Small cable to bigger cable. Can I do it?

I'm upgrading my solar system. It's about 45 ft from my panels to my MPPT. The first five feet from the panels has very difficult access. Would it be acceptable to use the existing wire for that section and then splice on bigger cable for the rest of the run? The various wiring size calculators say the remaining first section would be acceptable for voltage loss as would the spliced on section. I can provide more specifics if necessary. Thanks.

It's not a perfect solution. But better than not changing anything.

ABargeman said:

I'm upgrading my solar system. It's about 45 ft from my panels to my MPPT. The first five feet from the panels has very difficult access. Would it be acceptable to use the existing wire for that section and then splice on bigger cable for the rest of the run? The various wiring size calculators say the remaining first section would be acceptable for voltage loss as would the spliced on section. I can provide more specifics if necessary. Thanks.

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Consider what type splices to use. Twist on wire nuts, I would avoid and use something more positive. Crimped and soldered butt splices or split bolt are better than twist on nuts.

Put ring terminals on the ends of the two wires. Then install a single post terminal to put the two ring terminals on. I connected two wires of different sizes in a butt connector. It worked, but it's kind of permanent. See my write-up here. If I had to do it over, I probably would go with the ring terminals and single post.

I would use Polaris connectors. Polaris is the brand , electricians sometimes call them squids.
They are insulated , weatherproof , have good wire range sizes , no crimping required and are reusable. They can get a little spendy, but they are quick and reliable. You only need a hex key and strippers to install them.

Best option for the splice, really depends on where the splice will be located. (Box, free air, underground, ect...) and the size of the wires.

Thanks for all the replies. The system is on my dutch barge so it's stranded wire; no wire nuts allowed. I I like the idea of the post terminal suggested by HRTKD, like a mini bus bar but I already have some Polaris-type connectors (mine just use a screw driver) and that is certainly where I am headed. The connection will be inside and I was thinking of heat shrink on it for protection. The difference in wire size is maybe from 14 to 10 AWG and I would use ferrules. Of course the best solution would be the bigger cable all the way but that would require hot steel work and I really didn't want to deal with that in this small village in France.
Again, thanks for all the replies. I don't post much; I lurk alot, and the forum has been an invaluable resource.