Wire sizing for MPPT

photovoltaic
photovoltaic Solar Expert Posts: 37 ✭✭
in PV Installers Forum - NEC, Wiring, Installation #1
Hi,
I am trying to determine the correct wire size to go from my combiner box to MPPT controller.

I have a Midnite Solar Classic 150 MPPT controller and 10 Kyocera KD135 watt panels - two arrays of 5 panels in series.

Do I use the Kyocera Open circuit voltage Voc 22.1v to calculate the voltage drop of my wire going from the combiner box to the MPPT controller - which would be 22.1 volts x 5 = 110.5 volts ?

I used the panels Imp 7.63 amps in wire size calculator and get this result

A maximum distance of 196.55 feet will limit the voltage drop to 3% or less with a 10 AWG Copper conductor delivering 8 amps on a 110 volt system.

Did I use the correct panel voltages and amp specs to do this calculation ?
Thank you for any advice.
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Comments

  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #2
    Re: Wire sizing for MPPT
    photovoltaic wrote: »
    Did I use the correct panel voltages and amp specs to do this calculation ?
    Thank you for any advice.

    Just as you use Imp to calculate the voltage drop at maximum current, you should use Vmp rather than Voc to calculate the percentage that the voltage drop is of the total voltage.
    That will make your results slightly worse.
    Are you using 196.55 feet as the single-wire one way distance? Or are you using the calculation for two wires each running 196.55 feet distance? As you would expect there is a factor of two difference there.

    I believe that for this particular calculator, if you select single phase, it will calculate the voltage drop for two wires, while if you select three phase, it will calculate the voltage drop for a single wire. (Don't ask why, it's complicated. :-))
    SMA SB 3000, old BP panels.
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #3
    Re: Wire sizing for MPPT
    photovoltaic wrote: »
    Did I use the correct panel voltages and amp specs to do this calculation ?

    You have two strings and the current in each string is 7.63 amps. When you combine them the current is 15.26 amps.

    How far are the panels from the controller? #10 AWG will result in a 3% voltage drop if the distance is 86 ft.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • photovoltaic
    photovoltaic Solar Expert Posts: 37 ✭✭
    #4
    Re: Wire sizing for MPPT
    vtmaps wrote: »
    You have two strings and the current in each string is 7.63 amps. When you combine them the current is 15.26 amps.

    How far are the panels from the controller? #10 AWG will result in a 3% voltage drop if the distance is 86 ft.

    --vtMaps

    Oh yeah - I actually woke up this morning and remembered I forgot to add the amps !
    My question is answered about which voltage rating to use - I will use the Vmp 17.7v in my calculation instead of Voc - Yes I am using single phase and understood that the calculator was giving me the two wire calculation .

    I am going to measure the distance for the wiring - I have a temporary solar setup now and it is good to the get the hands on experience before I make everything final.
    If I can get by with 10 gauge wire it is less expensive. Now I have to decide if I should use conduit and pull wire or if I should just use UF-B direct burial wire. From reading some other posts on here I should be okay to fuse 10 gauge wire with 30 amp DC rated breaker which will protect the wire as well as be sufficient for the 15.26 amp current potential. I have 15 amp breakers on each string in the combiner box.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,639 admin
    #5
    Re: Wire sizing for MPPT

    Yes, for solar we generally over size the wire for the current carried--So you can use the "full capacity" breaker, or one rated for 1.25x max continuous.

    The only caution--For very long wire runs--you can get enough resistance that a "dead short" will not draw enough current to trip the breaker. The one caution about using a maximum rated breaker for a wire run (V=I*R; I=V/R). Remember that breakers really do not trip quickly unless they have current flow much greater than rated current (a typical thermal breaker will pass 1.25x rated current for a 1/2 hour or more--and may never trip).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • photovoltaic
    photovoltaic Solar Expert Posts: 37 ✭✭
    #6
    Re: Wire sizing for MPPT
    BB. wrote: »
    Yes, for solar we generally over size the wire for the current carried--So you can use the "full capacity" breaker, or one rated for 1.25x max continuous.

    The only caution--For very long wire runs--you can get enough resistance that a "dead short" will not draw enough current to trip the breaker. The one caution about using a maximum rated breaker for a wire run (V=I*R; I=V/R). Remember that breakers really do not trip quickly unless they have current flow much greater than rated current (a typical thermal breaker will pass 1.25x rated current for a 1/2 hour or more--and may never trip).

    -Bill

    Yeah Bill I noticed that time delay last weekend - was testing a 600 watt inverter at full load and it was drawing 49 amps through a 40 amp breaker - it tripped eventually - took a few minutes
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