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BFL13's avatar
BFL13
Explorer II
Jun 19, 2015

Solar --Wire and Battery Fuse Size Confusion- How Big?

Can somebody please explain the math on this? I get really confused.

Previously, I have seen how you have to allow 1.25 oversizing for the wires from the panel, the controller, and the wires from controller to battery, but then the battery fuse size was to be even more oversized, so that fuse itself would be operating at no more than 80% of its rating. (When the 1.25 oversized stuff was running "filled up" to the 1.25 amount)

But then some blurbs on this warned against "counting the 1.25 twice", which made no sense concerning the battery fuse or CB (at least to me :( )

Now , reading the manual for my new Canadian Solar panel, it says,

" A module's Voc and Isc at STC should be multiplied by 1.25 when determining component voltage ratings, conductor ampacities, overcurrent device ratings, and the size of controls connected to the PV output." AND THEN, it says, "An additional 1.25 multiplier for a short-circuit current (for a total of 1.56), for sizing conductors and fuses MAY be applicable, as described in section 690-8 of US NEC."

(stresses mine)

So I need more coffee maybe, but where did that 1.56 come from doing two 1.25s ? And what is the difference between an "overcurrent device" as in the first list, and a "fuse" in the second part?

And-- does the second part's "conductors" for the 1.56 mean the same wires as in the first part's 1.25 "conductor ampacities"?

So if you did want to follow the US NEC (whether or not it applies to you--being not in the USA or because you are doing an RV not a house perhaps) what exactly do you do picking wire size and battery fuse size wrt panel Voc and Isc?

Does the solar controller size stay at 1.25 since it is not mentioned in the 1.56 part?

Thanks!
  • jrnymn7's avatar
    jrnymn7
    Explorer
    Jun 19, 2015
    I like to "fuse" at just above whatever current will usually be running thru the wire. A 75a charger, thru 2awg, means I would use a 90-100a c.b. I figure, this way, if the battery shorts, the c.b will (hopefully) blow long before the 2awg wire melts and catches fire, and voltage drop (thru c.b) will not be too outrageous. I've been running 45a thru a 50a c.b, and will be going bigger, as I don't like the v.d. I'm seeing at 45a.

    Same goes for solar controller output. If normally less than say 25a to bank, thru 6awg, I'd use a 30a c.b. Using the same 30a c.b. before the solar controller's input has shown zero v.d., to two decimal places, at 8a input.
  • pianotuna's avatar
    pianotuna
    Nomad III
    Jun 19, 2015
    Hi BFl13,

    Fuses are not slow blow devices. Use 1.56 multiplier, or if you are under 300 watts, no fuse at all from panel to controller.

    From controller to battery bank, fuse to the ampacity of the wire.
  • BFL13's avatar
    BFL13
    Explorer II
    Jun 19, 2015
    Thanks for the clarifications. Couple of niggling things:

    -Mex, it's not always the panel output that counts. With 24v panel and 12v battery, the controller-battery path carries almost twice the amps the panel-controller path does, so ampacity requirement goes up.

    -The wire gauge on the controller-battery path has to prevent voltage drop, have sufficient ampacity, and be fused to protect the wire "from the battery" but the current from the battery would be huge. I take it that if the wire is fused to the gauge for the controller output amps (and a bit more like 1.25 more) then that fuse will also blow if the battery shorts. That just leaves the piece of wire between battery and fuse to melt, which is why that piece is usually stated to be no more than 18 inches long (and not behind a wall I would think)

    -The operating at 80% NEC guide/rule is for "continuous" (yes?) but the 1.25 over STC idea is for "occasional" times when the panel is doing more than its Voc /Isc at STC. So why can't the fuse etc be at just 1.25 too, not 1.56, since it is not expected to be running at 1.25 continually?
  • Flyfasteddie's avatar
    Flyfasteddie
    Explorer
    Jun 19, 2015
    I agree that the engineers make this far more complicated than it needs to be.

    But to answer one of your original questions, the reason two 1.25's = 1.56 is because they are multiplied, not added. 1.25x1.25=1.56

    If you up-size the wire by 1.25, then up-size the fuse by an *additional* 1.25 over that, you are up-sizing the fuse by 1.56 over the original Voc and Isc.

    IMHO, just make sure you use large enough wire to get the minimum voltage drop you are comfortable with, then size the fuse sufficient to protect the wire size.
  • MEXICOWANDERER's avatar
    MEXICOWANDERER
    Explorer
    Jun 19, 2015
    Protect the wiring is job 1

    Unless somone botched the panel installation the panel to controller wire size is WAY bigger because of avoiding voltage drop. So you short panel output 'till the cows come home and the wire will laugh at it.

    Fusing from the battery is another thing altogether. A short can cause a fire. So a fuse must protect this side from the battery.

    But basically it is ignorance pure and simple to fit fuses rated way about array output potential.

    Some "tech writers" can't count past ten without unzipping their pants. Common sense should be your way point not hare-brained theory from a pimple-faced kid.
  • RJsfishin's avatar
    RJsfishin
    Explorer
    Jun 19, 2015
    Quit reading ! We know how it works, and its really very simple.
    I seldom learn anything new that is of any noticable value by reading all the hi tech confusing stuff. But if that is your "thing". go for it.
  • Homeless_by_Cho's avatar
    Homeless_by_Cho
    Explorer
    Jun 19, 2015
    I am also confused. I am presently installing solar on my truck camper and have been watching/reading many these solar threads and I am overwhelmed.

    Here is my conclusion: Many of the threads are making this incredibly complicated for the first time ordinary user/installer. This deep theory is good for the extended education of the dedicated solar enthusiast to be able to harvest the absolute most solar energy but what I need is a common everyday Rule-of-Thumb installation guide.

    1) Wire should be sized for no more than X% voltage drop based on XXX current.
    2) Install fuses between X and Y components.
    3) Install disconnect devices between XXX and YYY locations.
    4) Fuses should be sized for XXX current of the solar panel or controller output.
    5) Use series wiring between panels up to XXX voltage.
    6) Use PWM controllers for XXX. Use MPPT controllers for YYY. Use either for applications between XXX and YYY.
    7) Etc.

    Rules-of-Thumb are available for house and business AC installations. Many times they are also code requirements. I completely support the in-depth discussions and experiments by the many solar users but they are way overwhelming for the normal uses. I would like to see a set of Rules-of-Thumb.

    Thank you,
    LeRoy

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