Wiring

When I'm figuring what size wire and the distance I can run at a certain amp and voltage my question is this: According to my specs I can run 50' of wire at my ratings, from my panel (I have one) to my fuse box (combiner box, if I had more then one panel) is 45' so I am fine. Then from my fuse box to my charge controller is 30'. Now seeing I only have one panel the amperage is not increased at the combiner box, do I consider this two seperate runs under 50' or one run of 80"? Thank you

Steve

Comments

  • BB.BB. Super Moderators, Administrators Posts: 30,644 admin
    Re: Wiring

    Steve,

    You have lost me a bit...

    When you add solar panels, you can add them in parallel (increases current), or add them in series (add voltage).

    So, depending on how you add the additional panels, you may need heavier/more wiring or not.

    And, it also depends on the type of solar charge controller(s) you have...

    PWM (the inexpensive type) must match Vmp (voltage maximum power) of the solar panels to the battery bank voltage (example: Vmp~17.5 volts for a 12 volt battery bank)--You can only add more solar panels in Parallel, which increases your current flow. 2x the number of panels, you need 2x the copper (typically 3 AWG wire sizes thicker).

    For MPPT (Maximum Power Point Tracking) Solar Charge Controllers, you can add panels in series or parallel, your choice.

    If you add panels in series, the voltage will increase but current will remain the same. And you do not need to add more copper).

    Of course, in all setups, the Array Vmp/Imp must match the requirements of the Solar Charge Controller used.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: Wiring

    Maybe I explained it poorly, I'll try again. As an example let's say I have one solar panel, and at its rated amps and volts I can run a wire from it at a certain size up to 50'. Now lets say I have a combiner box 45' from the panel, in the combiner box I have a breaker for the panel, then from the combiner box to the charge controller I have 30', same size wire. Now my question is, is this considered one run for total length or is this considered two runs? You see the total is 75' for both runs which is to far for the amperage and voltage that I'm running on this size of wire, I can only go 50'. So do I have a 75' run or a 45' and a 30' because it is broken up by the combiner box?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Wiring

    You want to know if you should count the distance from panel(s) to controller plus controller to battery as one length, right?

    That would depend on what type of controller. Single or multiple panels with a PWM controller it's pretty much a straight run, as there is not much change in Voltage/current over the whole length. Multiple panels with an MPPT controller and you get into options of higher Voltage array with its lower line loss and lower current then down-converting to system Voltage and higher current.

    With one panel you can consider it one long run, but still keep the controller as close to the battery as possible as there will be some minor shift.

    Unless I'm not understanding the situation either. :confused: It happens.
  • BB.BB. Super Moderators, Administrators Posts: 30,644 admin
    Re: Wiring
    pls418 wrote: »
    Maybe I explained it poorly, I'll try again. As an example let's say I have one solar panel, and at its rated amps and volts I can run a wire from it at a certain size up to 50'. Now lets say I have a combiner box 45' from the panel, in the combiner box I have a breaker for the panel, then from the combiner box to the charge controller I have 30', same size wire. Now my question is, is this considered one run for total length or is this considered two runs?

    You have to look at several different factors... Of course, the wire has to be heavy enough to safely carry the current (i.e., 14 AWG wire with 15 amps maximum or 12 amps continuous).

    Second is the voltage drop... So you figure the voltage drop for the first 50' run based on 1xImp current.

    The second 45' run is based on 2x heavier wire and 2xImp current...

    So, for example.

    50' run, 4 AWG cable Imp=10 amps and Vmp=17.5 volts. Using a generic voltage drop calculator we see:
    • 50', 10amps, 4 awg cable => 0.27 volt drop
    And after the combiner, you have a second Imp=10 amp for 20 amps total on 45' run:
    • 45', 10amps, 0 awg cable => 0.21 volt drop
    Overall, for the 95' run, we recommend a maximum of 3% of 17.5 volt drop:
    • 17.5 volts * 0.03 = 0.525 volt drop maximum
    • 0.27+0.21 = 0.48 volt drop total
    So, the above example is just under our 3% recommended voltage drop limit.

    Note how heavy the wire gauge must be to support two ~175 watt solar panels on a 95' run (4 and 1/0 gauge cable)...

    If you instead placed the two panels in series for 95' run, 10 amps and Vmp=35 volts (1.05 volts maximum for 3% drop), the cable required would be:
    • 95', 10 amps, 10 awg cable => 0.84 volt drop
    Believe me, that is a whole lot cheaper to run 95' of 10 awg cable vs a bunch of 4 and 1/0 gauge cable for the same power (2x175 watts charging a 12 volt battery bank)...

    The only "extra cost" is you would need to use a MPPT type Charge Controller to run Vmp-array at 35 volts and charge the battery bank at 14.5 volts efficiently. (MPPT controller are not cheap).
    You see the total is 75' for both runs which is to far for the amperage and voltage that I'm running on this size of wire, I can only go 50'. So do I have a 75' run or a 45' and a 30' because it is broken up by the combiner box?

    If the "Combiner Box" is the Solar Charge Controller--You always have to mount the Solar Charge Controller very closely to the battery bank--Anything more than 5' or so becomes a problem (because of voltage drop).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Wiring

    basically, it is one run no matter how many smaller segments you add together for it. this is consistent from pvs to cc. from cc to batteries also is part of that same run, but often controllers can change the voltage as many mppt type controllers do. in any case the voltage drops are additive as are the voltage drop percentages for each section. now many consider the batteries as a load for the pvs and it is advisable to not stop there as the loads for the batteries and the wire runs for them also are part of the overall % of the system. in fact from cc to battery is very critical in that a few tenths of a volt dropped can fool the cc into thinking a battery has reached its proper charge voltage setpoints and cause a deficit charge and eventually bring about sulfation to the batteries and thus an early demise.

    now the nec does say for ac circuits that from source to load should be less than 5%. they state that due to heat build ups on the wires as that voltage drop at a current represents wattage dissipated at about 3 btus per watt. we have further incentives in keeping within those percentages as pv power is far more expensive and is often precious in many remote locations. some recommend a max of a 3% drop and still others recommend a max of a 2% drop, but is often representative from the pvs to the cc from cc manufacturers. this usually is an initial base investment made in the copper wires for the system. they can get quite large when high currents are meant to travel long distances and these long distances are quite a bit shorter than many think. i should also state that even pv interconnecting wires are also to be included in these calculations and many of these are for the small #12 used in these mc connections and the resulting higher losses there can force even larger wires for the run in order to keep voltage drops and the percentages down. some still have j boxes, but i'm not sure if the nec has forced the mc connections on us or not, but i believe they did. somebody can refresh my memory here on that, but the pv manufacturers will void a warranty if those cables/mc connectors are cut on their pvs.

    i know i went off again by getting carried away.:roll::cry:
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