I'm planning on replacing a 100A stab-lok (house burner) sub panel with a QO Mains box but I may need about 6 more inches of SER to get connected comfortably. The original box is already about 12 in from ceiling (I know, too high, but it's neigh impossible to run a new service feed). Can the SER conductors be extended, inside the box, with a butt connector and shrink tubing? Or other thoughts? This is a sub panel in the basement fed off 100A breaker at main box in garage. It's not my first box conversion, but the first time I've worried about service cable length. Thanks in advance..
It is acceptable to splice additional wire to a feeder, as long as that feeder is not an unfused conductor (service entry). Keep in mind that improperly installed splice can become a real nightmare. It might be better to try and raise the new panel up if there are no obstructions rather than try extend the feeder.
Also if an AHJ gets involved they may want to see if the panel has the proper clearances to legally install the splice. That would be wire bending area and proper fill.