I don't believe there is such a standard.
There are two practical concerns you may want to consider:
1. The joint will be of significantly greater diameter than the original cable. Depending on your arrangement, they may not sit side by side.
2. Access for repair. The majority of cable failures occur at points of 'workmanship', ie terminations and joints, so you should plan for your joint to have a higher likelihood of failure and be more readily accessible, if possible.