I am planning on splicing two 15A lighting only (no outlets) circuits in panel box in order to eliminate a tandem breaker in the panel.
This will be for a total lights replacement with new generation wafer LEDs, direct wire connections to LED transformer boxes after removing larger incandescent can lights.
This will result in twenty 14w LEDs single CB load (3A?) on 14/2 cable. I would still have a a third light circuit in hallway area to prevent floor blackout if a breaker tripped..
IMO replacing a properly installed tandem breaker with a single breaker plus a wire nut inside the breaker panel purely for aesthetic/elegance reasons is not worth pursuing. You aren't really reducing crowding and you're adding complexity.
I've been contemplating doing something similar to what you suggest, but in a full panel combining 8 circuits into two. Most of them are not lighting, they serve one socket that does nothing but charge phones, or perhaps once-upon-a-time served a window A/C (before central was installed).
My approach would be to install a splice box near the breaker panel, high up on the basement wall so there would be plenty of cable to work with, and to use 5-way lever nuts as neatly as possible inside that box, and connect it to the breaker panel with a conduit and four wires.
This would meaningfully reduce crowding in the breaker panel and regain breaker spaces.
The eight circuits use a mixture of 12 and 14 ga wire, so the two breakers serving them would have to be 15A and in one place where there is a 20A outlet I'd have to replace it with a 15.
Yes: LED lighting is a real game changer. My son and I wired his new house (3,000 sq ft) and we only used 2 15 amp lighting circuits, all lighting was LED. You are correct in that you'll be drawing about 3 amps on a 15 amp circuit.
Regarding splicing in a panel, it's not desirable, but is code legal. Do you need the additional breaker space? Or are you just trying to "clean things up"?
