Combiner basics and fuses

cb1000ridercb1000rider Registered Users Posts: 6
I'm working on designing a 3.1 kW "grid-tie" system. I'm an EE by education, have residential wiring experience (just finished my first custom home) and I'm working on a new project with a builder who employs a master electrician.

The electrician is fairly old school and doesn't like new things like solar.

Our design is currently 14 x 244 watt panels (24v each).
These will be "ground" mounted on a slight slope, no shade, optimum fixed orientation for our location.


Standard solar implementation seems to arrange groups of panels in what I believe are called "strings". I assume in my case this will be groups of panels wired in parallel.

Questions:
1) I see a lot of "Combiner" options - they all seem to have fuses setup for 150 V DC or 600 V DC at specific amperage. Are the voltage ratings simply standard for fuses.

2) How many panels are typical on a "string" (volts, amps, or watts are all fine for description)

Can someone give me a basic overview of connecting 14 panels to the typical combiner solution? I assume after the combiner, we do our "long" run to the inverter/grid tie...

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: Combiner basics and fuses

    Depends on your "load" for the solar array...

    Typically DC charge controllers (for battery banks) usually have a maximum working voltage of 150 VDC.

    For Grid Tied Inverters, they typically run up to 600 VAC Maximum (which is usually what the 120/240 VAC hardware we use around our homes is rated for also).

    So the box/fuse/breaker ratings depend on your usage.

    For the correct way to configure your series parallel strings, many vendors offer a website to help you define the optimum string for your installation (based on brand/model of solar panels, your local weather conditions, and how much total wattage you will be installing):

    Xantex XW 60 Amp MPPT Charge Controller String Sizer
    Xantex Grid Tie Inverter String Sizer

    Note that the terms of Off-Grid and Grid Tied are becomming more confusing... For example, the Xantrex XW product family Inverter is both Off Grid and Grid Tied capable (and also can fall back to Off Grid if the Grid fails). The XW Inverter uses the XW 60 amp Battery Charge Controller.

    The "standard" Grid Tied inverter is just solar panel-GT inverter-AC Breakers (no battery involved).

    So, just need to be clear on which GT type inverter you are looking at to give an more exact answer.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • cb1000ridercb1000rider Registered Users Posts: 6
    Re: Combiner basics and fuses

    Specifics of the implementation:
    Grid Tie (to residential power distribution)
    No battery, no storage, no generator back up.

    I'm not tied to a particular inverter (yet) - I've had the 3.0kW FRONIUS IG Inverter spec'd for me, but I'm up for suggestions and recognize that my panel / array output may go up to 130% of rating.

    Suggested inverters?

    Thanks for the calculators, BTW.
    There is something fundamental that I'm missing here. I'm seeing really high Voc voltages (circuit voltages)... Are the strings wired with the panels in series or parallel... I know that more current is bad when dealing with resistance over distance - which would be one reason to have more voltage and less current.
    Edit: It looks like grid tie inverters are designed to work with really high (150v-600v) DC voltages (yikes) - different from the charge controllers I've been playing with... Most likely wired in series.
  • mike95490mike95490 Solar Expert Posts: 9,359 ✭✭✭✭✭
    Re: Combiner basics and fuses
    There is something fundamental that I'm missing here. I'm seeing really high Voc voltages (circuit voltages)... Are the strings wired with the panels in series or parallel... I know that more current is bad when dealing with resistance over distance - which would be one reason to have more voltage and less current.
    Edit: It looks like grid tie inverters are designed to work with really high (150v-600v) DC voltages (yikes) - different from the charge controllers I've been playing with... Most likely wired in series.

    Exactly, GT inverters are 300-600V devices, 350-400V DC seems to be a good voltage for the down-conversion's best efficiency. Higher and you get more internal inverter losses, and too low, some inverters get flakey.
    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 ,

  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: Combiner basics and fuses

    Our host, Northern Arizona Wind and Sun, does sell GT inverters that they stand behind... You don't have to purchase from them, but it is a good place to start for pricing and good quality equipment. (yes--I will plug our host when asked an open ended question--they have been very friendly to the DYI support for everyone on this forum--Personally, neither Niel nor I have any business relationships with NAWS or in the Solar RE industry--we just help with spam control here).

    For somebody like me to recommend an inverter--I only have one home and one installation. It is a Xantrex GT 3.0 and has been working perfectly for 4 years now... A sample size of one.

    Things to look for--I personally like to avoid fans (mechanical devices that do not last forever and can be noisy) and open electronics where there is air flow (dust).

    Also, Xantrex has a simple RS 232 interface that you can get free software to run on an old low power PC (and even push to web--I don't use it). Many others require (expensive?) add-in cards/devices for data collection. A few appear to even need a yearly web subscription service.

    Some Inverters have multiple solar panel string inputs--nice if you have an east roof and and west roof (different Vmp based on temperature--better power collection).

    Most should offer 10 year warranty (mine came with 5 year).

    Look for good mfg.--but also compare pricing on a $$$/watt basis. Don't over pay unless there is some feature that you really want.

    Price with delivery and taxes included--shipping/insuring solar panels is not cheap.

    Personally, a ~3kWatt GT inverter/system is a good price point to start... Smaller inverters/systems can have a much higher $$$/watt cost.

    If you need backup power, the Xantrex XW system is hard to beat--but it is not cheap (no off-grid solar RE system is)... Personally, I chose to go with a small Honda eu2000i generator (and manual transfer switch) as we have few outages that last more than a handful of minutes. A small genset with backup fuel is more practical for me. Your mileage may differ.

    Your array can probably be done with one string of 14 panels with a GT Inverter... No combiners or fuses/breakers needed on the solar panel strings.

    Don't worry about the high solar panel voltages... It is really no different than your normal household AC wiring which is also rated to 600 VAC maximum too (all the basic stuff is interchangeable between the DC and AC side of the inverter, conduit, wiring, etc... Obviously, there are some things to watch out for like switches, circuit breakers and fuses--AC vs DC is a big difference).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • cb1000ridercb1000rider Registered Users Posts: 6
    Re: Combiner basics and fuses

    Thanks, you guys are awesome - I'll also look at our hosts products, I'm not tied to a particular solution other than buying "brand name" panels themselves.

    My only *yikes* at this point is wiring 400-500 VDC panels, that much DC voltage isn't good for you - we'll need to make sure the panels are covered when wiring and be very careful to stay away from potential arc/short situations.
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: Combiner basics and fuses

    Depending on your installation--covering the panels may be more dangerous (2nd story installation, wind, blowing paper/canvas/etc.).

    The current code and panels come with plugs where you have to work a bit at touching the conductors. And even then, you would only get a shock if you touched both the positive and negative leads at the same time. Because the solar panel output is "floating" (not ground referenced), even touching one lead is not going to shock you (unless you are working in the rain or a heavy salty fog on the coast).

    And--even covering the panels will reduce the amperage--but perhaps not the voltage (a thin tarp can still let light through)... It is not the 10's of amps at 500+ volts that will hurt you--it is the 0.01's of amps--which the panels may still be able to generate even when covered with a "blue tarp".

    When making the final connections, make sure the DC disconnect is turned off--the MC style connectors are not designed to make/break under load (current flow) and could be damaged if plugged/unplugged with a "hot" inverter or charge controller.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • cb1000ridercb1000rider Registered Users Posts: 6
    Re: Combiner basics and fuses

    Thanks, Bill.
    Right now the spec is for a ground mounted setup. I've suggested this over roof mounted simply for expandability - I'll run conduit that allows more panels / strings or offer the option to do a larger inverter.

    DC current is only "live" across conductors... I did note that the NEC requires the panels to be grounded - which I'm going to assume means a minimal ground gauge of the largest wire available and to the main breaker box ground.
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: Combiner basics and fuses

    Regarding "Grounding"--There are "two" grounds for solar panels... One is the metal frames which, generally, connect to a #6 awg wire to a local ground rod...

    And then there is "grounding" the negative or positive lead of the panels to local DC ground rod in the battery shed or house ground rod... That ground is a bit more of an issue. Most GT Type Inverters will require you to connect the Green Wire to their grounding block... And they will "take care of the solar panel grounding" (GT Inverter generally have a ground fault circuit to detect power to ground faults).

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