus bar (where-ever that is). Then, you would run your one 4 awg wire back from there...

    Or, depending on your actual current ratings, you can run 100' (completely made up numbers for example) of 6x 12 awg back to near the inverter (with fuse/breaker in positive leads back to panel), and combined into one 5' piece of 12 awg for the final run to the inverter--assuming the total currents from the 6 strings add up to less than 80% of the carrying capactiy of the 12 awg wire (typically around 20a*0.8=16a inside of conduit).

    Typically in solar installations, you are designing for minimum voltage drop (vs $$$$ spent). So, for example, my two 4.5 amp strings on my 3.5 kWatts of solar panels are running on 12 awg wire for ~30' (times 2) for low voltage drop, not because I needed to run 20 amps through the wiring.

    Does my "verbal" picture make sense?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Wiring Choices for PV to Combiner Box
    Wow, I didn't know you could do that! Do you then have to make that a much larger wire than the three or four positive wires? That really means that you could use the combiner boxes just for combining the positive wires and with the breakers.

    Also, a really basic question, then how much voltage and amperage do you calculate is going on the negative wire?

    And can you use these wire nuts or even split bolts to connect wire of different sizes without violating any NEC rules?


    the negative lead will need to be a larger wire and you can calculate it using the voltage drop calculator. i thought i was seeing bill mentioning 1-2 volts dropped for 12v and that would be high percentage wise as we like to keep system voltage drops as low as we can reasonably get, but nec specs 5% total from the source to the load. we generally opt for 2-3% or less if at all possible. you still will need to combine the negatives in a box to exit in one larger wire and it can be done in the same box your extensions for the positives off of the mc wires are housed in provided you've enough room. this can be a ground block like those found in circuit breaker boxes, but be sure the + and - will not touch even by the electrical box's metal housing if it's made of metal. here's an example ground block and they are available to accomodate both larger and smaller wire gauges and the number of connects. http://www.lowes.com/lowes/lkn?action=productDetail&productId=93985-82364-GBK10P&lpage=none

    yes, you can continue the run of wire using these connections. they use them in houses too although split bolts are less commonly used. if there will be many of these connections made with split bolts then make a provision to insulate these connections. one way is shrink tubing or some kind of sliding rubber boot among many other options. i believe split bolts to be good for only connecting 2 wires, but that's fine for your extension purposes. the negative combining will need something like the block in my example here or the one i sited earlier from windsun that has several smaller wires in and one larger wire out. this block is reciprocal in that it could be viewed as 1 large wire in and several smaller wires out, but not for our purpose here.

    i almost forgot to mention that they technically are for the same wire gauges in and out, but small excursions like joining a #10 and a #12 should work ok imho. excursions going too far are apt to create the possibility of the wires not securing well under the screw portion. maybe somebody could chime in on what the nec says about it.
  • MoeMoe Solar Expert Posts: 60 ✭✭
    Re: Wiring Choices for PV to Combiner Box
    niel wrote: »
    ...this can be a ground block like those found in circuit breaker boxes, but be sure the + and - will not touch even by the electrical box's metal housing if it's made of metal. here's an example ground block and they are available to accomodate both larger and smaller wire gauges and the number of connects. http://www.lowes.com/lowes/lkn?action=productDetail&productId=93985-82364-GBK10P&lpage=none

    Correct me if I'm wrong, but...

    If you use an uninsulated ground block in the box to join the negative leads, you're bonding the DC negative bus to ground at that point (assuming the box and conduit are grounded).

    That's okay if it's the one and only point you're doing that AND if you aren't using DC Ground Fault Protection. If you are using the latter, not insulating the negative leads from ground in the combiner box will bypass the DC-GFP.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Wiring Choices for PV to Combiner Box

    ground block is what the item is called, but in this case we aren't using it for grounding purposes. i guess i should've clarified that a bit more. this is not to imply it needs sent to ground because of the name of the part, however if you do ground it it may be easier to make the connection with this part.
    not all electrical boxes are made of metal noting that the metal of metal boxes are grounded so in that case please mount the grounding block to wood or plastic or some other insulated material to prevent electrical contact with the metal box. after that the insulating mount can then be attached to the box.
  • newenergynewenergy Solar Expert Posts: 291 ✭✭✭✭✭✭
    Re: Wiring Choices for PV to Combiner Box

    Some areas require a means of disconnection before entering any structure. Some are ok with it being right inside whenever you take the DC wiring inside. Some don't care either way as long as there's some disconnecting means somewhere. Yes, sometimes this disconnect is not readily accessible (whether on the roof or in an attic) but the code asks for a disconnect where it enters the house and the city doesn't necessarily want to force you to run conduit all over the exterior to get it to a readily accessible point. They do always want at least one disconnect (usually near the meter and inverter) to be readily accessible.

    690.14 for the relevant code

    Pretty much all of the inspectors I deal with want to see all of the wiring, PV or not, except maybe a very short length, either secure and under the array or in conduit.

    I like to use insulated butt end crimp splices to connect wire to wire. A well done wire nut is probably just as good, but it's a lot harder to screw up the crimped splice. Though, I see you are on a battery system and with the larger wire sizes, I don't know if these are available.

    BB - I've run bare in conduit and as far as I know it's ok with the NEC. When you say all of the insulation of the wiring should be of the same rating in a conduit do you mean just the voltage or temperature/wet-dry etc? I don't think I've ever mixed THHN with USE-2, but it doesn't seem like it would necessarily be wrong.


    You can buy waterproof strain relief for the wire that enters the junction/combiner box.

    http://www.aeesolar.com/catPDFs/low/15-Wire_Cable-low.pdf (sorry for the outside link - I looked for these on wind-sun's store and couldn't find them)
  • BB.BB. Super Moderators, Administrators Posts: 31,061 admin
    Re: Wiring Choices for PV to Combiner Box

    My understanding is that the insulation has to meet the minimum requirements (temperature, exposure, etc.) obvious stuff...

    One of the other requirements was that all of the wire in the same conduit had to have the same rating (IIRC).

    And, my assumption was that this was the voltage rating of the wire... So if you had a mix of 12 volt and 120/240 volt wiring in the same conduit, it all had to be wired with 600 Volt rated insulation--I guess under the assumption that if the insulation failed on a 120.240 volt wire, that the 12 volt insulation would not be good enough to prevent a short on the 12 volt bundle... There are a bunch of other caveats--how much current/energy is available, is the circuit double insulated, etc. that we won't worry about for this discussion.

    The regulatory engineer and I had lots of long discussions about whether all of the other ratings had to be the same too--It was years ago--but the argument was something like I could use regular temp rated insulation, but it sometimes was easier/cheaper to get higher temperature rated insulation in the colors I wanted--and the reg. engineer argued if I used the higher temp wire for some colors--I had to use the high temp wire for all colors (even though it was not needed)... I eventually won--but it was probably because the NEC (back 10-20 years ago) was not clear on the requirement that I could not do this... In the end, many times, that is how these discussions are ended--arguing dueling paragraphs from various regulations/requirements.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • sub3marathonmansub3marathonman Solar Expert Posts: 300 ✭✭✭
    Re: Wiring Choices for PV to Combiner Box

    The local inspector OK'd the combiner boxes on the garage roof. Disconnects on the roof are allowed in the NEC. But I agree that they aren't "readily accessable."

    And that brings me back to my initial dilemma, do I go for more readily accessable inside the house, or do I just put the combiner boxes on the roof and hope that they really are OK for outside use, as the manufacturer states?

    How often have people had to reset / turn off these breakers? Does anybody have the combiner box on the roof? And if so, have there been any major drawbacks?

    In regards to niel's suggestion, I did go to Lowes, and I did see those grounding bars and they look like they would work great at a relatively low price. However, now I've already ordered the wires long enough to accomodate the combiner boxes on the garage roof, (ordered through NAWS), so there isn't any potential monetary savings any more. It would just be convenience and protection of the wire connections if I relocated the intended combiner box location to inside the house.

    One other question I had was how to run the wiring for entering the house through the roof. The roofer wants to do a gooseneck, and from what people have said, you can't just turn the conduit down through the roof. So do you use the flexible EMT for that short run? And do you then need another disconnect either right before it enters the roof, or right after it comes into the house?
  • BB.BB. Super Moderators, Administrators Posts: 31,061 admin
    Re: Wiring Choices for PV to Combiner Box

    What are your AC/DC disconnects like at ground level and around the GT Inverter itself...

    Many inverters include the AC and sometimes the DC disconnect...

    Regarding how often needed to turn off the inverter... For a sample size of one--My Xantrex GT 3.0 I have not needed to turn it off in 3+ years.

    In general, if your disconnect gets water in it (because it is exposed, or because water followed the wires into the box), you will find you are going in much more often to clean corroded connections and damaged components.

    I had a similar problem with the AC Mains entry on my old home... Water was getting in through the roof flashing and dripping down into my breaker box--had to replace the guts after a few years of ignoring the problem (I could not figure out how the water was getting in... Eventually, I just took "600 MPH tape" -- aluminum heater duct tape -- and wrapped the flashing and "entry head" (?) and the leak was fixed).

    One thing to remember--assuming you stay in your home for the next 20+ years--you are going to get older and feel like climbing ladders as much any more (my 2nd story roof looks much more scary now than anything I remember from 30 years ago--kind of sucks getting old).

    Unless that box and wiring is really sealed well and not in full sun during the summer--I would vote to put it at ground level and shielded from the worst of the rain/sun.

    Breakers and their enclosures are relatively complex mechanical assemblies--they do not suffer heat, thermal cycling, and moisture well at all (thermal cycling + moisture is a sure way to kill just about any electromechanical device--worked well in my disk drive evaluation lab to sort out the weak performers--anything that did not die in a couple weeks was probably a good product).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • newenergynewenergy Solar Expert Posts: 291 ✭✭✭✭✭✭
    Re: Wiring Choices for PV to Combiner Box

    We install junction/combiner boxes on the roof on almost every job and haven't had a problem.

    If they are small boxes, we mount them on the rails, under the modules. That way they stay out of the sun and weather. They are outdoor rated and make a good seal and we keep all penetrations on the bottom or sides. We use waterproof fittings. If you have something that doesn't seem like a great seal, you can put silicone on it. If the box is a big combiner (like outback or midnite solar) I think it's fine to trust that they are good for outdoors, but of course minimize the exposure if possible.

    The breakers should really never need to be reset. The solar array is current limited and the wiring and overcurrent devices are sized based on the short circuit current, so they should never trip. If they do, something is wrong. Of course, it's possible for things to fail because of defects or corrosion.

    I don't understand: "I've already ordered the wires long enough to accomodate the combiner boxes on the garage roof" as opposed to combiner boxes inside. Shouldn't you need longer wires (PV wire, I assume) for combiners inside? And, if that's the way you are doing it are you running the PV wire in conduit? If not, I'd be more worried about protecting the PV wire than the combiner/junction boxes.

    Flex is what I call the flexible metal conduit that runs inside walls or an attic. It's not for outdoor use. Liquid tight, either metallic or non-metalic, is flexible and can be used outdoors, but we don't use it on the roof as it does not handle direct sun exposure very well, imo. We just use EMT on the roof and bend it as necessary. If you wanted to do a super high quality install, you could use aluminum conduit.

    There are a few ways to bring the conduit through the roof and seal it, depending on the type of roof. On a comp shingle roof I would generally just run it through, seal the whole and put a flashing over it. One company we subcontract for uses SolaDeck combiner boxes. It is the flashing and combiner box together and you can enter the attic/garage right under it. These are probably pretty good as far as waterproofing goes and they look pretty good, but they are pretty tight as far as inside space goes. It is a bit of a hassle to mount the buses and wire everything. Don't try to do this when the wires are hot! I took a shock from one hand to the other, through my chest (wearing dry leather gloves), when I was trying to do this.
  • sub3marathonmansub3marathonman Solar Expert Posts: 300 ✭✭✭
    Re: Wiring Choices for PV to Combiner Box
    newenergy wrote: »
    I don't understand: "I've already ordered the wires long enough to accomodate the combiner boxes on the garage roof" as opposed to combiner boxes inside. Shouldn't you need longer wires (PV wire, I assume) for combiners inside? And, if that's the way you are doing it are you running the PV wire in conduit? If not, I'd be more worried about protecting the PV wire than the combiner/junction boxes.


    OK, I guess I need to be a bit clearer. There were two options:

    1) Short PV wire, spliced in a box to regular larger gauge wire and run to the combiner box inside the house

    2) Longer PV wire, a bit more expensive, and run to the combiner box on the garage roof, with the fewer larger gauge wires then run to the charge controller.

    The plan now is to go with the longer PV wire (ordered from NAWS) and have the boxes on the roof. My electrician said splices on the roof might be OK for Arizona, but he didn't like that here in Florida. I don't understand why something can't be sprayed or painted on to prevent corrosion, but that is another debate.

    Then the wire will run in conduit across the roof, turn and run straight down a pipe into the garage and then into the charge controller. It was a debate about a "gooseneck" for the roof or a straight pipe, I'm still not sure about that. I don't want it raining inside the garage!

    So hopefully once this gets figured out it won't be long before everything can be hooked up.
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