GT install DC disconnect/ fuse?

Hello,
After so much time trying to acquire all the components, I think I am ready to start to put the project together. Last year I got a Sunpower SPX-4000x inverter and today just received 10 pc of Sunpower 320W panels. The specs are Voc 64.8; Isc=6.24;Vmp=54.7;Imp=5.86
I can’t find string sizing software for Sunpower inverters but I used SMA design. It shows that my minimum voltage on a hot day -234V that is lower than my MPPT (240v-480v). I am planning on adding another 2 extra panels for perfect configuration. I am going to do two strings of 5 panels, six in the future.
I am planning on using a IronRidge rails and hardware with Midnite solar Big box Baby enclosure to combine the two strings into a two MNTS touch safe fuse holders (my dc voltage will be high for any breakers) with two MNFUSE08 8A fuses. I am planning to ground the panels with bare copper wire and I am not sure what gauge it should be. In some answers here I found a minimum of 8AWG in other a minimum of #4. From the combiner box I am planning to use a galvanized conduit to the DC disconnect on the wall of the house. I am not sure if this disconnect should be fused, as I already have fuses in combiner box. My choices for a DC disconnects are Square D HU361RB. From there the dc cables go to inverter that is also outside on the wall. Right next to it I will put a AC disconnect and plan to use 10AWG cable to the breaker panel inside the garage. If my meter is right outside next to the inverter do I have to connect to the breaker box inside the garage or I can connect the inverter to the main disconnect panel next to the meter?

Comments

  • BB.BB. Super Moderators, Administrators Posts: 30,642 admin
    Re: GT install DC disconnect/ fuse?
    praseto wrote: »
    ...Sunpower 320W panels. The specs are Voc 64.8; Isc=6.24;Vmp=54.7;Imp=5.86
    I can’t find string sizing software for Sunpower inverters but I used SMA design. It shows that my minimum voltage on a hot day -234V that is lower than my MPPT (240v-480v). I am planning on adding another 2 extra panels for perfect configuration. I am going to do two strings of 5 panels, six in the future.

    That may be too small of fuse--You may get false trips or fuse failure in the future (fuses work by heating--Hot fuses in hot box running near rated current could be less than reliable).

    The data sheet (PDF) I found for a Sunpower E19 320 Watt panel says that the maximum series fuse is 20 amps... So, you could use 15-20 amp fuse/breaker just fine (use correct gauge wire).

    But if you are only installing two parallel strings, you do not need a combiner box + Fuses/Breakers at all (fusing is usually only required for 3 or more panels).
    I am planning on using a IronRidge rails and hardware with Midnite solar Big box Baby enclosure to combine the two strings into a two MNTS touch safe fuse holders (my dc voltage will be high for any breakers) with two MNFUSE08 8A fuses.

    If I understand correctly--No fuses required. You may need a DC disconnect on the outside of the home to meet the requirements of your local building inspectors (fuses holders probably do not meet that requirement).
    I am planning to ground the panels with bare copper wire and I am not sure what gauge it should be. In some answers here I found a minimum of 8AWG in other a minimum of #4.

    As far as I know, the requirement is 6 awg of bare copper wire at a minimum (I think it was OK to use #8 awg if it was insulated--But that was 30 years ago--don't know now).

    If you do not have one, you should buy a NEC or equivalent code book as used by your local inspectors. It is possible to find the NEC on-line (as I understand, since NEC is required by law--the cities have to put that on-line/made publicly available).
    From the combiner box I am planning to use a galvanized conduit to the DC disconnect on the wall of the house. I am not sure if this disconnect should be fused, as I already have fuses in combiner box. My choices for a DC disconnects are Square D HU361RB. From there the dc cables go to inverter that is also outside on the wall.

    Popping fuse holders does not usually meet the requirement for a disconnect (as I understand). Also, I think you have to either have a disconnect on the outside of the home or the wiring has to be in a metal conduit until it gets to the disconnect inside the home/inverter connection. If you use a DC disconnect on the outside of the home, you can use plastic (rated) conduit inside the home (again, you need to read the code for your area, and ask the inspectors when you pull your permit).

    The outside DC disconnect may be required by local fire code--You will have to check with your building department.
    Right next to it I will put a AC disconnect and plan to use 10AWG cable to the breaker panel inside the garage. If my meter is right outside next to the inverter do I have to connect to the breaker box inside the garage or I can connect the inverter to the main disconnect panel next to the meter?

    In my area, the utility got rid of the AC disconnect requirement... In the olden days, the utility required a lockable external AC disconnect so they could go to the homes with solar and lock off the solar GT inverter when they were working on the power lines--Or they would pull the meter to cut power to the whole home. Today, they recognize that Listed GT inverters do not need to be locked off for safety--So the requirement has bee deleted in our area (Northern California).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • prasetopraseto Registered Users Posts: 22
    Re: GT install DC disconnect/ fuse?

    Thank you for the fast reply. If i don't have a combiner box at the array, do my DC disconnect on the side of the house has to be fused or not.I am having problem find a dc disconnect to hold more then 300v DC and have fuses.
  • BB.BB. Super Moderators, Administrators Posts: 30,642 admin
    Re: GT install DC disconnect/ fuse?

    If the wire is large enough gauge to carry the rated current (per code), then I do not believe you need a fused/breaker type DC disconnect.

    If you are going to pull a permit, you should go over your design before you purchase any equipment. The local inspectors can require anything they want (and more requirements are being added to meet requirements for fire fighters).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: GT install DC disconnect/ fuse?
    BB. wrote: »
    If I understand correctly--No fuses required. You may need a DC disconnect on the outside of the home to meet the requirements of your local building inspectors (fuses holders probably do not meet that requirement).
    Correct. You should never use fuse holders as a disconnect. If you pop fuses while they are under load, you risk arcing and fire.
    BB. wrote: »
    As far as I know, the requirement is 6 awg of bare copper wire at a minimum (I think it was OK to use #8 awg if it was insulated--But that was 30 years ago--don't know now).
    #8 bare copper is acceptable in most jurisdictions.
  • prasetopraseto Registered Users Posts: 22
    Re: GT install DC disconnect/ fuse?

    Thank you to everyone for the info. I have to go to the permit dept to verify the design. I am trying to do the complete design by myself as a homeowner. The requirement for grounding wire on the panels in Orange County Florida is 4awg. The good thing is as a homeowner I can apply for all the permits. The bad part is they ask for engineered drawing of wind load on the roof structure. My plan drawing with the location of array, disconnects an connection was not enough.
  • jaggedbenjaggedben Solar Expert Posts: 230 ✭✭
    Re: GT install DC disconnect/ fuse?
    praseto wrote: »
    The requirement for grounding wire on the panels in Orange County Florida is 4awg.

    That's ridiculous. Are you sure it's not 6awg?
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: GT install DC disconnect/ fuse?
    jaggedben wrote: »
    That's ridiculous. Are you sure it's not 6awg?
    Maybe they consider it ballast. :D
  • jaggedbenjaggedben Solar Expert Posts: 230 ✭✭
    Re: GT install DC disconnect/ fuse?
    ggunn wrote: »
    Maybe they consider it ballast. :D

    Haha. Or maybe they're counting on it for structural support during hurricanes. To keep the panels from flying away.
  • prasetopraseto Registered Users Posts: 22
    Re: GT install DC disconnect/ fuse?

    I got the new paperwork and everything. I even got the AutoCAD drawing, wind calculation etc. They insist on engineer to sign on them. Tomorrow I am going to look on craigslist for some small company to verify and sign the schematics on mounting hardware. To tell you the truth about the grounding wire I am not sure that they know what they are talking about. When I show them the plans he said that everything I need is the engineers that made the plans to sign and put a seal. I explained to him that I made the plans and he started to look more closely, some kind of worried. I ask him about the grounding wire and he explained that this is not his dept. but I am welcome to ask the inspector for specific details 
  • BB.BB. Super Moderators, Administrators Posts: 30,642 admin
    Re: GT install DC disconnect/ fuse?

    If you do ask--Ask before you do any work. Inspectors can have their own take on things and make you change after you do the install.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • prasetopraseto Registered Users Posts: 22
    Re: GT install DC disconnect/ fuse?

    Thanks to Craigslist and 75$ I got an engineer signed plans. Two hours later I got a permit. I am so exited. I still have to wait on some materials and the project will be on the way.
    O, yes the inspector is the right hand of the law.
    Thanks again for all the help.
  • prasetopraseto Registered Users Posts: 22
    Re: GT install DC disconnect/ fuse?

    I have problem. I was thinking to use a Midnite solar Big box Baby enclosure to combine the two strings into a two MNTS touch safe fuse holders (my dc voltage will be high for any breakers) with two MNFUSE020 20A fuses. The reason was to use the combiner box so I can transition from PV wire to THHN wire and from the combiner box to start the conduit that is going thru the roof into the attic. I just received the big baby box that I ordered from Win&Sun, our sponsors, but the enclosure is rated for indoor use only and is not rated for more than 300v DC. So the question is where I can get a 3R enclosure with DIN rails? Or I can use any 3R box and just to have 2 bus bars that are insulated?
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: GT install DC disconnect/ fuse?
    praseto wrote: »
    I have problem. I was thinking to use a Midnite solar Big box Baby enclosure to combine the two strings into a two MNTS touch safe fuse holders (my dc voltage will be high for any breakers) with two MNFUSE020 20A fuses.

    Midnite makes combiner boxes that are designed for the purpose. They even have ones with built in MC4 connectors. --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • BB.BB. Super Moderators, Administrators Posts: 30,642 admin
    Re: GT install DC disconnect/ fuse?

    What is your solar array series/parallel configuration? If you only have one or two parallel strings--You really don't need fuses. If you have three or more parallel strings, then usually you need a fuse per string.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • prasetopraseto Registered Users Posts: 22
    Re: GT install DC disconnect/ fuse?

    I will probably not going to use a junction box. How you can get the USE-2 cable into the conduit? I mean the transition. I am going to use metal EMT.
    Here is the first rail going on the roof :
    http://i428.photobucket.com/albums/qq4/paseto/2012-11-19Firstrail.jpg
  • jaggedbenjaggedben Solar Expert Posts: 230 ✭✭
    Re: GT install DC disconnect/ fuse?
    praseto wrote: »
    I will probably not going to use a junction box. How you can get the USE-2 cable into the conduit? I mean the transition. I am going to use metal EMT.

    You kindof need a junction box. I'm skeptical that you can get away with it without a junction box if your inspector is requiring a #4 grounding conductor.

    To bring the USE-2 into a junction box, ask your electrical supply house for a "strain relief UF connector". You should be able to get them with 2-4 small holes to bring the USE-2 wires in. The ground can be brought in with the same or similar connector.

    I have seen it done without a junction box. Simply put a connector and a grounding bushing on the end of the conduit, but make sure the end of the conduit is pointing downward so that any water will flow out and not in. Then run your USE-2 directly into the conduit. The problems with this method are:
    -inspector may deem it insufficiently water tight
    -a bare copper ground conductor larger than #10 cannot be run inside conduit
    -Conduits need to be larger to handle the larger outer diameter of the USE-2
    -You use more USE-2 which is more expensive and less readily available than THWN-2
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: GT install DC disconnect/ fuse?
    jaggedben wrote: »
    -a bare copper ground conductor larger than #10 cannot be run inside conduit
    I did not know that. Got a code reference?
  • BB.BB. Super Moderators, Administrators Posts: 30,642 admin
    Re: GT install DC disconnect/ fuse?

    In our area, we had to have conduit (or metal flex) on external ground conductors to the ground rod.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jaggedbenjaggedben Solar Expert Posts: 230 ✭✭
    Re: GT install DC disconnect/ fuse?
    ggunn wrote: »
    I did not know that. Got a code reference?

    Man, you sent me hunting. Should have just said I had forgotten.:grr

    Seems it was in 310.3 in the '08 code but I don't know where it went or if it's still there in 2011. And to clarify, it prohibited a solid conductor not a bare one.

    EDIT: Found it. 310.106(C) in 2011.
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: GT install DC disconnect/ fuse?
    jaggedben wrote: »
    Man, you sent me hunting. Should have just said I had forgotten.:grr

    Seems it was in 310.3 in the '08 code but I don't know where it went or if it's still there in 2011. And to clarify, it prohibited a solid conductor not a bare one.

    EDIT: Found it. 310.106(C) in 2011.
    Thanks. That's useful info. I've never done that and I promise never to do it again.
  • prasetopraseto Registered Users Posts: 22
    Re: GT install DC disconnect/ fuse?

    Thanks to all for the suggestions.I found some strain relief UF connector in a local home depot. As the array is really close to the meter, less then 25 feet using PV wire is not too much more expensive. Here is the 4 rails ready on the roof.
    http://i428.photobucket.com/albums/qq4/paseto/2012-11-26Rails.jpg
    As my inverter is really next to meter can and the main 150A house disconnect should I look into tapping into the main feed or should I go to the panel that is in the garage 50'.To get to the panel that is in the garage is going to be difficult as I have a high ceiling and not too much space.
    Here is the pic of the inverter with the ac/dc disconnects.
    http://i428.photobucket.com/albums/qq4/paseto/meter1.jpg
  • jaggedbenjaggedben Solar Expert Posts: 230 ✭✭
    Re: GT install DC disconnect/ fuse?
    praseto wrote: »
    As my inverter is really next to meter can and the main 150A house disconnect should I look into tapping into the main feed or should I go to the panel that is in the garage 50'.To get to the panel that is in the garage is going to be difficult as I have a high ceiling and not too much space.
    Here is the pic of the inverter with the ac/dc disconnects.
    http://i428.photobucket.com/albums/qq4/paseto/meter1.jpg

    Is this going to be inspected? If yes, you might want to draw up a wiring diagram and submit it to your jurisdiction before doing any more work. As far as the NEC is concerned...

    I assume your inverter output requires a 20A breaker or fuses.
    -You can tap the conductors in the main on the load side of the breaker if they are rated for at least 141amps (typically 1/0 copper or 3/0 aluminum) It looks like your tap may be a bit more than 10ft, so you'd need to use #8 copper or larger.

    -Alternatively (if the load side conductors are too small) you could pull the meter and tap on the line side of the main breaker. Not sure how persnickety your power company will be about you pulling the meter.

    If the conductors in and out of your main are aluminum you'll need to tap using a method that's acceptable for that.

    For either method your AC disconnect better be a fused disconnect. EDIT: If you do the line side tap it had also better be service rated.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: GT install DC disconnect/ fuse?
    jaggedben wrote: »
    Is this going to be inspected? If yes, you might want to draw up a wiring diagram and submit it to your jurisdiction before doing any more work. As far as the NEC is concerned...

    I assume your inverter output requires a 20A breaker or fuses.
    -You can tap the conductors in the main on the load side of the breaker if they are rated for at least 141amps (typically 1/0 copper or 3/0 aluminum) It looks like your tap may be a bit more than 10ft, so you'd need to use #8 copper or larger.

    -Alternatively (if the load side conductors are too small) you could pull the meter and tap on the line side of the main breaker. Not sure how persnickety your power company will be about you pulling the meter.

    If the conductors in and out of your main are aluminum you'll need to tap using a method that's acceptable for that.

    For either method your AC disconnect better be a fused disconnect.

    I think there is a risk of some viewers misinterpreting what you (both praseto and jaggedben) are saying, so I will make some (hopefully) clarifying statements and questions:

    1. When you say hour "main 150 amp house disconnect" are you referring to a breaker panel which has individual load breakers going to local loads and also to the subpanel located elsewhere? Or a separate main breaker and a smaller feed breaker to the sub-panel?

    If there is a main breaker and a subpanel feed to which you will be connecting the GTI output, the 120% rule will apply to the combination of the main breaker and the amps limited by the actual backfeed breaker in the other panel. You will also need to apply the 120% rule to the sub-panel.

    If the entire load side of the 150 amp disconnect/breaker goes to the second panel, then you only need to apply the 120% rule in the sub-panel.

    2. A backfeed connection made to the load side of a breaker rather than through a separate breaker on the same panel bus may or may not be allowed by NEC, but if it is allowed should not remove the need to follow the 120% rule in that panel.

    3. A connection made to the LINE side of the main disconnect will be a line side tap, with allowed alternate possibilities being at the meter can or anywhere in between.

    4. A connection made to the LINE side of the sub-panel will be a line-side tap for the purposes of that panel but will be a breaker backfeed further up the line.

    If you are not familiar with the 120% rule, let us know. It is complicated. Generally it is more stringent than any ampacity rules applying to the feeder wiring, and it is intended to avoid overloading the bus which interconnects the breakers within the panel. It only depends on the current rating of all breakers which are capable of feeding power into the bus and has no relationship to the rating of breakers in the panel which are only feeding loads.

    5. You can only take advantage of the 120% rule if you put the backfeed breaker(s) on the opposite end of the panel from the main breaker.

    6. Although it is allowed by the NEC, in some cases either the local POCO or the AHJ will not allow actual line-side (service side) taps for residential installations.

    7. Licensed electricians who have not done GT before can be just as confused about this issue as you are. But hopefully they will have the resources to find out. :-)
    SMA SB 3000, old BP panels.
  • jaggedbenjaggedben Solar Expert Posts: 230 ✭✭
    Re: GT install DC disconnect/ fuse?

    inetdog, did you look at his picture?

    He has a outdoor main-breaker-only enclosure for a service disconnect. I'm pretty sure he's asking if he can tap the conductors inside it (line or load side). Might have saved you some words. I can't disagree with anything you say though. All those complicating factors are the reason I suggested he get it approved by his jurisdiction before doing any more work.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: GT install DC disconnect/ fuse?
    jaggedben wrote: »
    inetdog, did you look at his picture?

    He has a outdoor main-breaker-only enclosure for a service disconnect. I'm pretty sure he's asking if he can tap the conductors inside it (line or load side). Might have saved you some words. I can't disagree with anything you say though. All those complicating factors are the reason I suggested he get it approved by his jurisdiction before doing any more work.

    Thanks JB. I did look at the diagram, and figured that both on the off chance that the diagram is misleading and for the benefit of others who search the thread I might as well add some words. (They don't cost me anything, and my time is not worth much. :-) )

    His special case is why the size of the wiring from the breaker-disconnect to the inside panel has to be sized to carry the sum of the main breaker rating and the PV contribution. I am glad you mentioned that, as it is an uncommon enough situation that some would overlook it. (I suppose that if the main panel also has a 150 amp main breaker, the overcurrent protection for the feeders will be covered there except for non-solid wiring faults between the two points.)

    In practice, I would not expect his loads will approach that amount, but a good AHJ will pounce on the violation anyway. The diagram he submits should include the size of those existing conductors as well as the ones he is adding.

    The plan check (formal or informal) by AHJ is definitely the way to go. Whether the AHJ is correct in their interpretation of the applicable code or not, they are the ones with responsibility, and you never know when you might be dealing with local amendments. The AHJ will also probably have a good idea of what POCO will approve even if he does not run it by them too.
    SMA SB 3000, old BP panels.
  • prasetopraseto Registered Users Posts: 22
    Re: GT install DC disconnect/ fuse?

    I have one main 150A breaker right next to the meter can and the sub panel in the garage that feed individual loads in the house. I already got the permit from the county. On the drawing i stated that I will be taping to the main breaker(next to the meter) but when I actually open the breaker enclosure seemed little complicated.I was expecting a bus bar where I can add a breaker.Then looked into the permit plan, they approved the plan the way I did it but include the statement that any line side tap have to be done by certified electrician. I have plenty of #6 copper thhn to run to the panel in the garage,but the conduit have to go via the attic and I have high ceiling.Also I am also trying to keep him out of the garage as there I have a irrigation well pump that I don't have a permit. I am trying to weight my option before I go to the inspector for questions and review of the plan. Right now I did all the stuff that I am confident to be correct. Tomorrow I am going to call my electric company to see if they have any requirements.Thanks for the input.
  • jaggedbenjaggedben Solar Expert Posts: 230 ✭✭
    Re: GT install DC disconnect/ fuse?

    Praseto,

    From what you've said it sounds to me like the county approved your plans on the basis of your doing a supply-side connection (aka line side tap). (Too bad there wasn't an extra space for a breaker in your main disconnect enclosure, because that also would have counted as a supply side connection, and it would have been easy.)

    So with that said I think your best approach is to hire the certified electrician to do the line side tap. This also avoids all the issues with the ampacities and the 120% rule that inetdog is talking about. If you go with another method I think you'll need to re-draw and re-submit the plans.

    Again, double check that the AC disconnect you installed is a fused disconnect that is approved for use as service equipment.
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