Newbies setting up system - question on size of cables for our 4 Trojan 6v T105

TradiscantiaTradiscantia Solar Expert Posts: 31 ✭✭
Hi,
How do we determine what size wire to use for the battery cables? And what size fuses or breaker boxes, and where to put them?

our new RV system which will consist of

two 245 watt panels
Tristar MPPT 45 charge controller
Trimeter battery meter 2025 RV
4 Trojan T 105 to be wired in series (2) and in parallel
existing modified sine wave inverter 800 continuous watts 1600 surge

Any place online where we can see a schematic of a full installation similar to ours?

Janis and David

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Newbies setting up system - question on size of cables for our 4 Trojan 6v T105

    If you look in the manual for the inverter it should give recommendations specific to that inverter for wire and fuse size. Ordinarily it would be something along the lines of (800 Watts/12 Volts = 67 Amps) 2 AWG minimum with probably a 150 Amp fuse. Technically it is based on maximum Wattage at minimum input Voltage, taking into consideration wire length and the NEC 80% derating rule.

    Which is why I suggest looking at the inverter manual as they have figured this all out already. Saves work.

    Likewise the wiring for the charge controller is based on the same principals of maximum expected current, wire length, and Voltage drop. It can be sized either for the maximum of the controller or maximum of how much will be used, in this case about 32 Amps out so it may as well be wired for maximum capacity. Again the manual for the controller should give you the engineer's answer.
  • TradiscantiaTradiscantia Solar Expert Posts: 31 ✭✭
    Re: Newbies setting up system - question on size of cables for our 4 Trojan 6v T105

    Thanks Cariboucoot,
    We have both, but neither of us is 'up' on electrical terminology, and one chart is in ampH while another specifies ampacity and after a while, our minds are spinning.

    Our inverter manual says "use the cable provided." Not very helpful when we now need to wire the 4 batteries together and buy the correct wire size.
    Our batteries are not only going to power the inverter but also go directly into the 12 volt system box for the lights.

    Oh, I just found the cable supplied is 12 gauge. That seems awfully small to me! Does that mean we need to use the same size cables to connect the two batteries together?


    So does that mean we should use # 1 or #0 wire instead of #2?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Newbies setting up system - question on size of cables for our 4 Trojan 6v T105

    12 AWG?! Right; good for about 20 Amps continuous or (20 *12) 240 Watts. Uh, where are you supposed to get the rest of the power from? Did they at least give you a sensible fuse size?

    Let me put it this way: no one ever regretted putting in the biggest wire that would fit. So if you can fit 1 AWG or 0 AWG you will not only be able to handle the current (about 100 and 150 Amps respectively) but also minimize Voltage sag under load.

    The wires that connect the batteries should be the same size as feed the inverter. Some may call it overkill, but it works.

    Amp hours is Amps (rate) over time. Factor in Voltage and you have a quantity: Watt hours.

    And for wiring those four batteries up take a look at method #2 here: http://www.smartgauge.co.uk/batt_con.html
  • TradiscantiaTradiscantia Solar Expert Posts: 31 ✭✭
    Re: Newbies setting up system - question on size of cables for our 4 Trojan 6v T105

    What you say makes sense. The inverter that came with this rig is fairly minimalist, and now we have a pile of dough invested in solar components, but we don't want to buy another inverter unless we have to.
    So if we wire with #2, to be a bit more conservative, that means we will have to rewire the inverter, right? Could this get us into trouble with the operation of the inverter?
    I forgot to add that our main (for the DC system or when tied into shore power) is 30 amp.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Newbies setting up system - question on size of cables for our 4 Trojan 6v T105

    Okay, you are looking at three separate but inter-acting circuits:
    1). Panels to charge controller
    2). Charge controller to batteries
    3). Batteries to inverter

    Each of these will operate at a different Voltage and current over different wire lengths. The heaviest power will be between batteries and inverter. This is where the Voltage could be as low as 10 and the current as high as 80 (10 * 80 = 800 you only really need to consider the constant current as conductors will take a momentary surge well above their constant rating). This is where the heavy (2 AWG minimum) wire will be. The fuse/breaker would be 100 Amp. With 0 AWG you could go up to 150 (not likely you'll see that current).

    The circuit between the charge controller and batteries will handle up to 15 Volts @ 32 Amps and needs to be wired for that minimally. You might as well go for the full current because there isn't much difference here. I think you'll find the terminals will accept 6 AWG maximum. Thus fFusing on this circuit would be for the 45 Amps maximum of the controller 70 or 80 Amps.

    The wires from the panels to the controller are usually the highest Voltage and least current, which is good because they are also typically the longest wire runs (most affected by V-drop). Since it is an MPPT controller you can put the two panels in series or parallel; no fuses needed and the current at most would be 2X Isc of one panel (in parallel). Pretty sure these panels are about 8 to 9 Amps around 30 Vmp. So you are looking at maybe 18 Amps maximum @ 30 Volts or 9 Amps maximum at 60 Volts. For best efficiency keeping the panels in parallel is probably desirable here. The distance will have the biggest bearing here, but even 12 AWG could handle it for 12 feet at maximum power with <3% V-drop.
  • westendwestend Solar Expert Posts: 46
    Re: Newbies setting up system - question on size of cables for our 4 Trojan 6v T105

    You should size the battery cable to handle the maximum load of your inverter and all other DC draw from the DC distribution panel. Those loads would be a propane fridge, a furnace, lights, water pump, fans,and any hardwired alarms. You don't state if you have slides but those are a separate load that may be wired separately from the distribution panel. After you have the total, add 20% ampacity to correctly size the battery cables. You'll also have to take into consideration distances between inverter and battery bank, distribution panel and battery bank, and the slide wiring if you have it.

    FWIW, I just wired up a new DC distribution panel to handle solar, converter and 600w inverter. I used AWG #4 for the panel and the less than one foot to one bank of batteries. I also have a bank of batteries 6' away from the panel. That will be wired with AWG #2. I am trying to keep my voltage losses to less than 1%.

    The larger cable will only make the inverter happier as voltage drop will be less.

    Your main ampacity for the shore power AC is 30 amp? That is typical for most travel trailers. If you're referring to an AC circuit breaker or a main fuse on the DC distribution panel, that is not so relevant for cable sizes. Best advice so far in this thread is Cariboocoot's, "no one ever regretted putting in the biggest wire that would fit.".
  • TradiscantiaTradiscantia Solar Expert Posts: 31 ✭✭
    Re: Newbies setting up system - question on size of cables for our 4 Trojan 6v T105

    Thanks! This is now clear.
    I am going to call the inverter manufacturer (Cobra) tomorrow morning to make sure we can rewire the input to it with larger wire.
  • TradiscantiaTradiscantia Solar Expert Posts: 31 ✭✭
    Re: Newbies setting up system - question on size of cables for our 4 Trojan 6v T105

    We called the manufacturer (Cobra) and learned that they supplied a #12 cable with their 800 watt inverter "for the occasional use of a coffee pot." Duh? And then the manufacturer of the rig attached it to an 850 watt fridge and didn't rewire? Gives one lots of faith in the industry :grr
    So we are going to rewire with #2 AWG (although the company rep said we should use #6, but the wisdom and experience of this group WINS OUT over a manufacturer that has already demonstrated a high degree of ..... (I better stop before I say something I'll regret)

    Next question: the electrical supply shop has several different qualities of wire. What do you guys recommend?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Newbies setting up system - question on size of cables for our 4 Trojan 6v T105

    I think they must be confused about input and output. Although I don't see an 800 Watt inverter running a 1200 Watt coffee pot even with 12 AWG on the output. On the input it would fry in an instant (try putting 3X the rated current through a wire and see what happens. CAUTION: don't do this at home!)

    On the output side the current is approximately 1/10 the input.

    I think I'd be looking for a different inverter, if that's all the company knows. :roll:

    With 2 AWG you're looking at a couple of basic different types. The 'standard' multi-strand wire is easier to get ends on but the 'welding' wire is more flexible (lots of tiny little wires inside). If you can use the standard, do so. If you've got problems getting around corners you may need the welding wire.

    What are the choices available? Most of the wire differences will be in insulation, and you're pretty good there as it won't be outside (no UV exposure) and probably not subject to oil either.
  • TradiscantiaTradiscantia Solar Expert Posts: 31 ✭✭
    Re: Newbies setting up system - question on size of cables for our 4 Trojan 6v T105

    Actually it will be outside. We have a small cargo trailer made into a travel trailer by VRV. To go solar, we replaced the dinky 12v battery on the tongue and built a battery box suitable for 4 6v batteries. We chose solar instead of a new fridge (the fridge that came with this is AC only) since the cost of buying a propane fridge and remodeling the interior was just short of the cost of solar equipment. I think the price difference, before buying shunts and wires and fuses, was about $300, and we prefer not to use more fossil fuels than we already do in our vehicles.

    In addition to the battery cables, my husband was thinking of bringing the wires from the solar panels down the outside slope and then drilling one hole inside near the point of the curve so that we could have only one hole for the wires going into and out of the interior storage compartment where the controller and inverter are mounted. Only the wires to the battery monitor (Trimetric) will be fully inside the rig.

    If we had $$ falling from the sky, we'd buy a new inverter. But this one is here, and it works, and it's barely been used as the previous owner of the rig used it only to haul a load from So CA to Alaska. (At least we will have power falling from the sky soon ;)
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