Sometimes it's necessary to split power/signal from a single wire to multiple outputs. Normally I'd prefer to use a terminal/distribution block for something like this, but sometimes that's not feasible, such as when space is constrained or everything needs to be inline for a wiring harness. There actually aren't many inline connectors available for this purpose; a wire nut would do the job but isn't permanent, and there are some splice connectors like this Wago part which will accommodate up to five wires. This soldering method has worked well for me. Didn't think of the idea -- just documenting it.
Step 1: Identify and Strip "input" Wire and "output" Bundle
In this example I have one 16AWG wire feeding five 16AWG outputs for power distribution. Bundle the output wires together with whatever temporary method works (tape, zip ties, clamps, all of the above). Try to insert the end of the input wire into the center of the bundle.
Of course, when sizing your wires you'll need to consider the total current going through your input wire. Don't expect to max out multiple output lines with a single input conductor of the same size!
Step 2: Wrap the Entire Bundle With a Separate Strand
In this case I cut a separate length of about 80mm of my 16AWG wire and extracted a single strand. Attach the strand alongside the insulation of the input wire, and wrap it tightly around the bundle. Temporarily attach the other end to the output bundle if necessary to keep the wrap tight -- we'll cut off the excess later.
Step 3: Solder the Joint
There are many excellent soldering guides out there, so no need to rehash that. Since there are no sensitive components nearby we can use lots of heat and a broad chisel tip (in this case I set the iron to 430C). Heat up the joint and melt the solder with the wire, not the soldering iron. You should see the solder flow between the strands through the whole bundle. Work from multiple angles and make sure to get thorough coverage.
You should not be able to pull the bundle apart once the solder has cooled.
Step 4: Cover With Heat Shrink
Cover the entire joint with heat shrink. It can be helpful to use a 3:1 or higher shrink ratio tubing due to the size difference between the two ends of the joint.
All done! Thanks for reading.