Off Grid Well Pump Grounding and Backup Power Options

I am needing some advice/recommendations regarding an off grid well installation. We are going with the Grundfos SQFlex system, and will be dedicating a small 4 panel 860W array for our pump that will be installed close to the well. Our house will be about 400 feet from the well, and I would like to allow for a backup power source to power the well pump (generator and or AC from inverter) should the need ever arise. My direct question is regarding the grounding situation. The well casing and pump must share a common ground connection. If I want to run a dedicated buried power line from our house to the well pump as a backup, would the house ground and well ground need to be bonded. If I understand correctly from the NEC, the bonding wire would need to be no less than 2 gauge since the largest wire in the system will be 4/0. Has anyone done this, do you have any thoughts?

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Off Grid Well Pump Grounding and Backup Power Options

    Welcome to the forum.

    Can we just check some specs here? What sort of power are you running 400' through 4/0 wire to provide back-up for the well pump? That's pretty big and expensive wire. The SQF can run off 120 or 240 VAC and just a quick calc shows it would probably be less than 8 Amps on 120 VAC and only need 6 AWG to maintain full power.

    With 400 feet between them, I'd hazard that no ground connection between house and well is required. But ultimately it is a matter of inspection by the AHJ and what he says is needed.
  • BB.BB. Super Moderators, Administrators Posts: 30,637 admin
    Re: Off Grid Well Pump Grounding and Backup Power Options

    Grounding--I like to look at what is "practical" and "safe" regarding grounding--And what you are "protecting" against.

    So, how I would approach the problem:
    1. First I would use a double pole/double throw switch of some sort to either connect the panel +/- to the pump or to your remote MPPT (assumed) charge controller. Pump/panel grounding would be accomplished per pump installation instructions on the "pump" side of the DPDT switch.
    2. The solar panel frame/mounts would be properly grounded to the well casing/well ground rods/system.
    3. The home charge controller/battery bank would be grounded per the "normal" grounding instructions (note: NEC "requires" DC GFI system--Basically a 1 amp fuse/breaker between battery bank "-" and Earth Ground--If the fuse/breaker trips, then the charge controller stops converting energy--"Ground Fault". I have massive issues with this and recommend the old tried and true "hard ground" 6 awg or heavier jumper between Earth Ground and Battery Ground--can discuss more if you wish).
    4. The ground wire only needs to do two things... Safely pass current without overheating the maximum available current (something like Isc-array*1.56) and have low enough resistance to "trip" the fuse/breaker if there is a fault (i.e., very long wire/ground runs may have too much resistance to trip a breaker/fuse). In the case of solar panels, they cannot supply more than ~Isc-array into a dead short--So, the ground wire only needs to be heavy enough to safely pass current. And practically speaking, the current from a solar panel with a fuse at Isc-array*1.56 will never trip (solar panels do not output high current into shorts, unlike a battery bank). The minimum for buried ground is usually 6 awg or larger.

    Now, we get back into the practical issues. I assume your solar panels are around Vmp~30 volts? 4x in series would give you 120 VDC -- Very nice for a Grundfos type universal pump.

    However, for most solar charge controller, they usually only run at Vmp-array~100 VDC maximum (can over voltage MPPT type charge controller with Voc-cold >~150VDC). There are a few that can operate at higher voltages (Xantrex MPPT 60 amps Voc~600 VDC; Midnite has 200 VDC or greater controller available--I think).

    Then we need to look at the wire run of 400 feet from panel to charge controller... Assuming Vmp-array~120 VDC and 860 watts:
    • 860 watts / 120 volts = 7.2 amps Imp (silly to "require" 2 awg wiring for fault current)
    • Using generic voltage drop computer with 3% or 3.6 volt drop maximum at 7.2 amps and 400 foot one way run:
      • ->6 awg with 2.7 volt (2.3%) drop

    You could go with 2 panels in series, then two strings in parallel, but then you would need much heavier gauge wiring (~1/0 at 2.4% drop). But that would be a bit low for the pump (would work, but heavier wire required and a bit less efficient). And a lot of copper (or even heavier gauge aluminum) wire run... Also, depending on the solar panel Vmp ratings, this may not be high enough Vmp-array to charge a 48 volt battery bank (don't know what voltage bank you have).

    "IF" you still want to do this, I would check out the Midnite higher voltage charge controller (www.midnitesolar.com for their website and forum) and use 4 panels in series. Such a controller will probably cost more than $600... It may be better just to save the cost of copper and just buy a new array for your home system and use the power every day instead of $600+wire costs for emergency backup.

    Another option, it may still make sense for you to bury some extra wire--however, make it 120 VAC (or 240 VAC if you have it available) and run it from your home inverter to the well pump head so you can have backup power to run the pump if the array fails without having to haul a genset out to the well.

    Your comments?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Off Grid Well Pump Grounding and Backup Power Options

    Bill, he's talking about wiring in back-up power not the array. Evidently that is all set at the pump location.
  • BB.BB. Super Moderators, Administrators Posts: 30,637 admin
    Re: Off Grid Well Pump Grounding and Backup Power Options

    Marc,

    You are correct--Sorry about doing the lets connect the array for emergency power answer...

    However, the wiring solution for the pump should still be pretty close (based on ~7-8 amp pump current).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Off Grid Well Pump Grounding and Backup Power Options

    Bill;

    That's what I figured but the 4/0 wire size threw me for a loop. Pretty massive for 120 VAC @ 8 Amps. So I was wondering if that was right.

    If he runs a transformer and 240 VAC it looks like 12 AWG would handle it.
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Off Grid Well Pump Grounding and Backup Power Options

    Marc, Bill,
    He does not need heavy wire to power the pump. His problem is that he has two grounds, 400 ft apart. Bonding those two grounds together is what requires the heavy wire.
    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Off Grid Well Pump Grounding and Backup Power Options

    From the OP:
    If I understand correctly from the NEC, the bonding wire would need to be no less than 2 gauge since the largest wire in the system will be 4/0.

    See how I get confused?
  • logaholiclogaholic Registered Users Posts: 9
    Re: Off Grid Well Pump Grounding and Backup Power Options

    Thank you for the quick replies. Here are some details to clear up some issues:

    The well has a dedicated solar array that will be direct fed to the pump through the Grundfos controller. They are 4 215W panels, approx 35vmp. Our house will have a separate array feeding charge controllers and a 48V battery bank. The Grundfos control module for backup power is only rated for 120V according to the specs I could find from Grundfos, so that would be the voltage we would be sending to the well in the event of a shortage from the house.

    vtMaps is correct, the question here is regarding the separate grounds (well casing and ground rod at house), ~400ft apart. The 4/0 wire is the largest wire I anticipate using in the entire system, which would be between our house battery bank and inverter. The way I read the NEC is that the bonding wire between separate grounds is based off the largest size wire located anywhere in the system, so if I were to link the house and well system together, this would all be one system.

    As a separate but related issue, I am also wondering if there is any benefit in going this route and including the well casing as a grounding electrode bonded to the house grounding electrode, or if this is overkill and unnecessary. We do live on a ridge, so if there is a significant benefit with regards to lightning protection going this route, that also might push me in this direction. However, for close to the cost of copper involved I could pick up a spare generator to use in the event of an emergency shortage if our holding tank were to run low, which would mean no electrical connection between the well and house would be required. If we size the holding tank correctly I would hope we would never need this backup system, but it would be real nice to flip a switch in the basement rather than drag a generator down to the well in the event of a shortage. Perhaps my best bet is to contact the local inspector to see what he would recommend. I'd much rather do it right before back filling the trench! I appreciate all of your help and comments.
  • stephendvstephendv Solar Expert Posts: 1,571 ✭✭
    Re: Off Grid Well Pump Grounding and Backup Power Options

    If you really want to run a 120V cable between house and pump... then I'd think twice about going with the grundfos + separate panels. Why not use what you would have spent on the well system in upgrading your house's inverter and installing more panels? Then buy a much cheaper normal AC well pump. It might be cheaper to wire up a 240V split phase system and using a 240V deep well pump- than installing a separate grundfos system.

    One of the advantages is that with a single, bigger PV system you have more choice about what to do with the energy- e.g. on poor solar days you can choose to not run the pump and make use of the extra PV to service the house's loads. And on days when you really need more water pumped, you have your whole house array to use for the pump instead of being constrained by the small array for the grundfos.

    With regards to the grounding, I have a similar problem with the pump being about 1000ft from the inverter- I used separate ground rods- unbonded. I used a single PV system, 240V for 1000ft to a variable frequency drive which connects to a normal 3 phase AC well pump (1.5kW).

    (Note I'm not in the US so don't need to comply with NEC, and went for a TT grounded system http://en.wikipedia.org/wiki/Earthing_system#TT_networks)
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Off Grid Well Pump Grounding and Backup Power Options
    logaholic wrote: »
    for close to the cost of copper involved I could pick up a spare generator to use
    <snip>
    but it would be real nice to flip a switch in the basement rather than drag a generator down to the well in the event of a shortage.
    My preference would be to have a spare generator. Or maybe I would do this: Run the power wire in the trench (no bonding wire). Do not connect it at either end, and make sure that at each end it is not even near any wiring from the two independent electrical systems. In the event of an emergency you just need to make some temporary connections at either end.
    BTW, I have no inspector to contend with (but I am up to code, I think).

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Off Grid Well Pump Grounding and Backup Power Options

    If you've already got the Grundfos and its array there's no turning back. Otherwise like Stephen said; put the lot into a bigger household system and just run 240 VAC to the (less expensive) pump.

    As for the grounding issue itself, I believe you are interpreting NEC regs in respect to multiple grounding rods to create a single ground point. This really doesn't apply here in my opinion. I would not worry about running 2 AWG to connect the two ground points because they are each adequate for their respective jobs; you are not trying to add one to the other to obtain sufficient earth contact to satisfy grounding requirements.

    It's unfortunate that the regs are subject to opinion, and as I said before what ultimately matters is what the AHJ in this case says. Sometimes they know the letter and not the intent, if you know what I mean.
  • BB.BB. Super Moderators, Administrators Posts: 30,637 admin
    Re: Off Grid Well Pump Grounding and Backup Power Options

    Also, the 2 AWG wiring is on the battery bank, which is isolated from the AC loads by the Inverter. If you were running battery power directly to the well--that would be a different issue.

    The battery wiring has to be heavy because of the 12-48 volt battery bank voltage is much lower than your 120/240 VAC side (power = Voltage * Current). So--I would only read the NEC rule as applying to the AC side only.

    Also, review the code again... For example, on my home, I have 6 AWG ground wire even though I have something much heavier for the service drop from the pole.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • logaholiclogaholic Registered Users Posts: 9
    Re: Off Grid Well Pump Grounding and Backup Power Options

    Thanks for all the suggestions. I think what I may do is go the portable generator route, but go ahead and lay the conduit in case I ever change my mind after we have some experience with the system. We need a portable generator so it may be time to go ahead and get one. I'll try and keep this thread updated with how the system progresses this summer as we get it built in case anyone else faces a similar situation.
  • mike95490mike95490 Solar Expert Posts: 8,815 ✭✭✭✭✭
    Re: Off Grid Well Pump Grounding and Backup Power Options

    How have I not seen this till now ??

    Anyway, I'd had much the same issue, and decided to build one large PV array, large inverter, and generic, simple 240VAC pump and motor. I've been told that 3wire pumps start better than the 2 wire. And if something dies, I can get a replacement pump quickly and from anywhere.
    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 ,

  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Off Grid Well Pump Grounding and Backup Power Options
    logaholic wrote: »
    I think what I may do is go the portable generator route, but go ahead and lay the conduit in case I ever change my mind after we have some experience with the system.

    Conduits tend to fill up with water. Make sure you suck some plastic twine through it before it fills up. --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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