Wire 'codes' (for lack of a better term) are graded for the sheathing type. Meaning that THHN, etc means it is rated for a given condition (oil/gas, sunlight, exterior abrasion rating etc). Romex comes in an interior and exterior/direct burial versions for example. I too have used extension cords -- sometimes availability is the toughest part. Butt splices can be really good, but they require a decent crimp and proper sealing for exterior use. I have become a big fan of the 3M/Scotch brand silicon grease filled 3 wire crimps. I try to place them out of direct exposure and with the connector wedged pointing up (the wires/opening then point down). Most folks (often me included) don't do a very good job soldering. I have taken 2 soldering courses for flight rated soldering -- they are brutal (and I barely passed the second class). Another item that is tough: 'glued' heat shrink. What you almost always find is generic 2:1 (shrink rate) un-glued heat shrink. 3:1 with glue is like getting ice cream in the middle of the Sahara. Available, and the part that makes the whole job worth doing: sealed when done!
For those that may think "oh, I'll just use rtv". Even that has ratings fro exposure. Bought a house in the mid-90's. Tore out the drywall in the main part of the house and re-wired it (this is only one of MANY stories on that job). Got to where the romex looked modern and almost didn't tear out the next chunk of wall. Glad I did. Someone took three hanks of romex and soldered them together mid wall (looked like a spider web), then covered the joints with rtv. The rtv was crumbling from decay. Other problems I found were going to cause a fire sooner than this, but it was eventually going to. The lesson? Regardless of the improper (stressed wire junction not in a junction box) is that was not the correct use/rating for the RTV used.
Use the correct items the right way and it all works. The problem with butt splices is that they are either not crimped to spec or are used in exposed conditions (most are not rated for exposure). And that presumes you aren't using Harbor Freight grade shlock components (crimps; butt connectors, etc). You get what you pay for.
Another thing, as much as I am a fan of the round 6 pin connectors for anything under 4000#, I have to admit it is an old tech design that has been surpassed. The 7 pin blade type have one big advantage: the interface (blade to socket) tends to wipe (or clean) the common surfaces (faces?) as it is inserted/removed. The round pin connectors as used traditionally on trailers don't do that, nor is there a tension feature in the sockets or pins (spring shapes). The wiping affect is used in electronics connectors to encourage a good connection and to 'wipe' off corrosion. And yes, even in high quality central air only cabinets you get corrosion. And that is with known quality components (which to be fair seldom get exercised after systems are up and running -- versus a trailers connector).
Just my info.
