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Wire sizing double-check and wire selection advice needed

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  • Wire sizing double-check and wire selection advice needed

    My question is in reference to the wire run from an rv-roof-mounted panel combiner box to an MPPT controller located inside the cabin.

    GT panels are arranged in a 2x3 array (3 parallel strings of 2 series-wired panels each). Each panel 300W @ 32V. Panels and combiner box on top of an RV roof in AZ desert (hot, hot, and then hotter still).

    My math gives me 14 combined amps (@63V) from the array. 10 awg 90-degree copper de-rated by .41 for high temps (159 - 176... the highest temp de-rating value in the table I'm referencing) gives me 16.4 amps.

    My run will be ~ 20 feet max, and the only portion of the wire that will be actually be exposed to the extreme temps I expect will be at or near the combiner box itself.

    So I'm thinking 10 AWG should be a good choice. Would you agree? Or should I go bigger still? For some reason it just seems small. Is my math screwy?

    Also, what specific wire type/brand would you choose for this run and/or anything between the panels and the combiner box? The higher heat-rating the better, I'd think, but I don't really know what there is to choose from over 90.
    Last edited by magicbus; 05-16-2019, 12:19 PM.

  • #2
    Originally posted by magicbus View Post
    So I'm thinking 10 AWG should be a good choice. Would you agree? Or should I go bigger still? For some reason it just seems small. Is my math screwy?
    .
    What is the voltage drop you'll see for the given amps, wire size and length?
    What is that voltage drop as a percentage of your expected voltage?

    The cons of going with larger than necessary wire are extra cost and difficulty in getting the wire to fit in a given conduit or bend around tight corners.
    The pros are less voltage drop, less heat, more safety margin w.r.t. having too much current flowing through the wire.
    So going a little larger on the wire is OK.

    And you're probably looking at NEC for the wire sizing - but an RV I think falls under different codes (if any code) because it's a vehicle. And I have no idea which type of wire are acceptable for use in an RV. (Is THHW2 OK? Don't know. Is there some some wire/wire-insulation that's specific to vehicles? I think so, but don't know)

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