We need to splice or extend existing AWG 1 main service panel wires and the splice would be left open to rain. What kind of connectors that would create watertight waterproof splice?
I have this (but putting tape all over it would not create waterproof splice because water can leak in:
Unless you're the power company I don't think you can do that. You'd have to have a water tight/weatherproof junction box and then use split bolt connectors, if they're still allowed. In my experience the cable is usually replaced.