NEC question - PV rated wire for underground conduit run??

Hi folks,
A little background, I organized and installed a 12kw system in West Africa. The system works great, we run a school and metal shop from it, but I didn't have to deal with any regulation over there. (There's a little video showing the system on youtube, if anyone is interested: http://www.youtube.com/watch?v=FgjKH7oFna0. As stated in the video, I know it's not up to code here, but we're still working on closing it all up.)

I live over there but am back home in Michigan for the summer. My parents asked me to put up a system for them while I was here. I did the fab work for the support system, and got it all mounted and hooked up. We just had the inspector out and his list of changes was pretty long! I'm not an electrician, although I did do the system in Africa with no real troubles. Anyway, along with a list of small and fixable complaints from the inspector, we have one big disagreement.

Our system is ground mounted, and the wiring comes together in a junction box on the mounting structure where two strings of ten panels are wire nutted together connected to an underground run that's in conduit to the inverter. All of the wiring from the panels into that box is PV rated cable. Through the conduit, from the junction box to the inverter, we used aluminum 6 gauge service entrance cable, because it's what we could find for a decent price. I know copper is a better conductor and find stranded wire is better, but we ran the system for several days and just like the wire loss calculator predicted, the fact that the cable is so oversized makes up for the aluminum and big strands. I saw a 5040W peak out of our 4600W of panels - so in practice the wiring obviously isn't a limiting factor.

So, the inspector is making us pull the wire out of the conduit to replace it. He cited an NEC rule saying that service entrance cable isn't allowed to be used underground, even in conduit, for anything. Ok, I accept that. I'm not trying to skirt the rules. However, he's citing another NEC section which talks about connections between panels and the inverter to say that the cable we put in the conduit has to be not only DC rated, but PV rated! Not a single inch of that run will ever see sunlight, why would we need to use UV protected cable? That section is talking about exposed connections.

So, although my SE cable is working perfectly fine, it appears that we need to replace it to satisfy the NEC. What exactly does the NEC really demand from this part of the connection though? It is a connection between the panels and the inverter, but the part of the connection we're talking about is completely enclosed in conduit.

I'm having trouble uploading a picture of the system from my phone, but I think you get the picture. Panels mounted on a steel system on the ground, connected to a box on the mount, underground conduit from there to the inverter about 100' away mounted on the side of our barn, where our electrical service entrance is.

So, here are my questions about the wire in the underground conduit section. Does the NEC absolutely require the cable to be:
-PV rated?
-DC rated?
-Fine stranded?
-Copper (aluminum is specifically allowed by our Fronius inverter)

Comments

  • JasonPAtkinsJasonPAtkins Registered Users Posts: 3
    Re: NEC question - PV rated wire for underground conduit run??

    Attachment not found.
    pic finally attached!
  • BB.BB. Super Moderators, Administrators Posts: 30,645 admin
    Re: NEC question - PV rated wire for underground conduit run??

    Beautiful area!

    Regarding PV Wiring... I think the new "PV rated" wire is "double insulated" (???) to protect against shock hazard for exposed wiring around solar panels. You may be getting some grief from that requirement (but double insulation is not required for cabling in conduit)... From this link:

    http://www.southwire.com/products/PVPhotovoltaicWireOEM.htm

    It can also be used as direct burial...

    Anyway, my concern is that (at least from this vendor) the wire is only available in 8 awg maximum. This is not really large enough for many larger installations (higher current/longer wire runs).

    I would agree with you that this wire is fine (and required) for wiring the solar array where it is exposed to sun and customers, but I would not see any reason for the home run back to the main building/solar charge controller. Any appropriately rated cable for buried/wet location should be fine (per NEC)--And PV/UV resistance would not have any requirement (just because it is dedicated to a solar array).

    I would just point at the NEC using the appropriate tables for current/voltage/temperature/location.

    Regarding stranding... Normal NEC rated cable with relatively heavy standing is fine (something in the NEC says that wire heavier than ~10-8 AWG must be stranded???).

    Find stranded cable is used in high flex situations. Welding cables, Diesel Locomotives, etc. where bending/vibration can work harden the copper. There are two major hassles with fine stranded cable... First it can be difficult to find NEC rated insulation. Second is that the very fine cables are hard to terminate correctly. Normal crimp connectors don't crimp down far enough (lots of "air gap" between strands) and directly terminating in breaker/screw terminals are not very solid.

    Lastly, if you use aluminum wiring--I would suggest using outdoor/wet location rated crimp aluminum to copper terminations/transitions... And use the copper pigtails for termination in equipment/breakers. I am not a big fan of aluminum in normal wiring (even if it was/is legal in some situations). Using sealed/crimped connections keeps oxygen from the connections and holds the aluminum better).

    http://inspectapedia.com/aluminum/aluminum.htm

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mtdocmtdoc Solar Expert Posts: 600 ✭✭
    Re: NEC question - PV rated wire for underground conduit run??

    So, here are my questions about the wire in the underground conduit section. Does the NEC absolutely require the cable to be:
    -PV rated?
    -DC rated?
    -Fine stranded?
    -Copper (aluminum is specifically allowed by our Fronius inverter)

    Standard THHN/THWN should be fine. My very "by the book" inspector had no prblem with it.
  • WNY DaveWNY Dave Registered Users Posts: 23 ✭✭
    Re: NEC question - PV rated wire for underground conduit run??

    Does your service entry cable have all the conductors already insulated together into a single cable? if so, its probably not good to put them in conduit, that is made to be exposed outside of conduit for heat dissipation, etc. Typically you run individual wires thru conduit and not overfill the conduit based on the AWG size of the wires. I think it's like 40-50% fill rate depending on your AWG wire size and trade size conduit. If it's in conduit underground, I think it has to have a "W" for WET location, like THWN, or XHHW
  • JasonPAtkinsJasonPAtkins Registered Users Posts: 3
    Re: NEC question - PV rated wire for underground conduit run??

    Ok, so I think what I'm hearing is:
    -Wire doesn't need to be PV rated in a conduit
    -We just have to follow general NEC rules that apply to any system underground

    So:
    -Does the NEC require anything different of DC systems than AC? (stranding, cable specifically rated for DC, etc)
    Or can I just use any underground rated cable that is rated for enough voltage? (could be alu or cu, could be coarse or fine stranded, etc)



    The other issue tied into this is that the inspector is claiming the array needs to be grounded to the inverter. He's claiming the code dictates it. The array is grounded and the inverter is grounded, but I'm fighting the idea of tying those grounds together because of the distance. It's DC, so there's no issue with differential grounding - so why waste copper and increase lightning exposure tying something together that is already adequately grounded apart?

    Thanks for the help, I appreciate it.
  • BB.BB. Super Moderators, Administrators Posts: 30,645 admin
    Re: NEC question - PV rated wire for underground conduit run??
    -Wire doesn't need to be PV rated in a conduit

    I would agree... As long as the voltage is less than 600 VAC (or srt(2)*600VAC=848.5VDC peak), and the rest of the wiring/hardware is rated to at least 600 VAC.
    -We just have to follow general NEC rules that apply to any system underground

    And, NEC "does not like" sharing circuits in the same conduit... Say you have some telephone/networking/RS 232 cables along with your power cables--they are not usually allowed to share the same conduit. It gets a bit "iffy" if they will allow you to, for example, share your DC Array Wiring to the home and, for example, a 120/240 VAC branch circuit for an outlet/well pump, etc. near the array (I think they should, as long as all the insulation is 600 VAC code rated for the application--But an inspector may make you run three different conduits--DC, AC, communications).
    -Does the NEC require anything different of DC systems than AC? (stranding, cable specifically rated for DC, etc)
    Or can I just use any underground rated cable that is rated for enough voltage? (could be alu or cu, could be coarse or fine stranded, etc)

    It has been decades since I last studied/work with the NEC for AC vs DC power systems (I used to do some Telephony equipment design). But, other than looking at the VAC vs VDC ratings for switches/connectors/etc. and the srt(2) difference between VAC and VDC (peak) voltages--There was nothing that "cared" if AC or DC was sent through the power cables.

    There are some detail issues that apply to AC but not DC (you can put a pair of +/- DC cables through two different holes in a metal box--AC you cannot do that because the AC current will cause circulating currents in the metal box--So Hot/Return pairs of AC cables always have to go through the same hole).

    There are some deratings that the NEC applies to solar power from arrays... Basically a 1.25 standard NEC derating (i.e., 60 amps max current * 1.25 = 75 amp minimum rated wire/breaker). There is a second 1.25x derating for power for solar power/solar array maximum current--i.e., that 75 amp circuit from the solar array * 1.25 2nd derating = 93.75 amp minimum rated circuit/controller).

    I have a few issues with the double deratings--They are not "that simple". It does depend on controller type too (and it is voltage that varies with temperature, not current for solar arrays).

    For larger wiring, you will only get "standard" stranded cables... use it, works well. Do not use "high flex" cables unless you have a very specific need. It is a pain to "do right" and is usually more problem than it is worth (and difficult to find any UL/NEC rated cable for house wiring--it is there, but not common).
    The other issue tied into this is that the inspector is claiming the array needs to be grounded to the inverter. He's claiming the code dictates it. The array is grounded and the inverter is grounded, but I'm fighting the idea of tying those grounds together because of the distance. It's DC, so there's no issue with differential grounding - so why waste copper and increase lightning exposure tying something together that is already adequately grounded apart?

    If this a Grid Tied system (GT Inverter + Solar Array, no battery bank+inverter)--Tell the inspector that the solar electrical wiring (typically negative, sometimes positive) return is grounded at the GT inverter via the Ground Fault Detection circuit (on all GT inverters mfg. in the last 10+ years). If you ground the negative (or positive lead) of the solar array at the array location, you are violating the UL Listing for the Inverter and potentially bypassing the DC GFD circuitry in the Inverter. Mention to him that this is "single point ground bonding" just like is done with 120/240 VAC split phase circuits (single point bond in Main Panel between Neutral and Earth/Green wire safety ground).

    If this is an Off Grid or Hybrid power system (solar+charge controller+battery bank+AC inverter), the answer is a bit more complex. The newest models of solar charge controllers have the DC GFD/GFI circuitry inside (Midnite, Schneider/Xantrex, etc.). Some of the older MPPT charge controllers do not. You can buy DC GFI breaker assemblies to put between the Array and the Charge Controller (or between charge controller and battery bank).

    http://www.solar-electric.com/mndc-gfp80.html

    I have a whole bunch of issues about DC GFI--But the powers that be don't understand the issues or chose to ignore them.

    The solar array frames and metal structure should be grounded to a local ground rod (lightning protection). And it would be a good idea to run a third cable from the Array Ground Rod to the Home's main ground rod. This is done so that you do not get "energized metal" at the array metal frame work--The current is returned to the main ground to the DC ground in the solar power system (which gets back to the DC GFI--A long/boring discussion. :blush:).

    I am not sure if your AHJ (authority having jurisdiction) will require a ground bond between the Array ground rod and the home ground rod.

    You have to find the sections (chapter and verse in the NEC--the version/date used by your AHJ) that will allow you to use non-PV rated wire, grounding, etc. needed for your system.

    "Asking" for help from the AHJ will probably get you further than tossing photo copies of code on their desk. In the end, if you can convince them that your installation is "safe" and meets the code--You should be OK to go.

    There are a few issues that AHJ may require that you may choose to "bypass" after the inspection (cough... DC GFI for off grid power systems)--But that is for another day. Obviously, you are not going to pull new cable just to not use PV rated wiring.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jaggedbenjaggedben Solar Expert Posts: 230 ✭✭
    Re: NEC question - PV rated wire for underground conduit run??

    I think I'm concurring with what's already been said, but nevertheless...:D
    So, the inspector is making us pull the wire out of the conduit to replace it. He cited an NEC rule saying that service entrance cable isn't allowed to be used underground, even in conduit, for anything. Ok, I accept that. I'm not trying to skirt the rules.

    I believe he's correct on that issue.
    However, he's citing another NEC section which talks about connections between panels and the inverter to say that the cable we put in the conduit has to be not only DC rated, but PV rated! Not a single inch of that run will ever see sunlight, why would we need to use UV protected cable? That section is talking about exposed connections.

    Yeah, he's totally wrong there. You can use any NEC approved wiring method for DC PV circuits that are not exposed to sunlight.
    Ok, so I think what I'm hearing is:
    -Wire doesn't need to be PV rated in a conduit
    -We just have to follow general NEC rules that apply to any system underground

    Generally speaking, yes.
    So:
    -Does the NEC require anything different of DC systems than AC? (stranding, cable specifically rated for DC, etc)

    Let's limit the discussion to just DC PV source and output circuits. In that case the answer is no, nothing different. 690.31 specifically says that all approved wiring methods included in the are allowed (i.e. anything in Chapter 3). There are some restrictions based on certain circumstances (notably 690.31(E), circuits inside a building) but they aren't relevant to your situation here.

    Or can I just use any underground rated cable that is rated for enough voltage? (could be alu or cu, could be coarse or fine stranded, etc)
    The other issue tied into this is that the inspector is claiming the array needs to be grounded to the inverter. He's claiming the code dictates it. The array is grounded and the inverter is grounded, but I'm fighting the idea of tying those grounds together because of the distance.

    Unfortunately for you, not only is he correct about what the code says about that one, but the inverters ground fault detection won't operate correctly if you don't do this.
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: NEC question - PV rated wire for underground conduit run??
    he's citing another NEC section which talks about connections between panels and the inverter to say that the cable we put in the conduit has to be not only DC rated, but PV rated!
    jaggedben wrote: »
    Yeah, he's totally wrong there. You can use any NEC approved wiring method for DC PV circuits that are not exposed to sunlight.

    Jason, you may find that you must do what the inspector approves, not what the NEC approves :cry:

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: NEC question - PV rated wire for underground conduit run??

    A polite way would be to get yourself a copy of the NEC and ask him to explain the sections discussed above, because you are confused by the contradiction of part XXX to part YYY
     
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  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: NEC question - PV rated wire for underground conduit run??
    westbranch wrote: »
    A polite way would be to get yourself a copy of the NEC and ask him to explain the sections discussed above, because you are confused by the contradiction of part XXX to part YYY

    yes, you can do that, but as vtmaps said the inspector calls the shots, not the nec. the inspectors are free to accept or deny any or all of what the nec says as the nec is not law unless the inspector says so. he can change it if he so wishes to so if he says he'll approve it if you jump on 1 foot then that's what i suspect you'll be doing. get on his bad side and he may expect you to jump with no feet.
  • solar_davesolar_dave Solar Expert Posts: 2,366 ✭✭✭✭
    Re: NEC question - PV rated wire for underground conduit run??
    niel wrote: »
    yes, you can do that, but as vtmaps said the inspector calls the shots, not the nec. the inspectors are free to accept or deny any or all of what the nec says as the nec is not law unless the inspector says so. he can change it if he so wishes to so if he says he'll approve it if you jump on 1 foot then that's what i suspect you'll be doing. get on his bad side and he may expect you to jump with no feet.

    This is so true as our inspector required several changes, yet the NEC was the basis for original design criteria. In fact the permitting disallowed a line side tap but that is covered in the NEC.
  • jaggedbenjaggedben Solar Expert Posts: 230 ✭✭
    Re: NEC question - PV rated wire for underground conduit run??
    niel wrote: »
    yes, you can do that, but as vtmaps said the inspector calls the shots, not the nec. the inspectors are free to accept or deny any or all of what the nec says as the nec is not law unless the inspector says so. he can change it if he so wishes to so if he says he'll approve it if you jump on 1 foot then that's what i suspect you'll be doing. get on his bad side and he may expect you to jump with no feet.

    I wouldn't put it that way (the red part). In most states and localities the NEC or something close to it is the law, and the inspectors are not within their legal rights to require something different. Your state or locality may have amendments to its code that differ slightly from the NEC, and in that case it is your right to see that document, but the document is still likely the law. Not all states are like this, but most are.

    With that said, you are right in that the real issue comes down to how much time and money you are willing to expend to get an official to follow the law when they are not inclined to do so, whether from stupidity, stubbornness, or whatever other motive. Too many inspectors don't like being told they are wrong and won't change their mind just due to being reasoned with. In some cases it is worthwhile to go 'up the foodchain' by calling the inspector's boss, which may be another inspector with more seniority, or an elected official. But then again you need to find someone inclined to take your side. If you don't, usually it is not worth hiring a lawyer to pass an electrical inspection, compared to just giving them what they ask for.
  • solar_davesolar_dave Solar Expert Posts: 2,366 ✭✭✭✭
    Re: NEC question - PV rated wire for underground conduit run??
    jaggedben wrote: »
    I wouldn't put it that way (the red part). In most states and localities the NEC or something close to it is the law, and the inspectors are not within their legal rights to require something different. Your state or locality may have amendments to its code that differ slightly from the NEC, and in that case it is your right to see that document, but the document is still likely the law. Not all states are like this, but most are.

    With that said, you are right in that the real issue comes down to how much time and money you are willing to expend to get an official to follow the law when they are not inclined to do so, whether from stupidity, stubbornness, or whatever other motive. Too many inspectors don't like being told they are wrong and won't change their mind just due to being reasoned with. In some cases it is worthwhile to go 'up the foodchain' by calling the inspector's boss, which may be another inspector with more seniority, or an elected official. But then again you need to find someone inclined to take your side. If you don't, usually it is not worth hiring a lawyer to pass an electrical inspection, compared to just giving them what they ask for.

    This is a painful process which might be easily worked around if known ahead of time. Drawings that spec the wiring are your friend.
  • stanraestanrae Registered Users Posts: 1
    Re: NEC question - PV rated wire for underground conduit run??

    The question is Why does the NEC require Conduit for underground situations ? The NEC addresses this issue in many instances, and the basic statement is earth (dirt) reaction to electrical conductors. The basis is set on how much Current is being carried, Insulation that surrounds the conductor. Direct burry cable is allowed but is discouraged by many local Inspectors (old school thinking type especially). Usually Service Entrance cables are designated "W" (THWN or XHHW) as Dave Stated. You can search the net for the direct answer to the NEC code requirement and find it with out Buying a code book.
  • jcheiljcheil Solar Expert Posts: 722 ✭✭✭
    Re: NEC question - PV rated wire for underground conduit run??
    stanrae wrote: »
    The question is Why does the NEC require Conduit for underground situations ? The NEC addresses this issue in many instances, and the basic statement is earth (dirt) reaction to electrical conductors. The basis is set on how much Current is being carried, Insulation that surrounds the conductor. Direct burry cable is allowed but is discouraged by many local Inspectors (old school thinking type especially). Usually Service Entrance cables are designated "W" (THWN or XHHW) as Dave Stated. You can search the net for the direct answer to the NEC code requirement and find it with out Buying a code book.

    When I was building my other on-grid house here in Florida the inspector said it needed to be in conduit to prevent damage from someone digging in the area and also had a minimum depth that it had to be buried to take into consideration any "weight" that might drive over it.

    Considering this was in my backyard and I didn't think a semi could ever drive across my lawn I argued that point but never won. So my guess is that it is more for protection.
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