Rv wiring size

lipetslipets Solar Expert Posts: 61 ✭✭
I have a 600 watt system with #10 wire from roof to combiner box then to Kid about 4' , then from the Kid again #10 about 8 feet to 4-6 volt batteries, works fine all summer.

this was recommended by Midnight.

Now I getting a new RV, the factory will pre-wire it, similar setup, will use likely 3 200 +- watt panels, 4-6 volts.
this will be again about 4' from roof to combiner box & controller so I was going to have a #10 wire there.

But the distance from the controller to batteries will be about 17', so I was thinking a #8 or #6.

opinions?

Also may in future add 2 more 6V total of 6.

Lastly are there any higher efficacy panels on the market that I may want to look at?
and what specs may be best, I know 600 watts worked for our needs, but I can mount 3-200's or any other size equaling about 600W

tks

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,521 admin
    Re: Rv wiring size
    lipets wrote: »
    I have a 600 watt system with #10 wire from roof to combiner box then to Kid about 4' , then from the Kid again #10 about 8 feet to 4-6 volt batteries, works fine all summer.

    Assuming this is a 12 volt battery bank:

    600 Watts * 0.77 panel+controller derate * 1/14.5 volts charging = 32.5 amp

    Controller is rated for a maximum of 30 amps (you will get a little clipping of available charging current on a nice/sunny/clool day--But no problems). Using a generic voltage drop calculator:

    30 amps by 8 feet on 10 AWG cable => 0.6 volt drop

    My 2 cents--Your voltage drop from the battery bank to the charge controller should be around 0.05 to 0.1 volts a maximum drop so that the controller can accurately measure the battery voltage. In your case, the controller may think i is charging at 14.7 volts (pretty aggressive charging), but the battery is only seeing 14.1 volts -- A pretty leisurely charging rate for flooded cell batteries.

    So, beside the wire heating losses (0.6v*30a=18watts of losses in battery wiring), you are not getting the quick/high current charging in Bulk and early Absorb phases for your battery bank that you would really like.

    My first suggestion is to move the controller much closer to the battery bank. And, if you can, use heavier cable (will 6 AWG cable fit, or at least splice in 6 awg next to the controller). 2 feet and 6 awg gives you 0.06 volt drop--Much better.

    Now I getting a new RV, the factory will pre-wire it, similar setup, will use likely 3 200 +- watt panels, 4-6 volts.
    this will be again about 4' from roof to combiner box & controller so I was going to have a #10 wire there.

    But the distance from the controller to batteries will be about 17', so I was thinking a #8 or #6.

    opinions?

    Noooo.... 17 feet @ 30 amps on 6 awg cable gives you 0.5 volts of drop again.

    In my humble opinion--Short+heavy cable from the charge controller to the battery bank is critical for fast/aggressive/accurate battery charging.

    Note to Midnite and other controller manufacturers... Most controllers do not have a remote battery sense lines option... However, virtually all of them have the ability to measure output current current and offset their output voltage (temperature compensation, meter calibration, etc.).

    There is no reason for remote voltage sense... Just add a screen setting for battery cable resistance. Voltage Drop = Cable Resistance * Current. Simply subtract the battery charging current * cable resistance to get the proper battery voltage AT THE BATTERY. Saves the issue of problems with sense leads/connections. And gives the customer a much better chance at quickly/accurately charging the battery bank with difficult wiring/placement issues.
    Also may in future add 2 more 6V total of 6.

    3x 220 AH battery strings = 660 Amp*Hour at 12 volt capacity
    30 amps charging / 660 AH = 0.045 = 4.5% rate of charge

    That is really too low of charging rate for an off grid full time system... For a weekend use cabin/dry camping RV--5% is about the minimum rate of charge I would suggest. For full time off grid living, 10% to 13%+ rate of charge with solar is usually a better choice.

    Do you have a sepecific need for that amount of battery bank (~660 AH @ 12 volts)? If you have shore power and genset available to help keep the battery bank "Happy" -- That would help.

    On the other hand, keeping with your 440 AH battery bank and simply firing up the genset when needed (bad weather, perhaps power to run skill saw/other larger power tools) may be a better choice. Batteries that are subject to low rates of charge generally need more baby sitting/more generator run-time/etc. or you run the risk of "deficit charging" and sulfating them--Leading to several years less of battery life.

    Not to say that a 4.5% rate of charge won't work--But look at battery bank to see if you really need that 660 AH bank or not--And make sure you have genset/other power sources to help keep the bank well charged (i.e., run the genset in the early AM to 80% or so State of Charge, then let the solar finish charging the rest of the day).
    Lastly are there any higher efficacy panels on the market that I may want to look at?
    and what specs may be best, I know 600 watts worked for our needs, but I can mount 3-200's or any other size equaling about 600W

    You might see if you can find HIT Photovoltaic - Panasonic module. When Sanyo used to make them--They were very well regarded panels. They may be hard to find in the US (last I heard a year or so ago--I am not in the solar business).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • zonebluezoneblue Solar Expert Posts: 1,218 ✭✭✭✭
    Re: Rv wiring size

    As bill says you need to mount the controller next to the disconnect, right next to the batterys. Then all you have to do is get the cable to the array right, and youre good to go.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


  • ReedReed Solar Expert Posts: 55 ✭✭
    Re: Rv wiring size

    Higher voltage from panels means less amperage and lower voltage loss. Larger cabling always is a good plan.
  • lipetslipets Solar Expert Posts: 61 ✭✭
    Re: Rv wiring size

    Spoke to one of the engineers at Midnite he said with 600 watts #10 is fine 4 panels maybe #8 but #10 will suffice at the given parameters
  • BB.BB. Super Moderators, Administrators Posts: 29,521 admin
    Re: Rv wiring size

    Voltage does matter... For a 100 Volt Vmp array:

    600 Watts / 100 Volts = 6 amps with ~3+ volts drop just fine (for wire run)

    For a 12 volt battery bank:

    600 Watts / 12 volts = 50 amps -- On a 12 volt device, 0.5 to 1.0 volt drop max, for charging 0.05 to 0.10 volt drop recommended

    Details matter when you go from paper high level design to actual hardware--A LOT.

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