A properly done splice must be done to where the final result equals or exceeds the original cable. Depending on local conditions you might be able to skimp but it can and will come back to bite you.
If you test joints which is done more in breaker testing and sometimes bus bars, there are various specs but the big one is less than 1 milliohm resistance. It is usually in the microohms. We also usually compare phases because the book number is a pretty “loose” spec. Typically I get around 50-250 microohms depending on joint construction. You cannot totally eliminate SOME drop even if you TIG or Cadweld the copper together. It has to do with constriction resistance and with mechanical and crimp lugs, alpha spots. Constriction resistance plays a role not only when you change cable diameters but at ANY joint. In terms of proper splices I would go crimped barrel connectors or if you are really paranoid, Cadweld or TiG weld it. I Cadweld grounds but not phase conductors...too much work insulating big chunky welds, and you have to cut back and reinsulate anything heat damaged. There is a reason automotive went from solder joints to crimped years ago. SAE sponsored a lot of PhDs doing joint testing both mechanical and electrical . The barrel is marked with the MINIMUM number of crimps. Some manufacturers have connectors that allow multiple cable sizes. If you don’t have these you need to use bolted crimp lugs instead. This is a permanent connection. Don’t take chances with mechanical fittings underground. Those stupidly expensive insulated terminals are rated for underground but you are risking having to dig it back up. Temporary only. When Burndy, Polaris, etc., stop having failures we can revisit that. Those are temporary.
My splices have been literally underwater as in shore power for a dredge, for months at a time. Not just moist but full on submerged and subjected to mechanical force on occasion. The cable isn’t even really rated for that (water trees) but it does work especially at 600 V duty. No failures unless they are literally mechanically pulled apart and the failure happens in the cable next to the splice. It’s all workmanship. Every splice I ever repaired had workmanship issues. Like wire nuts buried in giant tape balls or one half attempt at a crimp using lineman pliers on a barrel marked for 3.
Second is insulation. Follow 3M guidelines. 2 layers of something to act as a separator...they recommend varnished cambric. Add putty if needed to smooth out odd shapes. Two layers of rubber splicing tape 130C. Two layers of vinyl stretched to half width. The tension causes it to pull against itself keeping everything packed in tight. Tape is cheap. If you want more, use more. I’ve also used adhesive lined shrink wrap over barrel connectors instead. Works good.
In multi conductor cable make sure to stagger the splices.
Finally you need to restore the outer jacket. Unless you put it in a rated vault use adhesive lined heavy wall shrink tubing. Again...I have done years of “field” testing in a marine environment. One rule to remember. ALWAYS put the jacket on loose BEFORE you connect anything. If you think it might not fit (cut and dry test first), slide a second one on. If it doesn’t work you have to cut everything up and start over. At that point the only other thing you can do is swear a lot when you realize just how far ahead of yourself you got.
There is more to do if it’s shielded but you don’t have to deal with that.
In terms of testing, micro ohm testing can only be done before insulating. A Megger tells you if you have some kind of defect somewhere. It picks up actual weak insulation in SOME instances as well as contamination and moisture. It tells you the condition today. It has almost zero predictive power even if you temperature correct or use PI. The one and only time to test workmanship is when the splices are being made by watching what is done. It’s not hard. Anyone can be trained. But there are a lot of steps, far too many short cuts, and it is 100% dependent on the skill and knowledge of the person doing it. Crimped barrels and lugs and die or dieless crimpers help with one part of it by taking the skill aspect away. Heat shrink tubing does too. But at the end of the day once it’s buried it’s a mystery.
