Off Grid wiring Question

Hey everyone,

This board has been a great educational resource for best pv practices.

I'm new to the PV arena and am setting up a small system on my currently unpowered garden shed/ham shack.

I've done my power budget math and think I have that covered but have a wiring question.

The major components of my system are as follows:
1 - Evergreen ES 190
Morningstar mppt
Morningstar Suresine
12V battery bank consists of 3 parallel power battery SLF12205 agm (179AH)
Baby box to tie inverter, panels and cc together.
20' 6Ga run from pv to baby box

Now for my questions:
Can the Midnite Solar 63 amp ground fault protector double as a pv disconnect?

Do I need additional lower rating fuses between the pv panel and the CC?

Thanks in advance for any help.

Comments

  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Off Grid wiring Question

    Hopp,

    1) The GFI breaker is actually two breakers in one. There's also a small mechanically-parallel breaker (typically 0.5 A) that opens if there's a system ground fault, and it's this action that causes the 63 A breaker to open.

    2) A circuit breaker's basic job is to protect the circuit wiring. Allowing for wire specs, conditions of use, various deratings, and the NEC (Article 310 in particular), you'd have to be using some pretty large wire for just the 63 A circuit breaker to be appropriate.

    3) The Evergreen ES-190's Isc spec is 8.05 A. The design spec per NEC 690.8 is 8.05 A x 125% = 10.0625 A. You'll need a DC rated circuit breaker rated for 100% continuous duty at that value or the next higher standard value, which is 12 A. This breaker should be wired in series with the GFI breaker.

    4) You could use this 12 A breaker as the PV DC disconnect, but, technically, you'd have to run the PV wires in EMT from the point of entry to the breaker box.

    HTH,
    Jim / crewzer
  • BB.
    BB. Super Moderators, Administrators Posts: 33,432 admin
    Re: Off Grid wiring Question

    Unless you are getting a building permit on this small system where they will require a GF system--I would not bother with any breaker on the Panel to CC connection... Size the wire per Jim's suggestion, and there is no need for a breaker/fuse at all with the single panel at that low of current.

    Also, I am not a believer in the Ground Fault detection circuitry... Assuming a properly installed system, the GF circuit will add no safety to the system (with that low of voltage and current ratings and the size wiring you are proposing to use), and can cause other issues (like ground loops and more sensitivity to damage from nearby lighting strikes--and GF breaker will do nothing to limit direct lighting strike damage--nothing can)...
    All in my very humble opinion. :cool:

    Using properly sized fuses/breakers in your connections from the battery to the loads and charger will take care of the major sources of problems.

    Also, make sure your battery box is properly vented, the battery connections are protected from falling metal objects (battery box is OK), and that you purchase the Remote Battery Temperature Sensor for your Charge Controller.

    Lastly, AGM batteries are "near perfect" for solar--but since you cannot check their specific gravity (for state of charge) and since you cannot add water if they are overcharged and vent--you will need either (or both) an accurate and calibrated voltmeter (measure battery voltage after 3+ hours of resting to estimate state of charge) and/or a battery monitor...

    Spec. wise--I very much like the new Xantrex LinkLite and LinkPro models--they both have a contact that can be programed to "turn on" when the battery is discharged below xx% state of charge... Very nice to protect your investment in your battery system (setup to sound an alarm, or turn off your inverter/loads)--and will help you understand/predict how much power you can use from your system. The closest thing to putting a "gas tank gauge" on your battery bank you can do...

    The Xantrex models are very pricey--the Trimetric at 1/2 the price is not a bad deal--and one or two others are using it with good results.

    And, take a look at getting a kill-a-watt meter... You will be very glad to have it to monitor the loads on your inverter--and you can also use it around the house to measure the energy usage of your 120 VAC 15 amp max appliances (fridge, stereo, TV, etc.)...

    At best, you will get between ~200-500 Watt*Hours per day 6-9 months of the year from your system... Don't fall into the trap of deficit charging your batteries--they will die an early death.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2
    System2 Posts: 6,290 admin
    Re: Off Grid wiring Question

    Thank you BB and crewzer for the helpful input.

    This is going to be a small non permitted system. This is just a test system before I add a serious solar/wind system to my camp in rural Northern MI. I'm only going to be living my current location for the next 4 years and when I move this system will be disassembled and moved to my next home.

    BB. The shed is ~100' from any other structure or any power source so I didn't see the benefit of a breaker on the panel to CC link either. Since it's a commercial panel, I don't believe I'll see any dangerous internal panel ground fault so it seems reasonable that I can forgo GFI. The panel feed will be enclosed in PVC conduit to prevent a bird from messing it up badly. The ground will not be.

    I understand the importance of the remote BTS and have one. The largest component I don't have is the Baby Box and breakers. As soon as I've figured out exactly what I need I plan to head on over to store.solar-electric.com.

    I'm also looking for a Morningstar modbus converter to hook a pic data logger to the CC. I've looked around and haven't been able to find it available. Until then the DMM will have to do for battery health. When my system is complete, I'll have the pic fire up a telemetry link every hour and update a webserver with both my weather station data and power parameters.

    Attached you'll find a first rough draft of my system -- I'm still unclear on the exact schematic wrt the baby box wiring but you can be sure I'll be certain of it before I proceed. I also need to check and make sure that the SureSine grounding point doesn't make a loop. If you see anything awry please let me know.

    Glad to see you think I can hit a minimum net power input somewhere around my projected minimum of .25kWh a day in Dec. If I don't hit that target I'll simply use less power -- no need to run the batteries ragged.

    Thanks again everybody.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,432 admin
    Re: Off Grid wiring Question

    Using this link (for International Falls, MI, and derating of 0.52 and 90 degree vertical panel for snow shedding)--you will average around 0.21 kWhrs per day for your worst month of November with your system... December, the next worst month, will average 0.25 kWhrs per day (250 watt*hours per day). You should get 10% better than I predicted above because you are using AGM batteries.

    Your grounding--I would "save the copper" and just ground the solar panel frame with a ground rod (or several, depending on your soil conditions) driven next the the mount...

    Also--if not stated before, assuming you have snow--make the mount adjustable so that it can be set at 33 to 48 degrees during summer/spring-fall, and near 90 degrees (vertical, and above the snow) during the winter. You will collect more energy in the winter, and not have to work as hard to keep the panels clear of snow.

    If you have lightning in your area--you might want to use a good connector from your panel to your solar charger--something you can unplug and move away from your solar charger area (or even out of the cabin). This will help prevent damage if your array is hit, or there is a nearby strike.

    Remember that this information from the Charge Controller only shows what has been collected--not what has been used or is available in the battery bank. If you have very little energy usage--then the amount of energy collected by the panel/charge controller will also appear very small too (since the batteries are full--the charge controller simply stops charging the battery--and there is no current flow to log).

    For safety grounding--to prevent ground loops think of a star... Every electrical circuit you intend to ground should only have one connection from the circuit/device to ground rod... Avoid things like grounding the Negative side of the battery bank, and grounding the negative chassis of a 12 volt car radio to your earth ground (as and example)... The problem is that for a car, anything metal is considered "earth/battery" ground... So--in this case your car radio chassis should just be connected to the negative 12 volt lead--and there should be no "safety" ground nearby. Otherwise, the negative/return current will have two paths to return on (through the normal negative lead to the battery and through the chassis to ground rod to battery negative terminal). In itself--multiple ground paths are not a huge problem--what becomes a problem is if one return path is a small negative lead for carrying a small current (say a car radio), and it shares a path with a large water pump (20+ amps)... It is possible to fry the small black lead if the safety and black lead both share the current load.

    Also, your safety ground is intended to be where lightning current/energy is dumped... Keeping that separate from your +/- power network is also a good idea (and, as always, the 12 volt circuits should be kept short--and if you have to send power any "large" distance, use 120 VAC + inverter -- if you can.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset