Not sure what gauge wire to go with

K1ngN0thingK1ngN0thing Solar Expert Posts: 27
I just got a Tristar TS45 charge controller and it's a lot more complicated than I expected. I'm not sure what gauge wire to buy, and what else I have to get, if anything. I thought all I'd have to do is connect it to my battery and solar panel with wires, but I'm reading stuff about fuses and resistors and it's a little overwhelming. A sticker inside of the controller recommends 2-4 gauge wire but according to the calculations, but given my setup (one 100w, 12v panel with the controller less than 10 feet away from the battery) I don't need wire that thick. Would it be safe to go with a thinner wire as per the calculations? for example.


  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    If you never put more than a 100 watt panel on the TS45, then you do not need as heavy cable.

    Assuming 10 feet, 0.10 volt maximum drop, 5.71 Amp Imp panel, and using a generic voltage drop calculator:

    10 AWG cable:
    Voltage drop: 0.072
    Voltage drop percentage: 0.50%
    Voltage at the end: 14.428

    Your minimum wire/breaker rating would be:

    5.71 amps * 1.25 NEC derating * 1.25 NEC solar derating = 8.9 amps ~ 10 amp minimum fuse/breaker

    Of course, you could easily go up to 25 amps for the fuse/breaker based on 10 AWG cable.

    In general, you want to keep the wiring short and heavy from the charge controller to the battery bank for minimum voltage drop. You want the "longer wire run" to be from the solar array to the charge controller where voltage drop/variations are not as critical.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ImurphyImurphy Registered Users Posts: 16 ✭✭
    Assuming an almost impossible 10 amps from your single panel at 12volts with 10 ft of wire 10ga keeps you under 2% volt drop. You can always go bigger. If you plan on adding more panels down the road I suggest you buy the wire now instead of redoing it all later. If you max out the charger at 45 amps @ 12volt with 10 ft you would need a min of 4ga to stay under 2%. I would suggest going with 2ga myself as that gets you almost down to 1%. And a fuse rated to protect the wire should always be used.
  • K1ngN0thingK1ngN0thing Solar Expert Posts: 27 page 6 diagram on the left, are those curved breaks in the positive lines the fuses? How do I install fuses into the line? Why do I need a fuse if the solar panel can't output more than the wire can handle?
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Yes, those are fuses/breakers.

    The battery is the source of high current--So the fuses protect the wiring if there is ever a short circuit (in the controller or controller wiring). Solar panels by design cannot output high surge current. So, the fuses/breakers are sized large enough to not trip during normal operation.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • K1ngN0thingK1ngN0thing Solar Expert Posts: 27
    So something like this where I cut the wire and connect it to both ends with the fuse holder + fuse completing the circuit

    Also, what's the best way to pass my wires through my roof? I understand there are pass-throughs designed to minimize potential leaks, but don't know what search terms I should be using.
  • Wheelman55Wheelman55 Registered Users Posts: 140 ✭✭✭
    What does the group think of using a circuit breaker instead of a fuse? Cost is about the same and if the breaker trips it can be reset. With car audio I put the fuse/circuit breaker as close to the battery as I can to protect as much of the wire as possible.
    Building Off-Grid in Terlingua, TX
    14 CS 370 watt modules. HZLA horizontal tracker. Schneider: XW6048, Mini PDP, MPPT 80-600, SCP. 1 Discover AES 48 volt LiFePO4 battery 130 ah
  • PhotowhitPhotowhit Solar Expert Posts: 5,653 ✭✭✭✭✭
    Breakers are fine, but use DC rated breakers!
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former, 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Circuit breakers are usually the way to go. A bit more expensive in the short term, but can be less costly and make it much easier to service your system in the long term (over current protection and handy on/off switch for each load/circuit/charging source). And placing them near the battery, is best practice.

    Fuses can be much smaller/easier to install in limited spaces. For example, BlueSea makes a really nice/small fuse holder that is nice to use on a battery bank:

    Very nice for small to medium size systems and RV's.

    Many people do not fuse/protect the battery wiring. This is from the older days and car/marine battery use. These vehicles have very heavy wiring (to crank the motor) and the single battery is not large enough to cause the starter leads to start a fire before the battery goes "dead".

    With large battery banks to store several days worth of power for an off grid system--The battery banks are large enough to cause serious smoke and fire if the main bank wiring is shorted. Having a fuse per battery string is very handy.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    BB. wrote: »
    Having a fuse per battery string is very handy.

    But you need to keep more spare fuses on hand since a short circuit will blow all of them at once.
    It also allows you to protect each string at a somewhat lower current level instead of having one big fuse.
    The drawback to that approach is that anything which causes unequal current distribution may lead to the fuses blowing one after another under normal load or small overload until they are all blown.
    SMA SB 3000, old BP panels.
  • semperfi123semperfi123 Registered Users Posts: 6
    I use 8 strand from home depot at .43 a foot works good for me .
  • K1ngN0thingK1ngN0thing Solar Expert Posts: 27
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    That is a self resetting circuit breaker and has no manual on/off.

    Self resetting--You should only use that if you need the function (remote location with nobody around, etc.). Generally when you trip a breaker, you really want it to stay off until the problem is fixed (there are some designs where you want to limit maximum current for a motor or similar, but want auto reset to try again).

    Also, not having a manual on/off still leaves the need for a manual switch to be installed (generally, it is really handy to turn off all loads when you leave for an extended period--Also, it can be handy for debugging).

    Those smaller/automotive breakers are frequently not really rated for turn off 1,000's to 10,000's of Amperes of current that large Lead Acid battery banks and supply into a short circuit.

    For anything much more than a small RV or cabin (lights, laptop, cell phone charger, etc.), you should really look at full size circuit breakers.

    Here are some from our host NAWS--But you are welcome to buy them elsewhere/other types if it is better for you (marine supply store, etc.):

    Some High Amperage breakers (good for larger AC inverters, charge controllers, etc.):

    And some smaller breakers:

    Standard DC Fuses & Breakers

    Depending on the complexity of your system--It can be very nice to buy pre-wired DC Panels that put all the bits and pieces together in one engineered/tested unit. Here is an example from Midnite Solar (I believe they have more variations, and there are other vendors too--Talk with NAWS or other suppliers for further information):


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • scrubjaysnestscrubjaysnest Solar Expert Posts: 175 ✭✭✭
    I used manual reset breakers under the hood of the truck and auto resets back in the camper for wiring run protection. Blue Sea fuse from battery to distribution panel in the camper. The distribution panel has fuses with indicating leds for a blown fuse. Breakers bewteen the solar panels and the CC and the CC to the battery. Got the breakers at my favorite auto parts store.

    All wiring is either 8 or 10 AWG marine wire. This system has worked without problems for decades prior to adding solar. Added solar 3 years ago and to date no problems.
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