"doubling up" wires is not recommended because it's very easy for one wire to hog all the current if the splice is not perfect. Much better to take old wire out and replace it with proper size, if your conduit is too small, you are going to have to increase PV array voltage to be able to use safe wire fill in conduit. And then that means a MPPT controller that is rated for the array voltage and wattage. May need to pull 2 pairs, one for each array and use 2 controllers. Midnight and Xantrex can get up to 600V MPPT if needed, but I think you can avoid that,
From your wire gauge site:
B) It is not advisable to run important/critical current wires using multiple wires, due to the possibility that one or more wires could become disconnected or degraded due to corrosion, etc and the remaining wire(s) would have to bear the load without the extra help of the now defunct adjacent wires.
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