Drawing review please!

Hello folks,

Attached is a three line PDF of my simple 6kw grid tie system. Let me know if my selection of wire sizes are appropriate and if there are any flagrant violations of the NEC that needs to be corrected.

One question I haven't figured out is what is the rating of my service panel bus bar! I'm adding a 40amp breaker to back feed power. If I've read the NEC 2008 correctly, the bus bar needs to be 1.20 x (200a+40a) = 288amps. I wonder if Cuttler Hammer would have this info?

Here's a picture of the label inside the service panel door. I didn't see any info on the bus bar. Looks like it's from the mid 80's. Let me know if there's a site that may contain this info.

100_0480.jpg

Thank you kindly.

Comments

  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Drawing review please!

    the buss is rated 20% higher than the rating on the box itself. you have a 200a box so 1.2 x 200 = 240a total capacity for the buss.

    the drawing is somewhat difficult to see and i don't think you have enough info to make a comment on the wire choices unless i didn't see it. btw, the implementing of another ground rod is not advisable as there normally should only be 1 and that should be for the main service box. also, the main service box should be the only point where the neutral and ground are tied together.
  • alia176
    alia176 Solar Expert Posts: 33
    Re: Drawing review please!
    niel wrote: »
    the buss is rated 20% higher than the rating on the box itself. you have a 200a box so 1.2 x 200 = 240a total capacity for the buss.

    the drawing is somewhat difficult to see and i don't think you have enough info to make a comment on the wire choices unless i didn't see it. btw, the implementing of another ground rod is not advisable as there normally should only be 1 and that should be for the main service box. also, the main service box should be the only point where the neutral and ground are tied together.

    Thanks for your reply. I agree, the drawing is hard to read since I had to upload it as a Jpeg. I'll split it up and re-upload it from home tonight.

    So, my buss is good for 240amps. In that case, am I in trouble for wanting to add a 40amp back feed breaker to this 200a panel box or am I just skating by?

    The drawing shows the following:
    12 AWG USE-2 GREEN conductor for grounding the panels to the inverter.
    8 AWG bare copper for all AC grounding from inverter all the way to the 200a service panel and to the grounding rod.
    Currently, the house has no ground rod, but is using cold water pipe. The only ground rod my house has is located at the service entrance pole, which is 250' away from the house. At the service entrance pole, I have a utility meter and another 200amp load center along with a grounding rod.

    I read somewhere that it's good practice to use a grounding electrode near the inverter. Perhaps this was in another context?
  • Jburgess
    Jburgess Solar Expert Posts: 130 ✭✭✭
    Re: Drawing review please!

    The way the DC disconect switch is shown is not how my SB6000 works. It does not switch the negitive and it does not isolate the strings from each other when disconnected. Here's a drawing if it helps.

    Disconect.jpg
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Drawing review please!

    The dedicated 40 amp service panel (next to inverter) appears to be a duplicate and is not needed (at least from a safety point of view).

    The existing 40 amp branch circuit in the main panel is sufficient.

    Personally, I do not like to put redundant breakers/fuses in a single circuit. Over current protective devices are "unreliable" by definition (they are designed to "open/trip" when carrying ~100% of rated current or higher).

    One breaker/fuse is safe. Two breakers/fuses in series is just more places to check for trips/failures.

    -Bill

    PS: Also check with your utility/local inspectors... Some utilities no longer require the separate solar disconnect box on the AC side.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • alia176
    alia176 Solar Expert Posts: 33
    Re: Drawing review please!
    BB. wrote: »
    The dedicated 40 amp service panel (next to inverter) appears to be a duplicate and is not needed (at least from a safety point of view).

    The existing 40 amp branch circuit in the main panel is sufficient.

    Personally, I do not like to put redundant breakers/fuses in a single circuit. Over current protective devices are "unreliable" by definition (they are designed to "open/trip" when carrying ~100% of rated current or higher).

    One breaker/fuse is safe. Two breakers/fuses in series is just more places to check for trips/failures.

    -Bill

    PS: Also check with your utility/local inspectors... Some utilities no longer require the separate solar disconnect box on the AC side.

    Bill,

    From my understanding, the inverter needs to be isolated from AC and DC power. However, it does seem redundant to put a 40amp breaker and then a 60 amp disconnect. Perhaps I should put in a fused 60amp disconnect? I'll investigate further.

    As far as the separate solar disconnect - it is required by my local utility.

    Can you comment on the my choice of wire size please? I have 12awg USE-2 for POS, NEG & GND for the DC side. I'm using 8awg for all AC wire and grounding purposes.

    Cheers.
  • alia176
    alia176 Solar Expert Posts: 33
    Re: Drawing review please!
    Jburgess wrote: »
    The way the DC disconect switch is shown is not how my SB6000 works. It does not switch the negitive and it does not isolate the strings from each other when disconnected. Here's a drawing if it helps.

    Disconect.jpg

    EXCELLENT, thanks for catching that. I'll update my drawing.
  • alia176
    alia176 Solar Expert Posts: 33
    Re: Drawing review please!

    Per your suggestions, the drawing has been updated. Let me know if you see any issues.

    Thanks
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Drawing review please!

    Technically, you do not need the two fuses/breakers on the solar array outputs to the combiner boxes. Fuses/breakers are only need if there are 3 or more parallel strings (typically).

    By the way, the fuses/breakers (if used) in the combiner box are rated for the Series Fuse value per the solar panel specs?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • alia176
    alia176 Solar Expert Posts: 33
    Re: Drawing review please!

    the fuses are inside the inverter so perhaps I should just leave it out of the drawing? Yes, you're correct - the fuses are 15A, same rating per the panel specs.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Drawing review please!

    Or make the inverter "Box" bigger so that it is obvious it is part of the Listed Inverter (for inspectors).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • alia176
    alia176 Solar Expert Posts: 33
    Re: Drawing review please!

    Yeah, i should return to that large inverter box that I had previously.

    Thanks.
  • alia176
    alia176 Solar Expert Posts: 33
    Re: Drawing review please!

    Question on the 60amp fused disconnect. I was leaning toward using the Square D D22NRB and 40 amp fuses. Is this agreeable with everyone?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Drawing review please!

    Can you find a disconnect without fuses? They are not needed. Personally, I would use 60+ amp fuses so that these fuses would never blow (you have the correct breakers elsewhere). However, and inspector would probably not sign off on over sized fuses.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • alia176
    alia176 Solar Expert Posts: 33
    Re: Drawing review please!
    BB. wrote: »
    Can you find a disconnect without fuses? They are not needed. Personally, I would use 60+ amp fuses so that these fuses would never blow (you have the correct breakers elsewhere). However, and inspector would probably not sign off on over sized fuses.

    -Bill

    Bill,

    According to my drawing below, I don't have any breakers in the system as I was told that would be redundant. My initial design did have a subpanel with 40amp breakers (drawing in the previous postings). Now that the subpanel has been eliminated, I figured I need 40amp fuses in the fused disconnect. If not needed, sounds good to me as it's a savings of $100 that I can use elsewhere.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Drawing review please!

    You still have the 40 amp branch circuit breakers at the service entrance still (if I understand everything correctly).

    I can see no need for another 40 amp protective device in the wiring run. The only other source of current is the GT inverter itself which should have its own rated output limiting "stuff".

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • alia176
    alia176 Solar Expert Posts: 33
    Re: Drawing review please!

    Yeah, that's a good point - let the inverter output take care of the current limiting.

    Good, I'll remove it from my drawing before submitting tomorrow.

    Thanks Bill.
  • SolaRevolution
    SolaRevolution Solar Expert Posts: 410 ✭✭
    Re: Drawing review please!

    You have some grounding electrode conductor size issues.

    You need to have:
    #6 from the inverter to the ground rod
    #4 from the 200 amp AC panel to the ground rod.
    If either is in an area "subject to physical damage" they must be in conduit. NEC 250.64(B)

    The drawing lists # 8 bare. If the conductor is unprotected it need to be a #6 minimum.

    If there is more than one grounding electrode they must be interconnected my a minimum #6. There is an exception for a supplemental ground rod but it does not apply because this is your main GE for for the DC system.

    The 200 amp AC panel needs a minimum of #4 for the grounding electrode conductor bassed on a minimum current carrying conductor size of 2/0 for a sub feed.
    NEC Table 250.66
    NEC Table 310.15(B)(6)
  • alia176
    alia176 Solar Expert Posts: 33
    Re: Drawing review please!

    Thanks for your feedback. I've changed the design once again! My AHJ suggested that I don't add any new ground rods so they were deleted from my design.

    On this new design, a 100A subpanel was added. This subpanel is being fed from the main 200a service panel. 2awg wires will be used to connect between the new subpanel and the existing service panel. All wires will be in a conduit. I need to look up the conduit size for three #2awg + one ground wire.

    A 40amp/240v breaker will be used to back feed this new subpanel.

    I've attached the new design. Let me know your thoughts.

    Attachment not found.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Drawing review please!

    I am not sure, but you may have a typo in your Main Panel... You have listed 200 Amp main, and 100 Amp inside the box--Is this a 100 amp branch circuit to your 100 Amp sub panel with the 40 amp breaker? If so, this is not clear (to me).

    Also, I would be a bit surprised if your AHJ would allow a 100 amp branch circuit to a solar GT dedicated sub panel... I would have assumed the would only allow a 40 amp branch breaker to the GT sub panel (20% of the 200 amp main).

    Will the sub panel be marked "Grid Tied Power Only Connections" or will your AHJ allow other branch circuits (which gets into the whole 20% GT rating of the 100 amp sub panel which would allow allow 20 amp GT of connection if in a "mixed" use panel).

    I would guess that the Main Panel to GT Sub Panel circuit should be limited to a 40 amp branch circuit (the 2 awg branch wiring becomes "overkill" at this point as only 40 amp maximum).

    What is drawn right now is not really following the "intent" of the NEC (as I understand) and should not be allowed until a) the main panel branch circuit is changed to 40 amp branch breaker and b) the 100 amp sub panel is listed for connections to solar power source(s) only and c) the subpanel breaker (if there is one) is changed to 40 amps (this is a bit confusing to me--normally, as an engineer, I would not have a sub panel main breaker--but I do not know what NEC/AHJ would require--the upstream 40 amp maximum breaker should be enough--although, I could see a breaker rated to the box/upstream current limit being required since this is a "power source" circuit/box).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • alia176
    alia176 Solar Expert Posts: 33
    Re: Drawing review please!

    Your assumption is actually correct, I'm adding a 100amp breaker in my main 200a service panel to feed the new 100amp subpanel. I needed more branch circuits for future expansions like a detached workshop that's in the works.

    I was hoping to kill two birds with one stone and install a 100amp subpanel. In this new subpanel, I wanted to use 40amp back fed breaker for GT. However, I'm also confused as to the legality of this move. In this design, I don't end up having a dedicated "solar pv GT" subpanel.

    Would it meet NEC if I were to have a 100amp subpanel being fed by the main 200a service panel and then feed a 40amp subpanel (from the 100am subpanel) strictly for GT application? This would be easy to do but does seem a bit "cluttered" if you know what I mean.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Drawing review please!

    If you wanted a "generic" 100 amp sub panel... then I would "believe" that the proper way to do this is to install a sub panel breaker of:

    • (100 amp panel * 1.20 over rating) - 40 amp GT (your maximum?) circuit = 80 amp maximum sub panel breaker

    Would your AHJ allow a 100 amp breaker in the main panel, or 80 amp--Not sure. Would they even allow you to have a "generic mixed use" sub panel with a derated main breaker? Don't know that either... But I believe you will have to do a little "dance" for them to accept it (basically show that each piece meets code, and over all, if anything is added per code later, it all will still meet the intent of the code).

    That would be how I would "present" the design to the AHJ.

    Plus, I would guess there may need to be a label on the main panel that states something along the lines of "40 amps of GT solar power is supplied by sub-panel A1".

    Whether or not your AHJ would accept that--Don't know.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Thomas
    Thomas Solar Expert Posts: 291
    Re: Drawing review please!

    Bill! this is what I did! I have a sub line out to my inverter and 3 of 5 total lines, 2 lines bypassing inverter.
    I did this so my inverter would not be loaded by the outside lines.(washer/hot water)Tom
    Always troubleshoot with adequate sunlight.  Hi Ho Hi Ho
    2.3kW [10] Enecsys SMI-240-60 micro inverters

  • alia176
    alia176 Solar Expert Posts: 33
    Re: Drawing review please!
    BB. wrote: »
    If you wanted a "generic" 100 amp sub panel... then I would "believe" that the proper way to do this is to install a sub panel breaker of:

    • (100 amp panel * 1.20 over rating) - 40 amp GT (your maximum?) circuit = 80 amp maximum sub panel breaker

    Would your AHJ allow a 100 amp breaker in the main panel, or 80 amp--Not sure. Would they even allow you to have a "generic mixed use" sub panel with a derated main breaker? Don't know that either... But I believe you will have to do a little "dance" for them to accept it (basically show that each piece meets code, and over all, if anything is added per code later, it all will still meet the intent of the code).

    That would be how I would "present" the design to the AHJ.

    Plus, I would guess there may need to be a label on the main panel that states something along the lines of "40 amps of GT solar power is supplied by sub-panel A1".

    Whether or not your AHJ would accept that--Don't know.

    -Bill

    Yes, I'll have to discuss it with my AHJ to make sure all is good. I was thinking of installing a simple MLO (main lug only) panel so that there is no confusion as to which panel is the "real" service panel. Maybe I should install a 125a MLO panel but only use 100 amp or 80 amp breaker, not sure.

    Correct me if I'm wrong, but the bus bar in this new sub panel must be capable of: (100a + 40a) *1.2 over rating = 168 amp. Does this math sound correct? Furthermore, do panel vendors specify the bus rating typically?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Drawing review please!
    alia176 wrote: »
    Yes, I'll have to discuss it with my AHJ to make sure all is good. I was thinking of installing a simple MLO (main lug only) panel so that there is no confusion as to which panel is the "real" service panel. Maybe I should install a 125a MLO panel but only use 100 amp or 80 amp breaker, not sure.

    Not sure what an MLO panel is (I am not an electrician). But, if you have a current source in the panel, and can add more--It does sound like a local main panel breaker would not be the worst idea (although, probably not needed/overkill if the GT Inverter breaker remains small). While I do not like multiple breakers doing the same thing--I am not sure I would try very hard to argue the AHJ (authority having jurisdiction) out of it.
    Correct me if I'm wrong, but the bus bar in this new sub panel must be capable of: (100a + 40a) *1.2 over rating = 168 amp. Does this math sound correct? Furthermore, do panel vendors specify the bus rating typically?

    Yes, panels are rated based on the internal bus bar and the total current passing through the wiring/branch circuits that are connected.

    The maximum current rating of the panel is XXX Amps Maximum. There is a 20% extra for home panels with solar GT power circuits. That if you place the Solar GT breaker at the "far end" of the bus (away from input lugs/breaker), then you are allowed to have a total of 120% of rated current available in the box.

    You do not need to add another derating for the full box (see my previous equation for calculating of sub panel main fuse).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Drawing review please!

    "Correct me if I'm wrong, but the bus bar in this new sub panel must be capable of: (100a + 40a) *1.2 over rating = 168 amp. Does this math sound correct? Furthermore, do panel vendors specify the bus rating typically?"

    let me clarify that the bus rating is 20% higher than the panel rating and that would not be 40a on a 100a panel as that's 20a. a 100a panel will have a bus rating of 120a.