RV System sizing wire, and fuses

GreenhornGreenhorn Registered Users Posts: 11
Hello Folks,
I decided to give the rv a pv system for my own education. I located second hand, a 180watt,36 volt panel. A grid tie panel I believe. It VOC is 38.9 from my dmm. And just about 4amps from the dmm. So looks like around 140 watts real, no load.

I found a controller "BZ" mppt 250. Rated for 50v input and 25 amps.
I need to run approximately 6feet of wire to the controller input. Then the closest I can mount the controller to the battery is about 12 feet of wire.

It seems the most amperage the panel could send to the controller is 140w/12v= 12.6amps? Is this calc realistic?

My questions are regarding wire gauge and general hookup. I have quality stranded 10 gauge, but the sizing charts are telling me maybe I should run 8 gauge. I would like some input here please. Im on a budget and dont want to buy wire if Ive got free stuff in the garage.

Also, what type of fuses can I use that will take that gauge of wire. Im thinking I need a shut off from the panel as well? Or Where should it go.

How do I isolate the pv system from the standard converter charging system for hookups. Can they both be tied to the battery ; ie in parallel. Dont want to cook anything. Thanks for any input you can give.

Also, How do I ground the panel frame itself or is that necessary? It is aluminum.

I have not rec'd the controller yet and the directions may answer my questions, but I just want to be sure. Thanks again.
Charles

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,800 admin
    Re: RV System sizing wire, and fuses

    Charles,
    Greenhorn wrote: »
    Hello Folks,
    I decided to give the rv a pv system for my own education. I located second hand, a 180watt,36 volt panel. A grid tie panel I believe. It VOC is 38.9 from my dmm. And just about 4amps from the dmm. So looks like around 140 watts real, no load.

    I found a controller "BZ" mppt 250. Rated for 50v input and 25 amps.
    I need to run approximately 6feet of wire to the controller input. Then the closest I can mount the controller to the battery is about 12 feet of wire.

    Regarding the solar panel--4 amps short circuit sounds quite low (almost 1/2). If this was a "murky day"--then it is probably OK. The short circuit output current from a solar panel should be proportional to the amount of solar energy hitting the panel. Pointing at the sun at noon, on a very clear day--and you should be near Isc rating of panel.

    If the panel is only outputting 1/2 the current--then look for burned spots on the panel (bad wire, diodes, etc.). Flex the panel (just a little--it is glass) / run your hand up/down the back of the panel and see if the current changes. (Make sure the glass is clean--my panels with the local trees + a bit of water makes a brown stain that looks like a burned spot--washes off with soap and water).

    Sorry--My two cents is the BZ (at least from a couple years ago) was just a paper weight... Almost any controller (PWM or MPPT) would do better.

    However, in this case, with a 29 volt Vmp panel and a (I guess) a 12 volt battery bank--you really need to stick with an MPPT type controller.
    It seems the most amperage the panel could send to the controller is 140w/12v= 12.6amps? Is this calc realistic?
    If there is a name/model number on the panel, you will find Voc, Vmp, Imp, Isc.

    Assuming this is a 180 watt panel with Voc=36volts, then Isc (short circuit current) would be around 8 amps maximum:
    • 180 watts * 1/29 Vmp * 1.25 = 7.8 amps or so (guessing)
    The NEC would want another 1.25 safety margin (NEC does not control your RV--but it is still a safe/conservative set of rules to follow):
    • 7.8 amps * 1.25 = 9.75 amps
    • Round up to 15 amp circuit minimum (15 amp fuse, 14 AWG minimum).
    Assuming 7 amps maximum, 6 foot one way run (12 foot round trip--different wiring drop calculators use one or the other number). For yours, using this voltage drop calculator:
    • 6 foot one way run, 7 amps, 10 awg = 0.42 volt drop
    • 0.42/29 volts = 0.014 (1.4% drop)
    Perfectly OK.
    My questions are regarding wire gauge and general hookup. I have quality stranded 10 gauge, but the sizing charts are telling me maybe I should run 8 gauge. I would like some input here please. I am on a budget and dont want to buy wire if Ive got free stuff in the garage.

    Also, what type of fuses can I use that will take that gauge of wire. I am thinking I need a shut off from the panel as well? Or Where should it go.
    You don't need to fuse (or even switch) the solar panel side of the wiring... Just a fuse/breaker between the controller's battery output and the battery itself, near the battery bank, sized to the output rating of the controller or wiring (which ever is smaller). Keep the wiring run between the battery bank and the controller short--you don't want a lot of voltage drop here--it will interfere with battery charging if you have too much drop.

    You can use automotive fuses/breakers, or you can use ones like these:

    Fuses
    Midnite Breakers + Boxes
    Battery Bank Switch
    How do I isolate the pv system from the standard converter charging system for hookups. Can they both be tied to the battery ; ie in parallel. Don't want to cook anything. Thanks for any input you can give.
    Yes, assuming the solar charge controller is properly programed and operating--you can just parallel to the battery bank with the other AC charger/converter.
    Also, How do I ground the panel frame itself or is that necessary? It is aluminum.
    Aluminum is very easy to get electrolysis/corrosion. You can get grounding lugs that are designed for grounding aluminum:

    Grounding Lug

    But, in reality, you are mounting this panel to a (I would guess) a metal skinned RV with metal rails/mounts--You don't really need to ground the frame any more (if lightning hits the panel frame--you will have big problems anyway).
    I have not rec'd the controller yet and the directions may answer my questions, but I just want to be sure. Thanks again.
    Charles
    Depending one what you want to do, these would be better controllers for smaller arrays:

    Morningstar SunSaver 15 Amp MPPT Solar Charge Controller
    Morningstar TriStar 45 amp MPPT solar charge controller
    Rogue Power Technologies

    The above is a starting point--everyone has their own opinions and mine are no better than others'--much of it is just from reading here over the last few years (My system is Grid Tied--not Off-Grid; and I am not in the solar business).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • GreenhornGreenhorn Registered Users Posts: 11
    Re: RV System sizing wire, and fuses

    Thanks Bill for the detailed response.

    And thanks for the brutal honesty in my poor selection of a controller. It is not too late at this time for me to cancel the cheap paper weight and to fork up the addtional two hundred bucks if that will make the difference in real results. Ouch...but ok. I will make the adjustment and get the morning star controller, and display and return the unopened BZ(I did read a thread here about how bad it is).

    With that said, I need to give you a little bit better data this time.
    I measured the Voc and Isc again.... this time with direct high noon on a sunny clear day. I also located the manufacturer data:

    Data On Panel:
    Voc=44.2V
    Isc=5.58A
    Vmp=35.4
    Imp=5.08A

    My Readings: Solar Noon, Panel mounted Flat on RV roof.
    Voc=38.4
    Isc=5.3A

    So Following your rough calculations:
    Assuming this is a 180 watt panel with Voc=36volts, then Isc (short circuit current) would be around 8 amps maximum:

    * 180 watts * 1/29 Vmp * 1.25 = 7.8 amps or so (guessing)
    Then * 180 watts * 1/35.4 Vmp * 1.25 = 5.08 amps
    The NEC would want another 1.25 safety margin (NEC does not control your RV--but it is still a safe/conservative set of rules to follow):

    * 7.8 amps * 1.25 = 9.75 amps
    * Round up to 15 amp circuit minimum (15 amp fuse, 14 AWG minimum).
    Then * 5.08 amps * 1.25 = 6.35 amps
    * Round up to 15 amp circuit minimum (15 amp fuse, 14 AWG minimum)?

    Assuming 7 amps maximum, 6 foot one way run (12 foot round trip--different wiring drop calculators use one or the other number). For yours, using this voltage drop calculator:

    * 6 foot one way run, 7 amps, 10 awg = 0.42 volt drop
    * 0.42/29 volts = 0.014 (1.4% drop)
    Safe to assume 7amps max again....but Im thinking of moving the controller closer to the battery as you stated

    Ok, Am I better off to run a longer run (14 feet to the controller from the panel) from pv to controller and keep the controller close to the battery (3 feet). Without moving the battery inside the trailer I am thinking the morning star could go closer to the battery as you said, but it will increase the distance from pv panel to controller. Whats best? Im figuring things this way. I tried to follow your logic below.

    Keep the wiring run between the battery bank and the controller short--you don't want a lot of voltage drop here--it will interfere with battery charging if you have too much drop.

    ..so I have around 14 ft or a 28ft loop to the controller from the pv module. Following your model:
    *14ft one way, 7amps, 10awg =0.84 voltage drop
    *.84/35.4=0.024(or 2.4%) is this reasonable or should I go up to 8awg?



    This only from Panel to Controller, Correct? So from controller to battery we have a current variation due to the controller. Im guessing: Imp x Vmp = 5 x 38 = 177W max.
    177w/12v=14.75 amps max from controller to battery? So will 10awg be suitable to run about 3 feet to the controller?

    Then I will run separate wires from the controller back up to an easy to see display panel on the wall in the cabin(approx 10ft) . Any size restriction on the lcd?

    Thanks for correction on fuse placement. I dont need a fuse in series with the panel to the controller. Ok.


    Finally Bill, The pv panel frame is aluminum and the trailer roof is rubber (epdm I think). I mounted aluminum angle to the rv roof and bolted the panel to the angle, therefore there is no "equipment ground" for panel unless I run a ground wire. What do you suggest? Will I just create a ground loop with this wire? Thanks again for such fantastic direction.
    Charles
  • BB.BB. Super Moderators, Administrators Posts: 32,800 admin
    Re: RV System sizing wire, and fuses
    Greenhorn wrote: »
    Data On Panel:
    Voc=44.2V
    Isc=5.58A
    Vmp=35.4
    Imp=5.08A

    My Readings: Solar Noon, Panel mounted Flat on RV roof.
    Voc=38.4
    Isc=5.3A
    Data looks fine.

    And sorry about the BZ...
    So Following your rough calculations:

    Then * 180 watts * 1/35.4 Vmp * 1.25 = 5.08 amps
    ..so I have around 14 ft or a 28ft loop to the controller from the pv module. Following your model:
    *14ft one way, 7amps, 10awg =0.84 voltage drop
    *.84/35.4=0.024(or 2.4%) is this reasonable or should I go up to 8awg?
    Using 14 feet one way, 10 awg wire, and 5.5 amps current (in real life, your panels will not exceed Imp very often (a few times a year for less than an hour?)... Using the earlier voltage drop calculator, I get a 0.2 volt drop:
    • 0.2 volt drop / 35.4 Vmp = 0.006 = 0.6% drop
    I think there may have been a mistake/typo somewhere with the 0.84 volt drop... I think that is too high.

    Yes, a 5 amps load with 14 awg would be a good minimum wire gauge (you could go even smaller, but why bother).

    And yes, keep the wire run between controller and battery as close as practicable. Even if this makes the solar panel to controller longer/more voltage drop.
    This only from Panel to Controller, Correct? So from controller to battery we have a current variation due to the controller. Im guessing: Imp x Vmp = 5 x 38 = 177W max.
    177w/12v=14.75 amps max from controller to battery? So will 10awg be suitable to run about 3 feet to the controller?

    Yes, you are correct, you need to make calculations for each wiring run:
    • 180 watts * 0.95 controller losses * 1/12 volts charging = 14.25 amps typical maximum operational
    • 14.25 * 1.25 NEC derating * 1.25 NEC solar derating = 22.3 amps minimum circuit/fusing/breaker
    So, wiring for 25/30 amps (10 awg and 25-30 amp fuse/breaker) is fine.

    Voltage drop calculation; 3 foot one way run, 14.25 amps, 12 volt battery:
    • 0.1 volt drop
    • 0.1 volt drop / 12 volts = 0.008 = 0.8% drop
    By the way, make sure you get the remote battery temperature sensor -- Especially if you choose the MorningStar 15 amp MPPT charge controller (ir really needs the RBTS).

    By the way, MPPT type charge controllers have current limit on their output by design... So you can use the 15 amp controller with 14.25 amps and no safety margin (for controller selection)... You can even have more solar panels than 15 amp output--the MPPT controller will simply limit output to its maximum current rating.
    Then I will run separate wires from the controller back up to an easy to see display panel on the wall in the cabin(approx 10ft) . Any size restriction on the lcd?

    Yes--But I would be doing the same as you--read the manuals (I don't think you will hit the limit in an RV application).
    Thanks for correction on fuse placement. I dont need a fuse in series with the panel to the controller. Ok.

    Not for your application. If you had 3 or more panels (or panel strings) in parallel, then you would need a series protection fuse in each string.
    Finally Bill, The pv panel frame is aluminum and the trailer roof is rubber (epdm I think). I mounted aluminum angle to the rv roof and bolted the panel to the angle, therefore there is no "equipment ground" for panel unless I run a ground wire. What do you suggest? Will I just create a ground loop with this wire?

    OK--here is where we choose between practical and insanely safety conscious.

    Practical: Only a broken solar panel or lightning strike would possibly energize the frame. The frame is not in a touchable area--or at worst case the mounting screws hit some metal (frame, metal roof base) somewhere so there is a current path to frame/battery ground. It will be obvious if there is a solar panel to frame short--all the safety glass will be shattered.

    Safety: Piece of metal can become energized with lightning strike or 5+ amps at 35 volts. Need a dedicated safety ground wire from the panel to the frame/battery ground (you do have the battery frame grounded somewhere, right?) to prevent the metal frame from becoming energized and shocking somebody (at 24 VDC and higher, your wet hands can give you a shock) or causing some current flow through something (sheet-metal and mounting screw) and overheating/causing wood structure to catch fire.

    Your choice. ;)

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • GreenhornGreenhorn Registered Users Posts: 11
    Re: RV System sizing wire, and fuses

    Bill,
    Your help and this forum have been great. I cant thank you enough. I should receive the additional parts by Friday and will post pics of the project hopefully this weekend. Next week is a 4 day dry camp out trip out so hopefully Ill be ready. Thanks again for the fantastic input.
    Best Regards
    Charles:D
  • BB.BB. Super Moderators, Administrators Posts: 32,800 admin
    Re: RV System sizing wire, and fuses

    You are very welcome Charles.

    We are here to help and learn from each other.

    Looking forward to the pictures of the project and of the trip!

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • GreenhornGreenhorn Registered Users Posts: 11
    Re: RV System sizing wire, and fuses

    Hello Again,
    Looks Like I have a new question after receiving my shipment. Im about to start running wire and such, but I have a question.

    It's about the load terminals on the sunsaver 15 amp controller. This may seem stupid, but must these terminals be used? I realize that they would go to a load distribution for the various loads...I.E. at the dc end of the fuse panel. However, it seems to me that since the dc end of the fuse panel is already connected to the battery and I am running the controller (battery terminals) to the battery this would be a redundant connection to connect directly to the distribution? :confused:

    I am thinking this would be a good connection to run to an inverter if I get one. Please advise on my logic. My gut is telling me Ive got an analytical error here. Im going to re-read the literature and maybe that will solve my dilema.

    Thanks ahead of time for the help!
    Charles
  • mike95490mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: RV System sizing wire, and fuses
    Greenhorn wrote: »
    It's about the load terminals on the sunsaver 15 amp controller. This may seem stupid, but must these terminals be used?

    For an inverter, NEVER. Any inverter will draw too much current for the internal load terminal wires. (should be a limit stated in the paperwork)

    For a small load, like lights or something that may be left on accidentally, it's to help protect the battery, but they generally cut off at 80% discharged.
    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 ,

  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: RV System sizing wire, and fuses

    as was said, never for an inverter and no you do not ever have to use it at all if you do not want to. usually these have a low voltage disconnect associated with them, but often it disconnects far too late anyway so it is somewhat useless in my eyes.
  • GreenhornGreenhorn Registered Users Posts: 11
    Re: RV System sizing wire, and fuses

    Yes Guys,

    Thanks for the input. Upon a real review of instructions it is all clear. The load terminals are essentially for a low voltage cutoff to save the battery as you stated, so if i bypass those I wont have this feature. I may wait on hooking up this feature. Im in a time crunch to be on the road early next week with the pretty much disassembled at this point.

    And thanks for the NO NO on an inverter tied there. It clearly states Do Not Connect an Inverter to the Load Terminals. I should always read things ten times before asking questions. Thanks again for the notes. :D

    Charles
  • GreenhornGreenhorn Registered Users Posts: 11
    Re: RV System working!: Battery Sizing

    Well we now have a working RV- solar system up and running. Yippee! Everything appears to be doing what it should. Pretty Cool. Taking off for festival tomorrow.
    Finished in nick of time.

    I have only one question picking my brain. I read that the Morningstar Controller states a 100ah battery minimum and I dont actually know the rating on my battery? Its not labeld. It is a large Interstate battery; labeled deep cycle marine. no crank amps or ah rating. So far the system has charged it to just over 50ah. Im thinking that is the limit. If so.,,,any overcharge issues I should be concerned about and should I disconnect (remove fuse) at this level to avoid damage/other issues. I intend to upgrade batteries in a few paychecks. Thanks for any advice as usual. I will post pics of install apon return from trip.
    Thanks all
    Charles:D
  • BB.BB. Super Moderators, Administrators Posts: 32,800 admin
    Re: RV System sizing wire, and fuses

    Either weigh or take the exterior measurements of the battery and compare it with some standard deep cycle batteries and their AH rating... Will be close enough for government work.

    If the charger is doing what it is supposed to do (hold 14.x volts, and if a 3 stage charger, back down to 13.7 volts for float)--then you should be fine for now and I would not disconnect the battery.

    If you are seeing more than 14.8 volts for long periods of time and/or the battery is "bubbling" a lot (for several hours)--then it is getting over charged and you need to look at the controller's programming and output voltage to see what is going on. The battery will eventually be damaged by over charging.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • dwhdwh Solar Expert Posts: 1,341 ✭✭✭
    Re: RV System working!: Battery Sizing
    Greenhorn wrote: »
    So far the system has charged it to just over 50ah.

    Um...well, that's not quite right.

    "50ah" would indicate how much total electricity has flowed into the battery, but it doesn't tell you anything about how close the battery is to full charge unless you know what size the battery is. 50ah into a 100ah battery is one thing, 50ah into a 80ah battery is something else and 50ah into a 2400ah battery bank is...well not enough to mention.

    Charging goes by voltage. So you would say that the system has charged the battery "up to" 13.2v or 13.8v or whatever (depends on your battery specs and what you set your charger to).

    The battery is full when the charger gets to float stage (battery full, maintenance charge to keep topped off). This goes by voltage, not amp hours. Depending on how much you drew down that battery, it might take 20ah to get to float, it might take 60ah.

    So, if you know that you have a 100ah battery, and the charger is regularly pushing 50ah of electricity into the battery to get the voltage up to float stage, then you know that you are regularly drawing the battery down by 50%.

    For a 100ah battery (average "deep cycle marine" size is usually around there), if you consistently drain over 50ah out of it, then it won't last nearly as long (number of cycles it can do before it fails) as it would if you regularly drain only 25ah out of it.
  • GreenhornGreenhorn Registered Users Posts: 11
    Re: RV System sizing wire, and fuses

    Thanks for the guys. I think I'm good to go for now. I'm off the the show. Will post some pics and update apon return.
    Regards
    Charles
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